United States
Environmental Protection
Agency
Great Lakes National
Program Office
536 South Clark Street
Chicago, Illinois 60605
EPA-905/3-82 001
Zooplankton Community
Composition in the
Nearshore Waters of
Southern  Lake Michigan
                  r

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                                            EPA-905/3-82/OQ1
                                            July  1982
      ZOOPLANRTON COMMUNITY COMPOSITION

                      IN

  NEARSHORE WATERS OF  SOUTHERN  LAKE MICHIGAN


                      by
      John E. Gannon, F. James Bricker,
                     and
             Rathryn S.  Bricker
             Biological  Station
          The University  of Michigan
          Pellston, Michigan  49769
              Grant 1005337 01
               Project Officer

              David C. Rockwell
     Great Lakes National Program Office
           536  South  Clark Street
          Chicago,  Illinois  60605
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                  REGION V
          CHICAGO, ILLINOIS  60609

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                                  DISCLAIMER
     This report has been reviewed by the Office of Research and Development,
U.S. Environmental Protection Agency, and approved for publication.   Mention
of trade names or commercial products does not constitute endorsement or
recommendation for use.
                                      11

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                                   FOREWORD
     The Great Lakes National Program Office (GLNPO) of the United States
Environmental Protection Agency was established in Region V, Chicago to focus
attention on the significant and complex natural resource represented by the
Great Lakes.

     GLNPO implements a multi-media environmental management program drawing
on a wide range of expertise represented by Universities, private firms,
State, Federal, and Canadian Governmental Agencies and the International Joint
Commission.  The goal of the GLNPO program is to develop programs, practices
and technology necessary for a better understanding of the Great Lakes Basin
Ecosystem and to eliminate or reduce to the maximum extent practicable the
discharge of pollutants into the Great Lakes system.  The Office also
coordinates U.S. actions in fulfillment of the Agreement between Canada and
the United States of America on Great Lakes Water Quality of 1978.

     This study was supported by a GLNPO grant to the University of Michigan
at Ann Arbor for investigating the zooplankton community composition in
nearshore waters of southern Lake Michigan.
                                      111

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                                   ABSTRACT
     Zooplankton samples collected in 1977 in the nearshore waters of southern
Lake Michigan (0,4 km from shore) were analyzed to provide a bench mark on
zooplankton community composition for comparison with future studies.  Species
composition, abundance, and distribution were investigated to determine the
apparent response of the zooplankton community to water quality conditions.
It is difficult to establish long-term trends on changes in zooplankton
community composition commensurate with known changes in water quality in the
nearshore waters of southern Lake Michigan because of the lack of historical
zooplankton data.  Instead, the effects of water quality conditions on
zooplankton must be Inferred by comparing community composition in nearshore
waters impacted by pollutive discharges with less affected offshore waters.

     Distribution and abundance of zooplankton in the nearshore waters of
southern Lake Michigan is highly influenced by physical mixing of relatively
high quality offshore waters with variously polluted harbor effluents
nearshore.  Rotifers were overwhelmingly abundant, comprising about 95% of
total zooplankton.  Total rotifers and crustacean plankton generally were most
prevalent in nearshore waters exhibiting highest alkalinity, specific
conductance, and nutrient chemistry and lowest turbidity and Secchi disc
transparency.  The predominant species (i.e., Keratella cochlearis, _K. crassa,
Polyarthra vulgaris, Conochilus unicornis, and Bosmina longirqstris) also were
most abundant in nearshore waters.  The distribution of these species often
was significantly correlated with physicochemieal variables.  The apparent
response of the zooplankton community to nutrient enrichment was an increase
in density of indigenous, eurytopic species rather than species shifts toward
more eutrophic forms.  This feature seems to be Indicative of mesotrophy in
the Great Lakes.  Eutrophic indicator species (e.g., Brachionus spp.,
Euchlanis dilatata, Triehocerca spp. , and Acanthocyclops vernalis) were rare
and usually confined to harbor mouths.  Besides Bosmina longirostris, no
consistent statistically significant trends were noted between distribution of
crustacean species and physicochemieal variables.  However, there still was a
tendency for calanold copepods to be more prevalent in more oligotrophic
offshore waters.
                                      iv

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                                   CONTENTS


Foreword 	  iii

Abstract	   iv

List of I igures		   vi

List of Tables 	 viii

Acknowledgments 	   ix

     1.  Introduction 	,	,	    1

     2.  Materials and Methods 	    3
              Field 	    3
              Laboratory	    5

     3.  Results 	    6
              Physicochemistry 	    6
              Abundance and Distribution by Major Groups 	    6
              Species Composition	   28
              Seasonal and Spatial Distribution of Major Botifera ....   28
              Notes on Selected Minor Rotifer Taxa 	   43
              Seasonal and Spatial Distribution of Major Crustacea ...   53
              Notes on Minor Crustacea  Taxa 	   73

     4.  Discussion	   81

References	   88

Appendices
     A.  Contoured physicochemical data, 11 June 1977
     B.  Contoured physicochemical data, 20 August 1977
     C.  Contoured physicochemical data, 24 September 1977
     D.  Zooplankton mean and maximum abundance (number/m^) in the
         Indiana waters of southern Lake Michigan for all 1977
         sampling dates combined

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                                    FIGURES

Number                                                                 _Page
   1  Location of sampling stations in southern Lake Michigan,  1977 ,,     4
   2  Distribution of total rotifers,  June 	     8
   3  Distribution of total rotifers,  August 	    10
   4  Distribution of total rotifers,  September	    11
   5  Distribution of total crustaceans,  June	    12
   6  Distribution of total crustaceans,  August 	    13
   7  Distribution of total crustaceans,  September 	    14
   8  Distribution of total calanoid copepods,  June	    15
   9  Distribution of total calanoid copepods,  August 	    16
  10  Distribution of total calanoid copepods,  September	    17
  11  Distribution of total cyclopoid copepods, June 	    18
  12  Distribution of total cyclopoid copepods, August 	    19
  13  Distribution of total cyclopoid copepods, September 	    20
  14  Distribution of total cladocerans,  June	    22
  15  Distribution of total cladocerans,  August 	    23
  16  Distribution of total cladocerans,  September 	    24
  17  Distribution of calanoid/cyclopoid  + cladoceran ratio,  June ....    25
  18  Distribution of calanoid/cyclopoid  + cladoceran ratio,  August ,,    26
  19  Distribution of calanoid/cyclopoid  + cladoceran ratio,  September    27
  20  Distribution of Keratella cochlearis cochlearis, June  	    34
  21  Distribution of Keratella cochlearis cochlearis, August 	    35
  22  Distribution o f Ke ra te11a c och1ear is c och1 e a r i s, September	    36
  23  Distribution of Keratella crassa,  June 	    37
  24  Distribution of Keratella crassa, August  	    38
  25  Distribution of Keratella crassa,  September 	    39
  26  Distribution of Kellicottia longispina, June 	    40
  27  Distribution of Kellicottia 1ong i s p ina, August 	    41
                                      VI

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Number                                                                  Page
  28  Distribution of Rellicottij longispina, September 	    42
  29  Distribution of Polyarthra vulgaris,  June 	    44
  30  Distribution of Polyarthra vulgar is,  August 	    45
  31  Distribution of Polyarthra vulgar is,  September 	    46
  32  Distribution of Conochilus unicornis, June 	    47
  33  Distribution of Conochilus unicornis, August 	    48
  34  Distribution of Conochilus unicornis, September	    49
  35  Distribution of Bosmina longirostris, June	    54
  36  Distribution of Bosmina longirostris, August 	    55
  37  Distribution of Bosmina longirostris, September 	    56
  38  Distribution of Daphnia retrocurva, June	    58
  39  Distribution of Daj>hnia retrocurva, August	    59
  40  Distribution of Daj}hnia retrocurva, September 	,	    60
  41  Distribution of Daphnia galeata mendptae, August	    62
  42  Distribution of Daphnia galeata mendotae, September 	    63
  43  Distribution of Diaphanoson;a,  August  	    64
  44  Distribution of Diaphanosoma,  September 	    65
  45  Distribution of _Diaeyclops thomasi, June 	    66
  46  Distribution of Diacyclops thomasi, August 	    68
  47  Distribution of Diacyclops thomasi, September 	    69
  48  Distribution of diaptomid copepodids, June	    70
  49  Distribution of diaptomid copepodids, August	    71
  50  Distribution of diaptomid copepodids, September 	    72
  51  Distribution of Leptodiaptomus minutus, June 	    74
  52  Distribution of Leptpdiaptomus minutus, August	    75
  53  Distribution of Leptodiaptomus minutus, September 	    76
  54  Distribution of Leptodiaptomus ashlandi, June 	    77
  55  Distribution of Leptodiaptomus ashlandi, August	    78
  56  Distribution of Leptodiaptomus ashlandi, September	    79
                                      vn

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                                    TABLES
Number
   1  Zooplankton abundance by major groups  in  the  Indiana  waters
        of southern Lake Michigan during the  1977 sampling  season.
        Data are based on  the standardized net  tows  for micro-
        crustaceans and the pooled near-surface  and  bottom  samples
        for rotifers.  The average density in numbers  of  individuals
        per m-* and average relative abundance in percent  composi-
        tion of total Crustacea  (%C) and total  zooplankton  (%Z)
        are presented for each date 	
   2  Species composition and mean abundance  (number/in-*  x  10""-')  Of
        rotifers in southern Lake Michigan.   Data  are  pooled  near
        surface and bottom samples from  all stations  in  each  sampling
        period.  The abundance of species  less  than 100  individuals/in-'
        is represented by a plus sign  ( + )	   29

   3  Species composition and mean abundance  ( nutr.be r/trr) of crusta-
        ceans from standardized net  tows  in southern  Lake  Michigan.
        Data are from all stations in  each sampling period.
        Presence of a species in numbers  lower  than 10 individuals/m^
        is indicated by a plus sign  (*)  	   31
                                      Vlll

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                                ACKNOWLEDGMENTS
     We thank Theodore Ladewski for providing computer and statistical as-
sistance.  We also acknowledge the U.S. Environmental Protection Agency for
collecting the zooplankton samples and providing the physicochemical data.
                                      IX

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                                 INTRODUCTION
     The extreme southern end of Lake Michigan is an area of contrasting water
quality.  Relatively high quality offshore waters and comparatively polluted
nearshore waters mix in hydrodynamically complex patterns generated by
currents and seiche activity (Mortimer 1975).  There has been a long history
of pollutive discharges in the lake from the Chicago metropolitan area,
including industrial and domestic wastewater sources in the Indiana coastal
waters (Torrey 1976).

     This report concerns zooplankton community composition in the Indiana
waters of southern Lake Michigan.  The study was initiated by the United
States Environmental Protection Agency (USEPA), Region V, as part of its water
quality monitoring and surveillance program.  Phytoplankton data were
collected concurrently and are reported in Stoermer and Tuchman (1980).

     The purpose of this report is to provide a benchmark on zooplankton
composition, abundance, and distribution in the Indiana waters of Lake
Michigan for comparison with future monitoring and surveillance efforts.
Zooplankters have value as water quality indicators and have proved useful in
complementing phytoplankton data to assess the apparent effects of water
quality conditions on Great Lakes biota (Gannon and Stemberger 1978).

     The Indiana  coastline of Lake Michigan is intensely developed, with
urban and industrial complexes located at Gary, Burns Harbor, and Michigan
City.  Effluent data from industrial and domestic sources for Gary and Burns
Harbor are suEimarized by Snow (1974) and for Michigan City by JBF Scientific
Corporation (1978).  The only portion of relatively undeveloped coastline is
the Indiana Dunes National Lakeshore located west of Michigan City.

     Zooplankton investigations in southern Lake Michigan were reviewed by
Gannon (1974a).  In contrast with the rest of the lake, there is a com-
paratively long history of zooplankton investigations in the southern end of
Lake Michigan.   Unfortunately, sampling methods between studies have differed
and most early investigations were only qualitative.   Consequently, it is
difficult to establish any long-term trends on changes in zooplankton
community composition in comparison with known changes in water quality.  Most
information on the probable effects of pollution on zooplankton have been
inferred by comparing zooplankton community composition between impacted
nearshore waters and less affected offshore waters in more recent quantitative
investigations (e.g., Gannon 1975, Stemberger and Gannon 1977).  Johnson
(1972) sampled zooplankton in 1970 and provided the most comprehensive
information on zooplankton species composition, inshore distribution, and
abundance in southern Lake Michigan.  He reported a high biomass of
zooplankton in the Indiana waters of Lake Michigan in comparison with other

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studies in offshore waters, which is apparently a response by the zooplankton
community to nutrient enrichment.  In addition to nutrient loading, toxic
substances are known to be discharged into the Indiana waters of Lake Michigan
(Snow 1974, Torrey 1976).  The efects of toxic effluents on zooplankton have
not been investigated.  However, bottom sediments in Gary and Michigan City
Harbor were toxic to zooplankton in laboratory studies (Gannon and Beeton
1969, JBF Scientific Corporation 1978).

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                             MATERIALS AND METHODS
FIELD
     Zooplankton samples were collected by USEPA personnel on 11 June,
20 August, and 24 September, 1977.  Four transects were used with stations
located 1/2, 1, 2, and 5 miles offshore.  A 10-mile station on the Burns
Harbor transect was sampled only in June (Figure 1).

     Crustacean plankton were collected with a 0.5 m diameter, no. 6 (240 pm)
mesh conical net.  A standardized vertical tow was made from 10 m to the
surface (or bottom to the surface at stations less than 10 m deep).  A second
tow was taken from the bottom to the surface at stations deeper than 10 m.
Carbonated water was added promptly as a narcotizing agent (Gannon and Gannon
1975) and samples were preserved with 5% buffered formalin.

     The standard tow aided comparison of data between stations because
approximately the same volume of water was filtered by the net.  The no. 6
mesh net was chosen because the filtration efficiency of that mesh size is
near 100% (Gannon 1972, 1981), thereby improving the accuracy of abundance
data.  However, some of the smallest zooplankters (e.g., Chydorus sphaericus,
Bosniina longirostris, Eubostr.ina coregoni, Ceriodaphnia spp., Tropocyclops
pr as inus me x i c anu s, and cyclopoid copepodids) may escape through the mesh and
be undersampled.

     Rotifer samples were collected with 8-liter Niskin bottles at 2 m and
just off bottom.   The water samples from each depth were pooled and
concentrated in a filtering funnel fitted with 54 ym mesh screening (Likens
and Gilbert 1970).  The samples were narcotized with carbonated water and
preserved in 5% buffered formalin.

     It would be desirable to sample rotifers and crustacean plankton by the
same methods.  However, micro-crustaceans are too sparsely distributed to
collect with a water bottle.  Consequently, it was necessary to use a plankton
net to sample these organisms.  Rotifers are sufficiently concentrated so that
reliable samples can be collected with a water bottle.  Intercomparisons of
rotifer data with chemistry and phytoplankton are most valid statistically
since these limnological variables were obtained at the same depths using
identical methods.

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   87*20'W
  41* 50' N
    LMOl
    HMD 30
h 41 40
     IND3I
87IO'
                   *LM 02
87*00'
                       *LM03
                                        IND 38
                  BURNS!HARBOR
                         I	
                                              ^\
                                         AREA  ENLARGED
 Figure 1. Location of sampling stations in southern Lake Michigan, 1977.

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LABORATORY
     Prior to micro-crustacean counts, each sample was adjusted to a con-
stant volume in a graduated cylinder and poured into a 4-oz. jar.  The
cylinder was rinsed with an additional 10 to 20 mL of tap water and this was
carefully added to the rest of the sample to a final volume of usually
100 irJL.  The sample was then randomly and thoroughly mixed with a large-
bore, calibrated automatic pipette and the subsample quickly drawn from
the middle of the sample.  Aliquot sizes ranged from 1 mL to 10 mL depend-
ing on species numbers.  A second, larger aliquot usually was withdrawn for
enumeration of less common species.  Subsamples were transferred to a
chambered counting cell (Gannon 1971) and the entire contents, usually 150
to 300 individualsj were enumerated at 30 to 60x under a Wild stereo-
microscope.  Those organisms requiring higher magnificat ion for
identification were mounted in polyvinyl lactophenol, stained with
lignin pink, and examined at 100 or 430x under an American Optical com-
pound microscope.  Data were calculated in numbers of individuals per
m^ and percent composition.  The subsampling and enumeration
procedure has proved to be accurate and reproducible (Gannon 1972,
1981).

     Adult calanoid and cyclopoid copepods were identified to species
according to Wilson (1959) and Yeatman (1959), respectively.  Calanoid
copepodids were included with the adults of that species except those in the
family Diaptomidae.  Cyclopoid copepodids were not identified to genus,
although most were undoubtedly piacyclops.  Adult harpacticoid copepods were
identified to species where possible with the use of Wilson and Yeatman
(1959).  All cladocerans are reported at the species level except
p iaphanosoma.  Two species, D. leuchtenbergianum Fisher and D. brachyururo
(Lieven),were observed, with the former overwhelmingly most abundant.  Brooks
(1957) was used for Daphnia, and Deevey and Deevey (1971) for Eubosmina.  The
family Chydoridae was identified according to Frey (1959, 1962), Megard
(1967), and Smirnov (1971),  The remaining Cladocera were keyed according to
Brooks (1959).

     In preparation for rotifer counts, all samples were concentrated to
50 mL.  Each sample was thoroughly mixed with a calibrated automatic pipette
immediately before taking a subsample with the pipette from the center of the
jar.  Sub-samples of 1, 3, or 5 mL were taken depending on the density of
organisms so that the concentration of rotifers in each subsample included 200
to 400 individuals.  Subsamples were transferred to a 5-mL Plexiglas,
rectangular counting cell and all rotifers were enumerated under an American
Optical compound microscope at lOOx.  Each subsample was then replaced in the
jar, a second subsample was taken and enumerated, and the two counts averaged.
A minimum of 300 rotifers per sample was routinely counted.  Data were
calculated in numbers of individuals per m^ and percent composition at each
station.  The subsampling and counting procedure was tested and proved to be
accurate and reproducible (Stemberger et _a 1. 1979).

     Identifications were made to species for most rotifers.  Certain species
of the genus Synchaeta were indistinguishable by gross morphology because of

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their contracted state and, therefore, identification of these organisms was
determined by examination of the hard, chitinous mouth parts after sodium
hypochlorite bleach was used as a clearing agent (Stemberger 1973).  The main
references used in identifying rotifers were Jennings (1903), Ahlstrom (1943),
Voigt (1957), and Stemberger (1976).
                                    RESULTS
PHYS1COCHEM1STRY
     Physicochemical data were obtained concurrently with plankton collections
by the USEPA.  Contour plots for these data are presented in Appendices A-C.
Those physicochemical variables that may be important in interpreting patterns
of zooplankton distribution and abundance are discussed briefly here.

     On 11 June 1977, temperatures were warmest (18C) off Burns Harbor and
Michigan City and gradually decreased (<15C) offshore.  Specific conductance
was slightly higher (286 ymhos/cm) off Michigan City than elsewhere in the
study area (276-284 pmhos/em).  Highest Secchi disc reading (4-6 m) were
recorded at the outer stations off Gary, and lowest (>3 m) were near Burns
Harbor.  Nitrogen (aiunonia and nitrate) was slightly higher off Burns Harbor
(Appendix A).

     Temperatures on 20 August 1977 were slightly cooler (<20C) off Gary and
warmer (>21C) at the offshore stations.  Specific conductance was highest
(282 umhos/cm) off Gary and Burns Harbor.  Nitrogen (ammonia and nitrate)
concentrations exhibited the same pattern as specific conductance.  Secchi
disc transparency was lowest (3 ID) off the mouth of Michigan City Harbor and
highest (4-5 m) at the outermost stations (Appendix B).

     Most physicochemical variables were more evenly distributed on 24 Sep-
tember 1981.  Temperature (17C) and Secchi disc transparency (3 m) were
considerably uniform throughout the study area.  Specific conductance,
nitrate, and ammonia-nitrogen were slightly higher off Gary (Appendix C).
ABUNDANCE AMD DISTRIBUTION BY MAJOR GROUPS
     Although rotifer and micro-crustacean data are not strictly comparable
because different sampling methods were used, it is clearly evident that
rotifers were predominant over crustacean plankton in the Indiana waters of
southern Lake Michigan.  Mean abundances by cruise are given in Table 1 and
mean and maximum abundance for all cruises combined are presented in
Appendix D.  Rotifers consisted of over 90% of total zooplankton during the
sampling period.  Cladocerans were the predominant crustaceans in June and
August but calanoid copepods were slightly more prevalent in September.

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Cyclopoid copepods were a minor component of the plankton throughout the study
period.  Density of zooplankton (principally rotifers) was highest in June,
approximately twice as high as in August, and four times higher than in
September (Table l).
     TABLE 1.  ZGCPLAMTON ABUNDANCE BY MAJOR GROUPS IN THE INDIANA WATERS
          OF SOUTHERN LAKE MICHIGAN DURING THE 1977 SAMPLING SEASON.
       DATA ARE BASED ON THE STANDARDIZED NET TOWS FOR MICRO-CRUSTACEANS
         AND THE POOLED NEAR SURFACE AND BOTTOM SAMPLES FOR ROTIFERS.
           THE AVERAGE DENSITY IN NUMBERS OF INDIVIDUALS PER M3 AND
   AVERAGE RELATIVE ABUNDANCE  IN  PERCENT COMPOSITION OF TOTAL  CRUSTACEA (%C)
        AND TOTAL ZOOPLANKTON  (%Z)  ARE PRESENTED  FOR EACH SAMPLING DATE
Cladocera

  Total
  Crustacea

Roti fera
                       June

                 no./m3   %C    %Z
                             August

                        no./ro3   %C    %Z
  6,620  58.1   1.7
 11,390

366,700
 2.9

97.1
  Total
  Zooplankton  398,090
  3,010

190,800


193,810
                                   September

                               no. /m3   %C    %Z
Calanoid
Copepoda
Cyclopoid
Copepoda

2,980

1,780

26.2

15.7

0.7

0.5

870

130

28

4

.9

.3

0

G

.4

.1

4,010

360

49.9

4.5

3.8

0.3
         2,010  66.8   1.0     3,670  45.6   3.5
 1.5     8,040

98.5    97,700


       105,740
 7.6

92.4
     Rotifer populations were dramatically higher at the nearshore stations
off Burns Harbor and Indiana Dunes in the region where warmest water
temperatures and highest specific conductance were observed.  A maximum
abundance of over 900,000 rotifers per m3 was recorded at the 1 mile station
off Indiana Dunes.  In contrast, lowest rotifer densities (<100,000 per m3)
were observed at the offshore stations off the westernmost transect near Gary
(Figure 2).  Lowest temperatures and highest water quality conditions (i.e.,
lowest specific conductance, lowest nitrogen, and highest Secchi disc
transparency) were recorded at these same stations (Appendix A).

     Rotifer densities were highest (>200,000 per m3) in the nearshore waters
throughout the study area in August where highest specific conductance and

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00
        87 20'
        GARY
        87 10'
8 7 00'
BURNS I HARBOR
     Figure 2.  Distribution of total  rotifers, June.

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lowest Secchi disc readings were reported.  Lowest rotifer densities 100SOGO
per m3) were recorded at the outermost stations off Gary and Burns Harbor
where highest water quality conditions were observed (Figure 3, Appendix B).

     Total rotifers were reduced in numbers in September, ranging near 100,000
per rn3 at most locations.  Lowest numbers 50,000 per m3) were recorded at
easternmost stations 1 to 2 miles offshore.  A localized area of relatively
high abundance (>200,000 per m3) was at nearshore stations of Gary where
specific conductance and nitrogen also were highest (Figure 4, Appendix C).

     In contrast to the rotifers, patterns in total crustacean plankton
distribution usually did not exhibit as strong similarities to the
distribution of physicochemical variables.  However, some distinct patterns of
major groups of micro-crustaceans were discernible.

     In June, highest (>20,QOO per m3) total Crustacea were recorded at the
outermost stations off Indiana Dunes.  Crustaceans also were locally prevalent
(10 to 15,000 per m3) at nearshore locations off Gary.  Similar to the
rotifers, lowest crustacean numbers (<5,OOQ per m3) were observed at the
outermost stations off Gary (Figure 5).

     In contrast to rotifer distribution in August, crustacean plankters were
more prevalent offshore than inshore.  Crustacean densities generally were
less than 2,000 per m3 at 1/2 and 1 mile stations except off the Gary Harbor
mouth (3,120 per m3).  Offshore stations had densities from 4,000 to over
6,000 per m3 (Figure 6).

     In September, patterns of crustacean abundance most resembled the
distribution of rotifers and physicochemical variables (Figure 7, Appendix C).
Crustaceans were locally prevalent (10,200 to 14,400 per m3) off the Gary
Harbor mouth.  Elsewhere in the study area, crustacean  densities were more
uniformly distributed (4,700 to 8,200 per m3),

     In June, calanoid copepods were distinctly most prevalent (40-50% of
total Crustacea) at westernmost offshore stations in June, and became
relatively less abundant 20%) eastward off Indiana Dunes and Michigan City
(Figure 8).  Calanoids were relatively most abundant (73%) in offshore waters
in August.  They were an especially infrequent (<10%) component of the
crustacean plankton off Burns Harbor (Figure 9).   The pattern was less
distinct in September, with calanoids more evenly distributed (approximately
40-60%) throughout the study area.  However, the highest proportion of
calanoids was observed at outermost stations (Figure 10).

     Cyclopoid copepods were relatively less abundant than calanoid copepods
during the study period, but exhibited similar patterns of distribution.
In June, cyclopoids were most abundant (>30%) in the westernmost offshore
stations and were least prevalent (>10%) at all innermost stations
(Figure 11).  A similar pattern was observed in August (Figure 12).
In September, cyclopoids were relatively most abundant at eastern outermost
stations (Figure 13).

     As in the copepods, trends in cladoceran distribution were most distinct

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                               BURNS  I HARBOR
Figure 3.  Distribution of total rotifers, August.

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                               BURNS  I HARBOR
Figure 4.  Distribution of total rotifers, September,

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                              BURNS  I HARBOR
Figure 5.  Distribution of total crustaceans, June.

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   8?e20'
87 10'
8 7 00'
                               BURNS  I HARBOR
Figure 6.  Distribution of total crustaceans,  August,

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                               BURNS  I HARBOR
Figure 7.  Distribution  of total crustaceans, September.

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                              BURNS  I HARBOR
Figure 8.   Distribution of total  calanoid copepods, June.

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   87 20'
87*10'
                              BURNS  I HARBOR
87 00'
                                                                      ,*.   J.
                                             4P401
Figure 9.  Distribution of total  calanoid copepods, August.

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   87 20'
87 10'
8 7 00'
                               BURNS  I HARBOR
Figure 10.   Distribution of total calanoid  copepods, September.

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    87 20'
87*10'
8700'
                               BURNS  I  HARBOR
Figure 11.  Distribution of total cyclopoid copepods, June,

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    87 20'
87*10'
8 7 00'
                                BURNS  I HARBOR
Figure 12.   Distribution of total  cyclopoid copepods,  August

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                                BURNS   I HARBOR
Figure 13.   Distribution of total  cyclopoid copepods, September,

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in June and August and generally exhibited a reciprocal pattern in comparison
with the copepods.  Whereas copepods generally were most prevalent at offshore
locations, cladocerans were predominant inshore,  A distinct gradient from low
relative abundance 10% of total Crustacea) at outermost stations off Gary to
high abundance (>8C%) near Michigan City Harbor was discernible in June
(Figure 14).  In August, cladocerans were predominant, representing greater
than 70 to 80% of all crustaceans at inshore stations in comparison with 50 to
60% at offshore locations (Figure 15).   By September, cladocerans were less
prevalent throughout the study area, especially in offshore waters where
relative abundances of 30 to 40% were observed.  Highest abundances (50 to
60%) were located near harbor mouths (Figure 16).

     Gannon et aI. (1976) and Gannon and Stemberger (1978) suggested that the
ratio of calanoid copepods to cyclopoid copepods plus cladocerans may be
useful in detecting summertime trends in zooplankton distribution as related
to water quality.  Calanoid copepods generally are most prevalent in
oligotrophic conditions relative to the other major crustacean groups and,
therefore, the ratio may be greater in areas of higher water quality.
This trend was discernible in the Indiana waters of southern Lake Michigan.
In June, the ratio was highest at the outermost stations off Gary where
consistently higher water quality conditions were observed (Figure 17).
The ratio was highest off Indiana Dunes in August and lowest in the vicinity
of the mouths of Burns and Michigan City harbors (Figure 18).  As cladocerans
became less abundant relative to calanoid copepods in September, the ratio
increased in absolute value throughout  the study area (Figure 19).  However,
the absolute value carries little limnological significance.  Only the
relative differences in the ratio within a given data set may have
interpretive value.  The ratio was highest throughout the offshore waters and
on the transect of stations off Indiana Dunes.  As in August, lowest values
were observed off Burns Harbor and Michigan City harbor (Figure 19).

     A correlation coefficient matrix was examined to discern whether or not
some of the observed trends in spatial  distribution of zooplankton and
physicochemical variables were statistically significant.  Consistent trends
did not exist across all sampling periods.  Strongest correlations were
observed in June when spatial gradients in physicochertistry and zooplankton
densities were most distinct.  Total rotifers exhibited significant positive
correlations with temperature and specific conductance at the .01 level and a
significant negative correlation with Secchi disc transparency at the ,05
level in June.  Total Crustacea exhibited significant (.01) positive
correlations with temperature and specific conductance.  Total Crustacea and
Secchi disc correlations were not significant, undoubtedly because of
differences in distributions of calanoid copepods and cladocerans relative to
water transparency.  Cladocerans showed significant (.05) positive correlation
and calanoid copepods exhibited significant (.05) negative correlation to
Secchi disc depths.  Moreover, as a further reflection of this pattern, the
ratio of calanoid copepods to cyclopoid copepods and cladocerans showed
significant (.05) positive correlation  to Secchi disc transparency.  Cyclopoid
copepods did not exhibit any significant correlations with physicochemical
variables in June,

     In August, total rotifers showed significant (.05) negative correlation
                                      21

-------
NJ
          87 20'
87 10'
8 7 00'
                                     BURNS  I HARBOR
                                                                                            41 40'
      Figure 14.  Distribution of total cladocerans, June.

-------
                              BURNS  I HARBOR
Figure 15.  Distribution of total cladocerans, August,

-------
                               BURNS  I HARBOR
Figure 16.  Distribution of total eladocerans,  September,

-------
                               BURNS  V HARBOR
Figure 17.  Distribution of calanoid/cyclopoid + cladoceran ratio, June.

-------
   87 20'
87" 10'
87*00'
                               1URNS  I HARBOR
                                                                                    41*40'
Figure 18,  Distribution of calanoid/cyclopoid + cladoceran ratio, August.

-------
                               BURNS  I HARBOR
Figure 19.  Distribution of calanoid/cyclopoid 4- cladoceran ratio, September,

-------
with Secchi disc transparency, but no other correlations with physicocheniistry
were significant.  Cladocerans exhibited a significant (.01) positive
correlation, whereas calanoid copepods showed a significant (.01) negative
correlation to temperature.  In September, rotifers exhibited no significant
physicochetnical correlations, but total crustaceans showed a significant (.05)
positive correlation to specific conductance.  As in June, cladocerans had a
significant (.05) negative correlation with Secchi disc transparency whereas
calanoid copepods were positively correlated at the .01 level.  No consistent
trends in correlation of zooplankton with nutrient chemistry were observed
during the study period.
SPECIES COMPOSITION
     During the sampling period, 52 rotifer species were collected from the
Indiana waters of southern Lake Michigan (Table 2).  The predominant species
were Reratella cochlearis, JK. crassa, Kellicottia longispina, "PolyaTttira
retnat a, P. vulgar is, and Conochilus unicornis.  Congeneric occurrence of
species in southern Lake Michigan waters was common.  Nine genera were
represented by two or more species, including a maximum of five species each
in Keratella and Trichocerca.  Approximately 40 species, including all of the
predominant ones, are characteristic of limnetic waters of Lake Michigan.  The
remainder (e.g., Euclanis, Lophocaris, Stephanocercos) are primarily benthic
and littoral forms that occasionally appear in the plankton in nearshore
waters especially near river mouths.

     Crustacean plankters included 35 species in the study area, including
eight calanoid copepods, five cyclopoid copepods, one harpacticoid copepod,
and 21 cladocerans (Table 3).  As in the rotifers, congeneric occurrence of
micro-crustaceans was recorded in southern Lake Michigan waters.  Three
species each of Leptodiaptomus, Daphnia, and AIona and two species each of
Ce_r_iojdaphnia and IDiaphano soma were recorded.  Approximately 22 species are
characteristic of Lake Michigan limnetic waters, including predominant
Epischura lacus tr is, Lep^odJ.ap t omus a sh 1 and i, _L. roinutus, diaptomid
copepodids, Diacyclops thomasi, Diaphanosoma spp., Daphnia retrocurva,
D. galeata mendotae, Bosmina longirostris, and Eubosmina coregoni.  The
remainder (e.g., Eucyclops, Canthocamptus, Alona, Eyrycercus, Leydigia,
P1euroxi s, and Ilyocryptus) are littoral and benthic species which
occasionally appear in the plankton of nearshore waters.
SEASONAL AND SPATIAL DISTRIBUTION OF MAJOR ROTIFERA
     Keratella cochlearis cochlearis was the most abundant rotifer in the
Indiana waters of southern Lake Michigan.  This species had a mean abundance
for all sampling periods of 77,200 individuals per m-^ (Appendix D, Table D-l).
It was most abundant (mean of 160,700 per nP) in June when it constituted over
40% of total rotifers.  Its density was considerably less (about 40,000 per
                                      28

-------
      TABLE  2.   SPECIES COMPOSITION,  MEAN ABUNDANCE (NUMBER x 103/M3), AND
    TROPHIC  STATUS  OF  ROTIFERS  IN  SOUTHERN LAKE  MICHIGAN.   DATA ARE POOLED
  NEAR SURFACE AND BOTTOM SAMPLES FROM ALL STATIONS IN EACH SAMPLING PERIOD.
             THE  ABUNDANCE  OF  SPECIES LESS THAN  100 INDIVIDUALS/M3
                       IS  REPRESENTED BY A PLUS  SIGN (+)

Class Monogonata
Order Ploima
Family Brachionidae
Subfamily Brachioninae
Brachionus angularis Gosse
B. caudatus Barrois and Daday
Euchlanis dilatata Ehrbg.
Kellicottia longispina (Kellicott)
Keratella cochlearis cochlearis (Gosse)
K. cochlearis f. hispida (Lauterborn)
K. cochlearis f. robusta (Lauterborn)
K. cochlearis f. tecta (Gosse)
K. crassa Ahlstrora
K. earlinae Ahlstrom
K. quadrata (0. F. Muller)
K. valga f. brevispina
Lophocaris salpina (Ehrbg.)
Notholca foliacea (Ehrbg.)
N. labis Gosse
N. laurentiae Stemberger
N. sq^iamula (0. f. Muller)
Platyias patulus (0. F. Muller)
Trichotria tectractis (Ehrbg.)
Subfamily Colurinae
Lepadella patella (0. F. Muller)
Family Lecanidae
Lecane mira (Murray)
Monostyla closterocerca (Schmarda)
Family Tr ichocerc idae
Trichocerca cylindrica (Irahof)
T. multicrinis (Kellicott)
T. porcellus (Gosse)
T. pusilla (Jennings)
T. rousseleti (Voigt)

June
+
0.0
0.0
16.2
160.7
0.0
1.6
0.0
12.5
0.5
0.5
0.0
0.0
0.1
0.0
C.O
0.1
0.0
+
+
0.0
0.0
0.0
0.0
+
0.0
+
Aug.
+
0.1
0.0
2.4
40.1
0.1
2.3
0.2
15.2
0.2
0.2
0.0
+
C.I
0.0
0.1
0.1
+
0.0
0.0
0.0
0.0
0.1
0.6
2.9
+
0.8
Sept .
0.0
0.0
+
1.3
36.2
0.0
1.2
0.0
2.5
0.3
0.4
+
0.0
0.1
+
1.0
0.1
+
0.0
0.0
+
+
+
0.0
4.1
0.0
0.2
Trophic
Status*
E
E
E
0
ET
ET
ET
E
ET?
ET?
ET
I
E
0
0
0
0
E
I
I
E
E
E
E
E
E
E

(continued).
                                      29

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TABLE 2.  (continued).

Class Monogonata
Order Ploima
Family Gastropidae
Ascomorpha ecaudis Perty
Ascomorpha ovalis (Bergendal)
Gastropus stylifer Imhof
Family Tylotrochidae
Tylotrocha monopus (Jennings)
Family Asplanchnidae
Asplanchna priodonta Gosse
Family Synchaetidae
Ploesoma hudsoni (imhof)
P. lenticulare Herrick
P. truncatum (Levander)
Polyarthra dolichoptera Idelson
P. euryjptera Wierzejski
P, major Burckhardt
P. remata Skorikov
P. vulgaris Carlin
Synchaeta kitina Kousselet
S. lakowitziana Lucks
S, oblonga Ehrbg.
S. pectinata Ehrbg.
S. stylata Wierzejski
Family Testudinellidae
Filinia longiseta (Ehrbg.)
F. terminalis (Plate)
Pompholyx sulcata Hudson
Family Conochilidae
Conochilus unicornis (Rousselet)
Family Collothecidae
Collotheca mutabilis (Hudson)
C. pelagica (Rousselet)
Stephanocercos fimbriatus (Goldfuss)
Total Rotifers
June
0.5
0.1
0.8
0.7
4.8
0.1
0.0
0.2
0.3
0.0
0.2
62.8
17.4
0.6
+
0.1
0.0
1.2
0.0
0.0
+
102.1
3.1
0.0
0.0
386.7
Aug.
0.1
1.6
6.4
0.6
1.1
0.1
1.8
2.6
0.0
0.0
3.3
19.8
61.2
0.2
0.0
0.0
0.1
10.4
0.0
0.2
+
9.8
6.2
0.1
0.0
190.8
Sept.
+
0.2
1.2
+
0.1
0.1
0.0
1.0
0.0
+
2.8
6.5
32.2
+
0.0
0.0
0.0
2.6
+
+
0.0
1.2
2.2
0.0
f
97.7
Trophic
Status*
M
M
ET
M?
ET
ET
ET
ET
0
M?
M?
ET
ET
I
0
M?
M?
M?
E
0?
E
ET
M?
M?
I

* Trophic Status:  ET = eurytopic; E = eutrophic; M = mesotrophic; 0 -
  oligotrophic; I = insufficient information.   Compiled from Gannon and
  Stemberger (1978), Stemberger (1979), and other sources.
                                      30

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          TABLE 3.  SPECIES COMPOSITION, MEAN ABUNDANCE  (NUMBER/M3),
   AND TROPHIC STATUS OF CRUSTACEANS FROM STANDARDIZED NET TOWS  IN  SOUTHERN
      LAKE  MICHIGAN.   DATA ARE FROM ALL STATIONS IN EACH SAMPLING PEBIOD.
         PRESENCE  OF A SPECIES IN NUMBERS LOWER THAN 1C INDIVIDUALS/M3
                        IS INDICATED BY A PLUS SIGN (+)


Subclass Copepoda
Order Calanoida
Senecella calanoides Juday
Limnocalanus macrurus Sars
Eurytemora affinis (Poppe)
Epischura lacustris Forbes
Leptodiaptomus sicilis Forbes
L, ashlandi Marsh
L. minutus Lilljeborg
Skistodiaptomus oregonensis Lilljeborg
Diaptomid copepodids
Order Cyclopoida
Acanthocyclops vernalis Fisher
Diacyclops thomasi Forbes
Mesocyclops edax (Forbes)
Tropocyclops prasinus mexicanus Kiefer
Eucyclops agilis (Koch)
Cyclopoid copepodids
Order Harpact icoida
Canthocamptus staphylinoides Pearse
June
2,980
0
10
80
30
10
450
310
40
2,060
1,780
40
1,720
0
0
0
20
Aug.
870
0
70
90
290
70
340
130
10
120
0
0
10
0
0
Sept.
4,010
20
420
10
140
160
60
3,210
360
320
30
0
0
Trophic
Status*
0
0
I
M
0
M
M
ET
E
ET
ET
ET
I
I
Subclass Branchipoda
  Order Cladocera

  Family Leptodoridae
    _Le_p_t_o_dp_rji kindtii (Focke)

  Family Polyphemidae
    Polyphemus pediculus (L.)

  Family Sididae
    Diaphanosoma spp.

  Family Macrothricidae
    Ilyocryptus spinifer Herrick
                                             6,620   2,010   3,670
                                                30
                                                        20
10
                                                       430
        50
       260
 ET
 ET
ET?
(continued).
                                      31

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TABLE 3.  (continued).
                                                June
           Aug.  Sept.
                      Trophic
                      Status*
Family Holopedidae
  Holopedium gibberum Zaddach

Family Daphnidae
  Ceriodaphnia lacustris Birge
  C. quadrangula Muller
  Daphnia gal eatji jmendotae Birge
  ^' retrocurva Forbes
  D. ambigua Scourfield

Family Bosminidae
  Bpsmina longiros_tris (Kiiller)
  Eubosmina coregon i. ( Ba ird)

Family Chydoridae
  Alona affinis (Leydig)
  A. quadr angu 1 a r is (Miiller)
  A_. setulosa Megard
  Camptocercus re_ctirostris Schodler
  Chydorus sphaericus (Kuller)
  Eurycercus lamellatus (Muller)
  Leydigia quadrangu 1 ar_is (Leydig)
  Pleuroxus procurvus Birge

  Total Crustacea
                                                        20
                                                        20
                                                        10
                                                       ISO
                                                       930
                                                         0
                                                       310
                                                        80
                                                         0
                                                         0
                                                         0
                                                         0
                 0
                 +
               250
             1,170
                 0
             1,630
               310
   30
6,530
   10
 0
 0
 +
 0
10
 +
 0
 0
                                            11,390   3,010   8,040
                        ET
 E
 E
ET
ET
 E
 E
 I
                           ET
                           ET
                            I
                            I
                            I
                           ET?
                            I
                            I
* Trophic Status:  ET = eurytopic; E = eutrophic; M = mesotrophic; 0 =
  oligotrophic; I = insufficient  information.  Compiled  from Gannon and
  Stemberger (1978), and other sources.
                                      32

-------
m^)  in August and September, although  it still represented over 25% of total
rotifers  (Table 3).

     In June, i.. cochlearis was most abundant (222,600 to 384,400 per nP) at
nearshore stations off Burns Harbor and Indiana Dunes (Figure 20).  This
region was most turbid 3 m Secchi disc readings) and, indeed, the
distribution of this species exhibited a significant (.05) negative
correlation with Secchi disc transparency.  In waters with highest (>5 m)
Secchi disc readings, i.e., outermost  stations off Gary and Burns Harbor,
K_. cochlearis numbers were less than 50,000 per m^ (Figure 20).

      Kerate^lla cochlearis densities in August were highest (>400,000 per nr*)
off  Burns Harbor and Indiana Dunes.  As in June, numbers were highest in
turbid waters and this species had a significant (.05) negative correlation
with Secchi disc transparency and a significant (.05) positive correlation
with turbidity.  It was least abundant (254,000 per m^) at the outermost
station off Indiana Dunes (Figure 21).

     This species exhibited a west to  east decrease in density during
September.  It was most prevalent (>100,000 per m^) at nearshore locations off
Gary and was about ten times less abundant off Indiana Dunes and Michigan City
(Figure 22).  The waters off Gary had  higher specific conductance and
nutrients than elsewhere in the study  area during September (Appendix C),

     The second most abundant species  in the genus Kerate1la was K. crass a.
Its  average abundance during the study period was 10,000 per m^ (Appendix D,
Table D-l) and it was most prevalent in August (15,000 per m^) (Table 2).

     Seasonal patterns of distribution for K. cochlearis and K. crassa were
similar.  In June, K. crassa was least abundant (2,000 to 6,000 per m-5) at
offshore locations on westernmost transects and increased in numbers eastward
to a maximum (45,000 per m^) at the 1/2 mile station off Indiana Dunes
(Figure 23).  This pattern continued in August with numbers ranging from 2,500
to 11,700 per m^ offshore to near 30,000 per m^ at the 1/2 mile stations of
Indiana Dunes and Michigan City (Figure 24).  The distribution of JC. crassa in
August appeared to be related to water chemistry since this species showed
significant (.05) positive correlation with alkalinity and specific
conductance and a significant (.01) negative correlation with Secchi disc
transparency.  The pattern of west to east decrease in density was evident in
September with highest numbers (6,300 per m^) noted nearest Gary (Figure 25).
Kerate1la crassa, although considerably reduced in density in comparison with
June and August, exhibited a significant (.05) correlation with specific
conductance in September.

     In contrast to the distribution pattern for Keratella, Kellicottia
longispina generally was more abundant offshore than nearshore.  Kellicottia
was most abundant in June with a mean density of 16,200 per m^ (Table""fj,  It
had an overall mean density of 6,400 per m^ (Appendix D,  Table D-l).   Its
density was consistently low at 1/2 mile stations (Figures 26-28).  The
pattern does not appear to be specifically related to harbor outfalls since
densities also were low at the 1/2 mile station off Indiana Dunes.  Moreover,
its distribution did not correlate well with physicochemical variables.   For
                                      33

-------
u>
-O
                                   BURNS  I HARBOR
    Figure 20.   Distribution of Keratella cochlearis cochlearis, June,

-------
u>
Ln
                                    BURNS  I HARBOR
     Figure 21.   Distribution of Keratella cochlearis cochlearis,  August,

-------
                               BURNS  I HARBOR
Figure 22.  Distribution of Keratella cochlearis  cochlearis, September.

-------
LO
                                    BURNS  I HARBOR
     Figure 23.   Distribution of Keratella crassa, June.

-------
00
                                    BURNS  I HARBOR
     Figure 24.  Distribution of Keratella  crassa, August,

-------
VO
                                    BURNS  I HARBOR
     Figure 25.  Distribution of Keratella crassa, September,

-------
-p-
o
         8720'
        GARY
87 10'
87*00'
                                                                        DUNES
                                                                                      4I40'	
 BURNS HARBOR
     Figure 26.  Distribution of Kellicottia longispina, June.

-------
    8720'
   GARY
87 10'
87'00'
 BURNS HARBOR
Figure 27.  Distribution of Kellicott ia longispina, August,

-------
    8720'
   QARY
87*10'
87'OQ1
                                                           N
 BURNS HARBOR
Figure 28.  Distribution of Kellicottia longispina, September,

-------
example, it exhibited significant (.01) positive correlation with temperature
in June and significant (.01) negative correlation in August.  Either no
correlations or  inconsistent ones were obtained.

     Five species of Polyarthra were recorded in the study area, but only
P_. vulgar is and  P. remata were relatively abundant.  Respectively, they were
the second and fourth most abundant species during the sampling period.
Polyarthra vulgaris densities were highest (61,200 per m3) in August and
averaged 37,400  per m3 for all cruises.  Polyarthra remata was most prevalent
in June (62,800  per m3)  and averaged 29,000 per m^ for all cruises (Table 2;
Appendix D, Table D-l).

     Polyarthra  remata exhibited a similar spatial distribution pattern to
_P. vulgaris.  Consequently, only _P. vulgaris is illustrated (Figures 29-31).

      Polyarthra vulgaris exhibited a distinct  inshore-offshore gradient in
distribution in  June (Figure 29).  It ranged from 600 per m3 at the 5 mile
station to 49,000 per m3 at the 1/2 mile station off Burns Harbor.  It was
most abundant at nearshore stations off Burns Harbor and Indiana Dunes and
exhibited significant (.05) negative correlation with Secchi disc
transparency.  In August, it was most prevalent (70,000 to 80,000 per m3) near
Gary and Burns Harbor and at mid-transect stations off Indiana Dunes
(Figure 30).  No consistent distribution pattern was discernible in September
but highest densities (71,600 per m3) were recorded at the 1/2 mile station
off Gary (Figure 31).

     Conochilus  unicornis was overwhelmingly most abundant (mean of 102,100
per nHlin June  when it comprised about one-third of total rotifers (Table 2).
Its population dropped to 10% and 1% of June numbers in August and September,
respectively.  It was the third most abundant (mean of 36,200 per m3) rotifer
during the sampling period (Appendix D, Table D-l).  In June, the distribution
of Conochilus was similar to that observed for Keratella and Polyarthra
(Figure 32).  Densities ranged from less than 1,000 per m3 at westernmost
offshore stations to more than 300,000 per m3 at nearshore locations off
Indiana Dunes.   High densities (98,300 to 226,300 per m3) also were recorded
nearest Burns Harbor and Michigan City harbor.  It exhibited significant (.01)
positive correlations with temperature and specific conductance in June.
Patterns of distribution in August and September were less distinct and no
significant correlations with physicochemical variables were obtained.  In
general, Conochilus was more prevalent offshore than nearshore on the two
westernmost transects whereas its distribution eastward was irregular with
respect to distance from shore (Figures 33-34).
NOTES ON SELECTED hiNOR ROTIFER TAXA
     Besides Kellicottia, the only rotifers encountered in the Indiana waters
of southern Lake Michigan that appear to prefer colder waters were four
species of Notholca (Table 2).  Notholca labis was collected only at one
station off Michigan City in September, but _N. foliacea, _N. laurentiae, and
                                      43

-------
        87*20'
87*10'
87*00'
-O
                                   BURNS I HARBOR
                                                                                        41*40'
     Figure 29.  Distribution of Polyarthra vulgar!s,  June.

-------
                               BURNS  I HARBOR
Figure 30.  Distribution of Polyarthra vulgaris,  August.

-------
   8720'
87*10*
87*00'
                               BURNS  I HARBOR
                                                                                    41*40'
Figure 31.  Distribution of Polyarthra vulgaris, September.

-------
                               BURNS  I HARBOR
Figure 32.  Distribution of Conochilus unicornis,  June,

-------
00
                                                                              87'00'
                                    BURNS  I HARBOR
     Figure 33.  Distribution of Conochilus unicornis, August

-------
   87 20'
87*10'
87*00'
                               BURNS  I HARBOR
Figure 34,  Distribution of Cpnochilus  unicornis, September.

-------
H* squamula were obtained in low numbers throughout the sampling period.  They
were observed only at westernmost outer stations in June where coldest waters
prevailed (Appendix A).  The highest density  (2,700 per tn^) was recorded at
the 5 mile station off Burns Harbor for _N. squamula.  All three species were
rare (400 to 1,700 per m^) only at outermost  stations in August.  Notholca
foliacea and N. squanula were rare (100 to 400 per m-*) only at offshore
stations in September but N. laurentiae was more widespread, occurring at most
stations with a maximum of 3,400 per mj at the outermost station off  Indiana
Dunes.

     A few limnetic species, such as Filinia  longiseta, Keratella cochlearis
^" j :.e_c ta> Pompholyx sulcata, and Polyarthra euryptera, have apparent  value as
eutrophic indicators in the Great Lakes (Gannon and Stemberger 1978).  These
species were rare in the Indiana waters of southern Lake Michigan.  Only a few
individuals of Pompholyx were observed near Indiana (Gary) and Burns  Harbor
mouths in August and September.  Similarly, a few Filinia 1ongiseta were
collected near Burns Harbor  in September and  a few Polyarthra euryptera were
observed nearest Indiana Dunes in September.  Keratella cochlearis  f.  tecta
was found in low numbers (400 to 800 per m^)  near Burns Harbor, Indiana Dunes,
and Michigan City in August.

     The comparatively long  species list of rotifers  in southern Lake Michigan
(Table 2) is primarily because of the presence of many littoral and benthic
species in the plankton.  Collection of occasional  littoral specimens in the
plankton is to be expected in turbulent nearshore waters.  However, limnetic
appearance of large numbers  of littoral species (e.g., Brachionus, Euchlanis,
Trichocerca) is an apparent  response to eutrophication in the Great Lakes
(Gannon and Steinberger 1978).  None of the littoral species were observed  in
high densities during the sampling period.

     Two species of Brachionus were observed.  A few  B. angularis were
collected near Gary in June  and near Michigan City  in August and a  few
_B. caudatus were observed near Burns Harbor in August.  A single specimen  of
E uc hian is dilat a t a was collected nearest Burns Harbor in September.   Five
species of Trichocerca were  collected with _T. porcellus (overall mean
abundance of 2,400 per nr*) being most abundant (Appendix D, Table D-l).
Trichocerca multicrinis, the species of Trichocerca most often observed as an
eutrophic indicator (Gannon  and Stemberger 1978), was observed only in August
with a mean abundance of 600 per ir,3 (xable 2).  It was collected only from
stations near Gary and Burns Harbor mouths with a maximum of 2,900  per m-'
observed nearest Burns Harbor.  Trichocer_ca porcellus and JI. rousseleti were
present during all sampling  periods.  They were rare  in June; the former was
collected near Gary and Burns Harbor and the  latter only near Gary.   They  were
widely distributed in August and September but highest densities occurred
inshore.  Maximum densities  for T_. porcellus  were 7,300 per m-^ near Burns
Harbor in August and 10,400  per m-' near Gary  in September.  For T,  rousseleti,
maximum numbers were 2,500 per IE^ in August and 800 per m^ in September near
Indiana Dunes.  T r i c h qcere a  cy1indric a was found in low numbers (300  to 1,200
per m-^) near Gary, Burns Harbor, and Indiana  Dunes  in August and only near
Gary (300 per m^) in September.  A few specimens of T\ pus ilia were collected
off Indiana Dunes in August.
                                       50

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     Other littoral and benthic rotifers  (e.g., Lecane, Lepade11a, Lophpcaris,
Monostyla, Platyias, Stephanocercos, Testudinella, Trichotria, and Tylotrocha)
were collected as single individuals or a few specimens in nearshore waters,
primarily near harbor mouths,

     Other rotifers (e.g., Ascomorpha, Asplanchna, Collotheca, Gastropus,
Keratella, Ploesoma, Polyarthra, and Synchaeta)are limnetic  forms that were
observed  in comparatively low numbers in  southern Lake Michigan.  Only the
prevalent species (mean overall abundance of >1,000 per m-*) will be discussed
here (Appendix D, Table D-l).

     Seven species had mean densities of greater than  1,000 per m-* during the
sampling  period (Appendix D, Table D-l).  In general,  Synchaeta stylata,
Gastropus stylifer, and Keratella cochlearis f. robusta tended to be most
prevalent offshore whereas Asplanchna priodonta, Collotheca mutabilis,
Ploesoma  t rune a turn, and J?otyarthra jnajjor exhibited a tendency toward greater
densities near shore.

     Synchaeta stylata had a total mean abundance of 4,800 per m^ and was most
prevalent (mean of 10,400 per nH) in August when it comprised 5.5% of total
rotifers  (Table 2; Appendix D, Table D-l).  In June, it had a maximum density
of 12,800 per m^ at the outermost station off Burns Harbor.  Densities were
low (1,300 to 1,600 per m.3) nearest Gary and Michigan City and none were
collected nearest Burns Harbor.  Its distribution was discontinuous in August
with no individuals reported for the Burns Harbor and Indiana Dunes transects.
This species was found only near Gary and Michigan City harbors with maximum
abundance of 34,600 and 27,900 per m^, respectively.  In September, maximum
numbers (8,100 per m^) were obtained at the outermost station off Indiana
Dunes.  Densities tended to be slightly higher offshore (mean of 3,200 per m^
at outermost stations) than inshore (mean of 2,900 per nH at innermost
stations).

      Gas t ropus s ty 1 i f e r had a mean overall abundance of 2,900 per n>3 and also
was most  prevalent during August (mean of 6,400 per nH) (Table 2; Appendix D,
Table D-l).  Gastropus was relatively high (mean of 1,900 per m^) at outermost
stations  in June but maximum numbers (4,200 per m~) were recorded nearest
Indiana Dunes.  Its density nearest Gary and Burns Harbor was low (300 and
1,200 per m , respectively) and it was not collected on the Michigan City
transect.  Densities were higher at mid-depth locations in August with a
maximum abundance of 15,100 per rP recorded at the 1 mile station off Burns
Harbor.  Relatively high densities also were recorded at 1 mile stations off
Gary and Michigan City (9,200 and 7,100 per m-^).  Numbers nearest harbors
(mean of 4,800 per m^) and farthest offshore (mean of 5,100 per m^) were
similar.  In September, densities were highest offshore with the maximum
abundance (3,700 per m^) recorded off Gary.   Numbers were especially low 500
per m^) near Burns Harbor and Michigan City harbor.

     K e r a te1 la c o c h1ear is f. robusta had an overall mean abundance of 1,700
per mj and was most prevalent (mean of 2,300 per nH) in August (Table 2;
Appendix D, Table D-2).  In June, it was low in abundance (mean of 1,600 per
nP) at outermost stations and was most prevalent (15,400 to 17,200 per irH)
near Burns Harbor.  Numbers in August were high (3,700 to 5,400 per nH) at all
                                       51

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outermost stations except off Gary.  Numbers on the Gary transect were
consistently low (300 to 400 per m^).  Densities were low  (mean of  1,400 per
nr*) at harbor mouths but relatively high (7,100 per m-^) nearest Indiana Dunes.
Numbers in September were highest (mean of 1,900 per nH) at outermost stations
and the peak abundance was 2,500 per trH 2 miles off Burns  Harbor.   Low
densities (mean of 600 per m-*) were recorded at all 1/2 mile stations.

     Asplanchna priodonta had an overall mean abundance of 2,000 per rar and
was most prevalent in June (mean of 4,800 per m^)  (Table 2; Appendix D,
Table D-l).  It was infrequent (0 to 2,100 per m^) at outermost stations in
June and was most abundant (17,200 per m^) at the  Burns Harbor mouth.  It  also
was prevalent nearest Indiana Dunes (9,300 per m^).  A similar pattern was
observed in August but at much reduced densities.  Highest numbers  (2,100  per
m-^) were recorded nearshore at Indiana Dunes and Michigan  City.  Densities
also were relatively high (1,700 per m^) at the Burns Harbor mouth.  It was
low in abundance (800 per m-*) or absent from outermost stations.  Asplanchna
was collected from about one-half of the stations  in low abundance  during
September.  Highest numbers (700 per tip) were observed at  the outermost
station off Indiana Dunes.

     Ploesoma truncaturo also exhibited its highest mean abundance (2,600 per
m ) in August and had an overall mean density of 1,300 per nr* (Table 2;
Appendix D, Table D-2).  It was found only inshore near Gary, Burns Harbor,
and Indiana Dunes in June, ranging from 300 per nP near Gary to 1,300 per n;^
off Burns Harbor.  Relatively high densities were  obtained off Indiana Dunes
(11,900 per m-*) anct nearest Burns Harbor (7,900 per nP) in August whereas
outermost stations averaged 1,600 per nH.  The same trend  for higher densities
nearshore was apparent in September.  Numbers were especially high  (2,700-
3,700 per m^) near Gary.  Outermost stations averaged 600  per nH.

     Polyarthra major was rare in June (mean of 200 per nH) but was prevalent
in August and September (means of 3,300 and 2,800  per m^).  its overall mean
abundance during the sampling period was 2,100 per m^.  It was collected at
three scattered stations in June with highest numbers observed nearest Indiana
Dunes.  It was most abundant nearshore in August,  especially nearest Burns
Harbor (12,100 per m^).  Outermost stations averaged 1,000 per nP.  Densities
were similar inshore (mean of 2,500 per nP at 1/2 mile stations) and offshore
(mean 2,300 per m-* at outermost stations) on harbor transects in September.
However, numbers were considerably higher offshore (8,800  per m-*) than nearest
shore (2,700 per m^) on the Indiana Dunes transect.

     Collptheca mutabilis had an overall abundance of 3,800 per nH  and was
most abundant in August with a mean of 6,200 per m^,  Except off Gary,
densities of Collotheca were higher inshore.  In June, the lowest numbers  (200
to 600 per m-0 were observed at mid-depth stations off Gary.  Maximum
densities were obtained near Indiana Dunes (11.800 per m-').  Stations nearest
                            "3                      .                    Q
harbors averaged 3,300 per m-* while outermost stations were 1,100 per m-5.  In
August, there was a slight trend for increasing numbers shoreward to a maximum
at 1 mile stations and then numbers decreased nearest shore.  The highest
density (8,700 per m^) was recorded at the 2 mile  station  off Indiana Dunes
and the mean of all 2 mile stations was 7,400 per nP.  in  contrast, means  at
outermost and innermost stations were 6,700 and 5,900 per  nP, respectively.


                                      52

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Densities were more uniformly distributed  in September  but  the  trend  for
increasing numbers shoreward was still evident.  The highest  (6,600 per
was at  the 2 mile station off Gary.
SEASONAL AND SPATIAL DISTRIBUTION OF MAJOR CRUSTACEA
     An examination of  the crustacean plankton data  revealed  that  patterns of
distribution are more readily discernible using percentage composition rather
than numbers per unit volume.  A similar conclusion  was reached  in processing
micro-crustacean data from the Straits of Mackinac and northern  Lake Michigan
(Gannon _e_t _a_l.  1976).   Consequently, distribution of micro-crustaceans will be
discussed primarily in  terms of percentage composition in this section.

     Bosmina longirostris was the most abundant micro-crustacean in the
Indiana waters  of southern Lake Michigan.  It had a  total mean abundance  for
all cruises of  3,000 per m-* (Appendix D, Table D-2), representing  70.6% of all
Cladocera and 38.5% of  total Crustacea.  It was most prevalent (mean of 6,530
per m-3) in June and represented 57.3% of cladocerans and 58.1% of  total micro-
crustaceans.  Densities were considerably less in August and  September (means
of 310 and 1,630 per m-% respectively) but JJ. longirostris still comprised
15.4% and 36.4% of total Cladocera, respectively.

     In June, Bosm ina JLongjiro>s_tTi_s exhibited a northwest to southeast gradient
in abundance, with southeastern stations nearest Indiana Dunes and Michigan
City recording  the highest densities (4,900 to 15,200 per m^),   in contrast,
outermost stations on western transects were comparatively low in abundance
(430 to 1,070 per nP),  This species comprised less  than 10%  of  total
Crustacea offshore on western transects and over 70% near Indiana Dunes and
Michigan City (Figure 35).

     Bosmina longirostris densities in August were highest (530  per nH) off
the Gary Harbor mouth but, in terms of percentage composition, it was
relatively most abundant near Indiana Dunes (Figure  36).  It  comprised less
than 10% of total Crustacea at offshore locations and over 50% nearest Indiana
Dunes.  In September, densities (2,450 to 5,890 per m^) and relative abundance
(30-40% of total Crustacea) were highest near Gary (Figure 37).

     The spatial distribution of Bosmina 1qngirostris throughout the sampling
period strongly resembled the distribution of predominate rotifers, such as
Kerate1la cochlearis (Figures 20-22) and physicochemical variables (Appendices
A-C).Bosmina densities were consistently highest in waters with high
turbidity and specific  conductance.  This species exhibited a significant
(.05) positive correlation with specific conductance and alkalinity and a
significant (.05) negative correlation with Secchi disc transparency in
August.  Moreover, a significant (.01) positive correlation was obtained with
specific conductance and temperature in September.

     The other representative of the cladoceran family Bosminidae,  Eubosmina
coregoni,  was considerably less abundant than Bosmina longirostris.  Eubosmina


                                      53

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                               BURNS  I HARBOR
Figure 35.  Distribution of Bosmina longirostris,  June,

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                               BURNS  I HARBOR
Figure 36.  Distribution of  Bqsmina longirostris, August,

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Ln
                                     BURNS   V HARBOR
     Figure 37.  Distribution of Bosmina longirostris,  September.

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coregoni had a total mean abundance of  140 per m^ during  the  study  period
(Appendix D, Table D-2).  Its density increased as the season progressed from
a mean of 10 per irP in June  to a mean of 310 per iti-*  in September  (Table 3).
In contrast to _B. longirostris, the distribution of  E. coregoni was
sufficiently discontinuous from station to station that it was not  possible  to
contour these data.  Nevertheless, some trends in spatial distribution of  this
species are noteworthy.

     In June, it seems that  the Eubosmina population was  just  beginning to
appear in plankton.  This species was found in low numbers (10 to 30  per nP)
at most nearshore locations  in warmer water.  It was absent from  all  cooler
offshore stations.  In contrast, Eubosmina was more  prevalent  offshore in
August.  A maximum of 310 per 'nH was recorded at the outermost stations off
Gary.  Relative abundance offshore ranged from 3 to  6% of total Crustacea
while nearshore values ranged from 2.8% nearest Gary to none  near Indiana
Dunes and Michigan City.  Distribution of Eubosmina  was especially
discontinuous in September with two density peaks, one (940 per nP) 1 mile off
Gary and the other (600 per m^) 1/2 mile off Michigan City.   It tended to  be
slightly more prevalent at middle stations on the transects than  nearshore or
offshore.  Its relative abundance averaged 3,6% at the outermost  stations,
3.5% nearest shore, and 4.2% at middle  locations.

     No consistent statistically significant correlations between the
distribution of J5. coregoni  and physicochemical variables were obtained.
Moreover, no consistently significant correlations were obtained with any
other crustacean plankter except _B. longirostris.

     Daghni a retrocurva was the second most abundant cladoceran in  the study
area.  It had a total mean abundance of 690 per m^ for all cruises
(Appendix D, Table D-2).  Similar to _E_. coregoni, _D. retrocurva was apparently
just beginning to appear in the plankton during June (mean of  30 per n>3) and
became common in August (mean of 930 per nP) and September (mean of 1,170  per
trH), representing 0.5, 46,3, and 31.9%, respectively, of  total Cladocera
(Table 3).

     The spatial distribution of _D. retrocurva was similar to Eubosmina.   In
June, J). retrocurva was found only at warmer nearshore stations and was most
prevalent (10 to 150 per m-^) on the two easternmost  transects.  It  comprised
from 0 to 0.6% of total Crustacea at various stations (Figure 38).  In August,
its distribution was discontinuous, ranging from 10  per in 3 nearshore at
Indiana Dunes to 2,040 per m^ 2 miles off Burns Harbor (Figure 39),  In spite
of the anomolous high density at this Burns Harbor station, it was  relatively
more abundant on the outer transects (35.3%) than on the  inner ones (28.3%).
Daphnia retrocurva was most prevalent at middle locations on  each transect in
September.  It ranged from 240 per nr nearest Gary to 2,910 per np at the
1 mile station on the same transect.  Harbor mouth stations (mean of 570 per
ujS) Were lower in density than the station nearest Indiana Dunes  (1,140 per
nP).  This species averaged 10.7% of total Crustacea at outermost stations,
18.9% at middle stations, and 14.9% nearest shore (Figure 40).

     Daphnia galeata mendotae was considerably less  abundant  than
D. retrocurva.   It had an overall mean abundance of  140 per m^ (Appendix D,
                                      57

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oo
                                            BURNS HARBOR
     Figure 38.  Distribution of Daphnia retrocurva, June.

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                                       BURNS  HARBOR
Figure 39,  Distribution of Daphnia retrocurva, August.

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o
         87*20'
        QARY
87 10'
87*00
                                                              N
 BURNS HARBOR
                                                                                     4P401
     Figure 40.  Distribution of paphnia retrocurva,  September.

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Table D-2).  Only a few individuals were collected in June near Gary and
Indiana Dunes and the population became progressively larger in August (mean
of 930 per m3) and September (mean of 1,170 per m3) (Table 3).

     In contrast to _D. retrocurva, _D_. galeata mendotae was more abundant
offshore.  In August, it was collected at every station except the 1/2 mile
station off Indiana Dunes.  Densities at stations nearest shore ranged from 30
to 60 per m3 (mean of 2.6% of total Crustacea) while outermost stations were
from 410 to 490 per m3 (mean of 7,6%).  In general, there was a trend of
decreasing densities shoreward from northwest to southeast in the study area
(Figure 41).  In September, numbers also were greater offshore but the trend
of decreasing densities shoreward was from northeast to southwest.
Concentrations were highest (920 per m3) at outer stations on the Indiana
Dunes transect and lowest (0, 30 and 80 per m3) near the mouths of Gary
harbor, Burns Harbor, and Michigan City harbor, respectively.  Relative
abundance ranged from over 12% of total Crustacea at the easternmost outer
station to zero near Gary (Figure 42).

     Diaphanosoma spp. (primarily J>. 1euch t e nb ergianurn) was the only other
relatively abundant cladoceran in the study area.  It had a total mean
abundance for all cruises of 200 per m3 (Appendix D, Table 2).  As with most
other cladocerans, individuals were just beginning to appear in the plankton
during June; a few specimens were collected only at one station, 1 mile off
Indiana Dunes.  Mean abundance was 430 per m3 in August and 260 per m3 in
September (Table 3).

     The distribution of Diaphanosoma resembled that of Eubosmina and Daphnia
retrocurva, with highest densities recorded at stations 1 to 2 miles from
shore during August and September.  Low densities nearest shore did not appear
to be influenced overtly by harbors since numbers nearest Indiana Dunes (mean
of 145 per ro3) and nearest harbors (mean of 197.7 per m3) were appreciably
similar in August and September.   In August, highest densities (850 to 1,330
per rc3) were observed at offshore stations on the two inner transects where
Diaphanosoma represented over 20% of total Crustacea (Figure 43).  Densities
nearest shore ranged from 10 to 450 per m3 (mean of 13.6%).  In September,
Diaphanosoma distribution was more discontinuous but the tendency for higher
numbers at mid-depth locations still was evident.  Highest densities (near 500
per m3) were recorded 1 mile off Burns Harbor and 5 miles off Indiana Dunes.
Lowest numbers (30 to 60 per m3)  were obtained nearest Gary and Burns Harbor
and at the outermost station off Gary.  Densities nearest shore ranged from 40
to 280 per m3 (mean of 3.5%) (Figure 44).

     Diacyclops thomasi was the only relatively abundant cyclopoid copepod in
the plankton of southern Lake Michigan.   Its total mean abundance for all
cruises was 760 per m3 (Appendix D, Table D-2).  It was most prevalent (mean
of 1,720 per m3) in June when it  comprised 6.7% of total Crustacea and 36.1%
of all Copepoda (Table 3).  piacyclaps consistently was more prevalent
offshore during all sampling periods.  In June, it was most abundant (7,170
per m3) 1 mile off Gary and least prevalent (140 per m3) nearest Indiana
Dunes.  A general decreasing trend in relative abundance from northwest (30.9
to 45.8% of total Crustacea) to southeast (4.5 to 10.0%) was evident in June
(Figure 45).  Diacyclops was a minor constituent of the plankton in August,
                                     61

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                               BURNS  I HARBOR
Figure 41.  Distribution  of Daphnia galeata mendotae,  August.

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U)
                                      BURNS  I HARBOR
      Figure 42.  Distribution of Daphnia galeata mendotae,  September.

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    8720'
   GARY
87 10'
8700'
                                                         N
 BURNS HARBOR
                                                                  DUNES
                                                                                4I40'	J
Figure 43.  Distribution of Diaphanosoma, August,

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                                        BURNS  HARBOR
Figure 44.  Distribution of Diaphanosoma, September.

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    87 20'
87 10'
8700f
                               BURNS  I HARBOR
                                                                                    41 40'
Figure  45.  Distribution of Diacyclops  thomasi, June.

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comprising only 4.0% of total Crustacea, but it was still most prevalent
offshore.  Its mean abundance at outermost stations was 315 per nH in
comparison with a mean of 16.7 per m-^ at all other stations.  It was absent
nearest shore at Indiana Dunes and Michigan City.  This species was relatively
most abundant (8.7%) at the outermost station off Gary and ranged from 0-2%
nearshore (Figure 46).  Although the mean abundance of Diacyclops increased to
320 per irP in September, it remained at 4.0% of total Crustacea (Table 3).  It
was most abundant (1,150 per m^) at the outermost station off Indiana Dunes
and least prevalent (50 per m-*) nearest Burns Harbor.  This species was
relatively most abundant (5.0 to 15.5%) at outer stations northeastward and
was considerably less frequent (1.0 to 4.6%) elsewhere (Figure 47).  As with
all the Cladocera exept Bosmina longirostris, no consistently significant
correlations between Diacyclops distribution and physicochemical variables
were observed.

     Immature diapton.id copepodids were the most abundant calanoid copepods,
averaging 1,980 per nH during the sampling period and comprising 72.2% of
total calanoids, 55.9% of total copepods, and 24.8% of total Crustacea
(Appendix D, Table D-2).  The copepodids were not identified to species, but
based on relative abundance of adults, it is assumed that the majority of the
immature diaptomids were Legtodia pt omus ash1andi and L. roinutus.  Average
density of copepodids varied considerably; it was highest (3,210 per m^) n
September and was approximately 1.5 and 10 times lower in June and August,
respectively.  Even though density was so low in August, abundance remained
relatively high at 11.3% of total Crustacea as compared to 18.1% and 39.9% in
June and September, respectively (Table 3).

     Diaptomid copepodids exhibited a marked decrease in density eastward in
the study area in June and a decrease southward in August (Figures 48, 49).
In June, highest numbers (7,450 per ITH) were recorded at the outermost station
off Burns Harbor and lowest (50 per jH) were near Michigan City.  Relative
abundance ranged from over 40% of total Crustacea near Gary to less than 1%
near Michigan City (Figure 48).  They were slightly less abundant (19.3%) less
than 1 mile from shore in comparison with offshore stations (mean of 22.5%).
The copepodids also were relatively more abundant nearest harbor mouths (mean
of 22.4%) in comparison with the 1/2 mile station off Indiana Dunes (12.1%).
In August, numbers ranged from 920 per m^ at the outermost station off Burns
Harbor to 60 per nH nearest Michigan City.  Relative abundance was slightly
less (mean of 12.2%) nearshore (1/2 and 1 mile stations) than offshore (mean
of 11.1%) (Figure 49).  In September, the spatial distribution of diaptomid
copepodids was more discontinuous.  Highest numbers (6,950 per np) were
recorded near Gary and lowest (1,480 per nH) near Michigan City.  Abundance
(range of 21.8 to 52.9%) was relatively high at all stations.  No consistent
east-west or north-south trends in abundance were observed.  Relative
abundance was slightly higher (mean of 45.3%) on Gary and Indiana Dunes
transects than off Burns Harbor and Michigan City (32.8%) (Figure 50).

     Adults of Leptodiaptomus ashlandi and L. minutus were similar in overall
mean abundance (240 and 250 per mj,respectivelyT.Both species were most
prevalent (450 and 310 per nH, respectively) in June.  Leptodiaptomus minutu_s
(mean of 290 per nH) was about three times more abundant than L. ashlandi in
                                      67

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00
        87* ZO1
87-IO'
87*00'
                                   BURNS  I HARBOR
                                                                                        41*40'
    Figure 46.  Distribution of Diacyclops thomasi, August,

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                              BURNS  I HARBOR
Figure 47.  Distribution of Diacyclops thomasi, September,

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I
o
                                    BURNS  I HARBOR
     Figure 48.   Distribution of diaptomid copepodids, June,

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                               BURNS  I HARBOR
Figure 49.  Distribution of diaptomid copepodids, August,

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                               BURNS  I HARBOR
Figure 50.  Distribution of diaptomid copepodids, September,

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August and the two species were nearly equal in mean abundance (140 and 160
per TO^, respectively) in September (Table 3).

     Both L. ashlandi and L. minutus generally were more abundant at offshore
stations throughout the study period.  Leptodiaptomus minutus exhibited a
northeast to southwest decreasing trend in relative abundance on all three
sampling dates (Figures 51-53),  In June, densities ranged from near 500 per
m  at outer stations on easternmost transects to near 200 per irr at inner
stations on westernmost transects.  Densities in August ranged from 1,430 per
IT.-' at the outermost station off Indiana Dunes to 20-40 per m-^ at innermost
stations off Indiana Dunes, Burns Harbor, and Michigan City harbor.  Densities
were ir.ost disjunct from station to station in September with maximum numbers
(>300 per nH) recorded offshore at Gary and Indiana Dunes.  Lowest densities
(0-50 per nH) were near Gary and Burns Harbor.

     In contrast to _L. minutus, the distribution of L. ashlandi was
discontinuous from station to station but the general trend for greater
abundance offshore still was evident.  In June, L. ashlandi was most abundant
(2,400 per nH) at the outermost station off Burns Harbor and was least
abundant (40 per nH) nearest Indiana Dunes.  Relative abundance offshore (mean
of 5.8%) was slightly higher than at 1/2 and 1 mile stations (mean of 4.3%)
(Figure 54).  Numbers were highest (400 per nH) at the 2 mile station off
Indiana Dunes in August.  This species was not collected nearest Michigan
City; only a few individuals were obtained nearest Burns harbor and Indiana
Dunes.  Relative abundance was higher offshore (mean of 7.1%) in comparison
with stations less than one mile from shore (1.5%) (Figure 55).  In September,
densities were highest (mean of 290 per nH) on the Gary transect and lowest
(mean of 37 per m-*) on the Michigan City transect.  Besides relatively high
abundance (3.b%) nearest Indiana Dunes, abundances elsewhere ranged from 0.4
to 2.41 (Figure 56).
NOTES CN MINOR CRUSTACEA TAXA
     Limnocalanus macrurus, Senecella calanoides, and Leptodiaptorr.us sicilis
are cold stenothermic species that likely would be rare in the nearshore
waters of southern Lake Michigan during suirmer and early fall.  Indeed, they
were rare in standardized tows and in tows from the bottom to the surface at
deeper stations throughout the study period.  L imnoca1anus was present at
outermost stations on all transects in June and was most prevalent (50 per m^)
at the 5 mile station off Gary.  A few individuals were observed at nearshore
stations off Gary and Burns Harbor, but none were found near Indiana Dunes and
Michigan City harbor.  A few individuals were observed only at outermost
stations in August and September.  The distribution of Leptodia p t om_u s sicilis
was similar to Limnocalanus.  In June, L. sicilis occurred mostly at outermost
stations, reaching a maximum of 50 per m^ at the 2 mile station off Gary.  It
was absent from innermost stations except off Gary,  This species was rare
(maximum of 30 per m^) j_n August with most individuals occurring at outer
stations.  It was more prevalent (maximum of 300 per w^) at outermost
                                      73

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    8720'
87 10'
87*00'
                                                          N
                                                                                 4I40'	
   GARY
 BURNS HARBOR
Figure 51.  Distribution of Leptodiaptomus minutus, June.

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                                        BURNS HARBOR
Figure 52.  Distribution of Leptodiaptomus  minutus, August.

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                                        BURNS HARBOR
Figure 53.  Distribution of Leptodiaptomus minutus, September.

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   87*20'
  GARY
87IO'
87*00'
                                                                  DUNES
                                                                                4I40'_
 BURNS HARBOR
Figure 54.  Distribution of Leptodiaptomus ashlandi, June.

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--J
00
         8720'
        GARY
87 10'                             87*00'

                    N
                                                                        DUNES
                                                                                       4I40'	
 BURNS HARBOR
     Figure 55.  Distribution of Leptodiaptomus ashlandi, August.

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    87*20'
   GARY
87*10'
87*00'
                                                          N
 BURNS HARBOR
                                                                  DUNES
                                                                                 41*40'	
Figure 56.  Distribution of Leptodiaptomus ashlandi, September,

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stations, especially on western transects, in September.  This species was
absent from 1/2 wile stations in August and September,

     The other calanoid copepods occurring in low numbers were Skis tod lap t_omus
oregonensis, Epischura lacustris, and Eu_rytemora a f f i n i s.  In June,
S. oregonensis was present at most stations with a maximum of 90 per m? at the
2 mile station off Burns Harbor.  Only a few individuals were observed at
nearshore stations in August, and maximum numbers (>300 per m^) were found at
2 mile locations.  The same pattern was observed in September with maximum
numbers (>200 per HK) recorded at 5 mile stations off Burns Harbor and Indiana
Dunes.  Epischura was consistently more prevalent offshore during June and
August.  A northeast to southwest decreasing trend in abundance was evident in
June, with numbers of 50 to 90 per m-* offshore on the eastern two transects of
0 to 40 per m-* nearshore on the western two transects.  A similar northwest to
southeast decreasing trend was apparent in September, with a maximum of 360
per m^ at nearshore stations off Gary, Indiana Dunes, and Michigan City.
Numbers of Epischura were considerably higher in September (mean of 420 per
nP).  Densities ranged from 1,490 per m-5 1 mile off Burns Harbor to 30 per EI-^
nearest Gary, but no spatial distribution pattern was discernible.  In
contrast to Sk^istodiaptomus and Epischura, Eurytemora affinis was more
prevalent at nearshore locations.  In June, it was absent from outer
westernmost stations and was most abundant (150 to 190 per m-^) at 1 mile
stations off Indiana Dunes and Michigan City harbor.  It was rare (about 10
per m^) at scattered locations in August, with the stations on the two
westernmost transects most sparsely populated.  This species was absent from
outermost stations in September; it was rare at inshore locations except near
Burns Harbor (130 to 140 per m^).

     A can tho cyclop s v e r n_ a 1 is was low (20 per m^) in overall abundance
(Appendix D, Table D-2)but exhibited a noteworthy distributional pattern.
In June, it was absent from the westernmost transect off Gary where highest
water quality conditions were observed (Appendix A).  It was most prevalent
eastward off Indiana Dunes (430 per m^) and Michigan City harbor (450 per m-*).
Its distribution was restricted to harbor mouths in August and September.
This species was found in low numbers in August nearest Gary and Burns Harbor
(maximum of 110 per m-^) in September.  Other cyclopoid copepods, Mesocyclops
edax and Tropocyclops prasinus mexicanus, were extremely rare in southern Lake
Michigan.  Single individuals of M. edax were seen in August (1 mile station
off Burns Harbor) and in September (2 mile station off Gary).  The small
copepod, 1. prasinus mexicanus, was observed only in September at all stations
nearest shore except off Gary.

     Minor cladoceran taxa were extremely rare, usually comprising a mean
average abundance of less than 10 per m-* (Appendix D, Table D-2),  Only
Leptodora kindtii was relatively more abundant (total mean abundance of 20 per
nH for al1 cruises).  This species was found at all stations throughout the
study period.  It was slightly more prevalent (maximum of 60 to 100 per m^)
nearshore, especially off Indiana Dunes.

     Chydorus sphaericus was represented by a few individuals mostly at
locations nearest shore.  This species was most frequent at 1/2 and 1 mile
stations off the harbor mouths.  It reached a maximum abundance of 120 per m^
                                      80

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nearest Burns Harbor in June.  Similarly, Ceriod aphnia quad r angula and
C. lacustris were rare but relatively most frequent near harbor mouths.
Only C. quadrangula was observed in June, but in low numbers (10 to 20 per
m^), at 1/2 and 1 mile stations.  Both species were present in August but in
low densities (maximum near 100 per m-*).  Most individuals of both species
were observed near Burns Harbor.  In September, only a single individual of
C. quadrangu1a was found nearest Burns Harbor.

     Polyphemus pediculus was most frequent (mean of 30 per m-*) in June.
It was most abundant (maximum of 200 per m^) nearshore off Indiana Dunes and
Michigan City.  A few individuals (maximum about 25 per m^) were scattered
throughout the study area in August and September.  It was found at most
stations in August but was more frequent at outermost stations on western
transects, reaching a maximum abundance of 110 per UK at the 5 mile station
off Gary.  Only a few individuals were observed in September on Burns Harbor
and Indiana Dunes transects.  Only a single individual of Daphnia ambigua was
collected at the 2 mile station off Burns Harbor in June.

     The remaining micro-crustaceans are primarily benthic species that
infrequently appeared as single individuals at some stations.  Eurycercus
lamellatus was observed most frequently.  It was collected at all nearshore
locations in June and August.  Three species of Alona were observed nearest
harbor mouths in June and August (Table 3).  A single individual of
Ilyocryptus spinifer was observed nearest Michigan City harbor and a single
specimen of Leydigia quadrangularis was seen nearest Gary in August.  One
Pleuroxis procurvus was collected in August and several Camptocercus
rectirostris were observed in September near Burns Harbor.  The harpacticoid
copepod, C a n t h o_c amp t u s s taphy 1 inoides, was seen nearest shore at Indiana Dunes
and the cyclopoid, Eucjc1ops agi1is, was observed nearest Burns Harbor in
June.
                                  DISCUSSION
     The species composition of zooplankton in a lake, with few exceptions,
usually remains constant for many decades, perhaps for centuries, because
these species have adapted to the physicochemical environment and have been
successful in competing with other species.  A species which newly disperses
to that lake rarely can become established unless some environmental
disturbance occurs.  Perturbations which change the physicochemical milieu or
alter the balance of competition between species can cause extermination of
some species and allow the appearance of others.  At the present state of our
knowledge, eutrophication, size-selective predation by planktivorous fishes,
and toxic substances are the major factors that may cause changes in
zooplankton species composition and abundance.  Although monitoring and
surveillance programs primarily have been designed to assess eutrophication
trends, caution must be exercised in establishing one-to-one causal
relationships between changes in zooplankton community composition and
eutrophication (Gannon and Stemberger 1978).
                                      81

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     The importance of this investigation is to provide a benchmark on
zooplankton community composition for comparison with future studies.
Ideally, we would also like to compare results of this study with pre-
vious investigations.  Unfortunately, it is difficult to assess the impact
of past changes in water quality and lake ecology on zooplankton because of
the lack of comparable historical data for the Indiana waters of southern Lake
Michigan.

     Eddy (1927) reported on southern Lake Michigan plankton from qualitative
collections obtained near Chicago in 1887-1888 and quantitative samples
gathered near Gary, Indiana Dunes, and Michigan City in 1926-1927.  The
collections were from surface tows evidently made off breakwaters and jetties
and, therefore, these data are not strictly comparable to the present study
because of the different methods and sampling locations employed.
Nevertheless, it is readily apparent that many of the predominant zooplankton
genera (e.g., Keratella, Kellicottia, Bpsmina, Letodiaptomus, and Diacyclops)
present in 1977 were reported as common in 1887-1888.

     One of the most interesting results of the present study is the
overwhelming predominance of rotifers in comparison with crustacean plankton
in southern Lake Michigan in 1977.  Again, comparisons oust be made with
caution but it is interesting to note some obvious differences in the relative
abundance of rotifers and crustacean plankters between 1926-1927 and 1977.
Rotifers comprised an average of 95% of total zooplankton at stations nearest
shore off Gary, Indiana Dunes, and Michigan City in June and September, 1977,
and an average of 59.8% at similar locations in October, 1926, and May, 1927
(Eddy 1927).

     Increase in density of zooplankton without any appreciable shifts in
species composition is apparently an initial response by zooplankton to
nutrient enrichment (Fuller _e_t _a_l. 1977, Gannon and Stemberger 1978).  Density
of rotifers and crustacean plankters was approximately 16 times higher in
June, 1977, than in May, 1927, at nearshore locations off Michigan City and
Indiana Dunes.  Even considering that Eddy sampled somewhat earlier in the
growing season, an increase in zooplankton density probably has occurred in
southern Lake Michigan.  Indeed, counts of total plankton (primarily
phytoplankton) have increased in samples from water intakes in southern Lake
Michigan (Damann 1945, 1960) and such trends have been used as indications of
the response of the plankton community to eutrophication (Beeton 1969).

     The predominant rotifer species in 1926-1927 were Keratella cochlearis,
Polyarthra vulgar is (= _P. trig la), Kellicottia longispina (= Notholca
longispina), Synchaeta stylata, and jS. tremula (Eddy 1927).  Synchaeta was
relatively less abundant in 1977 but, otherwise, most of the predominant
species in 1926-1927 still were the prevalent ones in the present
investigation.  The most abundant crustacean plankters in 1926-1927 were
Bosmina sp., Diacyclops thomasi (= Cyclops bicuspidatus), Tropocyclops
prasinus mexicanus (- Cyclops prasinus), Daphnia retrocurva, Leptod iaptomus
jpinutus (= Diaptomus minutus), and JL. ashlandi (= J), ashlandi)^As with the
rotifers, most of the predominant micro-crustacean species were the same in
1926-1927 and 1977.
                                      82

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     The identity of the bosminid cladocerans  in Eddy's  (1927) plankton
collections has caused considerable confusion  in the interpretation of
zooplankton response to water quality changes  in Lake Michigan.  Certain
bosminid cladocerans have been used as indicators of trophic conditions.  The
classical species shift during eutrophication, as determined primarily from
paleolimnological studies, is from the oligotrophic "species," Bosmina
longispina, to the eutrophic species B. longirostris.  Eddy (1927)listed both
"species" in Lake Michigan during 1887-1888 and 1926-1927.  Wells (1960) found
only B. 1one ir ost ris in the lake during 1954-1955.  Based on the results of
these two studies, Beeton (1965) and Brooks (1969) suggested that
B, longispina was replaced by B. longirostris  in Lake Michigan, and used this
species shift as an indicator of advancing eutrophication.  This alleged
species shift cannot be verified because the exact identity of Eddy's
B. longispina cannot be determined.

     Williams (1966) presented quantitative 1961-1962 data on abundance and
composition of rotifers (predominant genera) from five water intakes around
the Great Lakes, including one station at Gary. . Because of different sampling
methods, this study cannot be compared directly to the 1977 data.  However, it
is noteworthy that the Gary station had the highest mean density of rotifers
in the Great Lakes and these data were correlated with relatively high
phytoplankton counts.  As in the present study, Keratella and Polyarthra were
the predominant genera in 1961-1962.

     More substantive information on zooplankton community composition as
influenced by water quality can be obtained by comparing recent quantitative
studies in regions of different water quality  in Lake Michigan.  Most pertinent
is the study of zooplankton species composition, inshore distribution, and
abundance in southern Lake Michigan in 1970 by Johnson (1972), who used nearly
the same sampling locations as in this investigation.   He used a 1/4 m diameter,
no. 20 mesh (76 urn) Wisconsin plankton net to  collect zooplankton on transects
of stations off Gary, Burns Harbor, and Michigan City during June through
October, 1970.  Both rotifer and crustacean data were procured but data analysis
and interpretation were more thorough for crustacean plankton.

     Johnson (1972) recorded 10 species of Copepoda, and 14 species each of
Cladocera and Rotifera.  Species lists, although more comprehensive for
rotifers in 1977, were basically similar in the two studies.  Bosroina
longirostris, Daphnia retrocurva, and Diacj'clops thomasi were the most
abundant crustacean plankters in 1970.  Johnson did not observe any consistent
patterns in zooplankton abundance and distribution between the three
transects;  however, biomass of crustacean plankton was generally higher at
Michigan City and Gary stations than at Burns Harbor.

     In contrast with the 1977 study, crustacean plankters were relatively
more abundant in 1970.   Johnson (1972) recorded mean numbers of over 100,000
per Hi3 for total crustaceans with ranges of 25,000 per m^ in June to 375,000
per iP in July.   These values are approximately ten times higher than density
figures reported in this study and in the offshore waters of Lake Michigan
(Wells 1960, Gannon 1972).  Moreover, Johnson's values are two to three times
higher than Eddy's (1927) historical data.   Comparable high numbers have been
reported only in Milwaukee Harbor (Gannon 1972), near  the Fox River mouth in


                                     83

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Green Bay (Gannon 1974b), and near the southwestern shore of Lake Michigan
(Roth and Stewart 1973).  Higher numbers of total Crustacea only have been
observed in the concurrent study of Green Bay in 1977 (Gannon et al.
in press).

     Contrasts in rotifer data betwen 1970 and 1977 in southern Lake Michigan
are interesting also.  Johnson (1972) noted the same predominant species
(especially Keratella _cochlearis and Polyarthra yulgaris) as in 1977.
However, density of total rotifers (mean of 120,000 per m3) was about seven
times higher than values reported by Eddy (1927) but only about one-half of
mean total numbers in this investigation.  Total rotifer densities were much
higher (>1,000,000 per m3) in Milwaukee Harbor (Stemberger 1974) but similar
(100,000 to 360,000 per m3) off Ludington in eastern Lake Michigan  (Duffy and
Listen 1978) and in Green Bay (230,000 per m3) during 1977 (Gannon et al.
in press).

     Because of the infrequency of zooplankton collections in southern Lake
Michigan, it is difficult to interpret the apparent differences in zooplankton
densities between investigations.  The increasing trend in both rotifer and
crustacean plankton abundance since 1926-1927 is probably real.  Differences
between 1970 and 1977 may be within yearly variation, but long-term seasonal
or annual zooplankton data are lacking for Lake Michigan.

     The sampling program in 1977 did not include stations at river mouths
and, therefore, it is difficult to resolve apparent impacts of harbor water
discharges on nearshore zooplankton community composition.  However, trends in
spatial distribution and abundance of zooplankton appeared to be related to
existing water quality conditions.

     Water quality patterns in the nearshore waters of southern Lake Michigan
are largely dependent on current and seiche regimes that mix relatively high
quality offshore waters with variously polluted harbor effluents from steel
mills, refineries, and municipal sewage plants (Snow 1974).  An offshore to
inshore decreasing gradient in water quality* was present during each sampling
period in 1977.  The gradient was most distinct in June, with highest water
quality offshore in the northwestern portion of the study area and decreasing
southeastward.  A similar but less distinct pattern was evident in August,
with a northeast to southwest decreasing trend.  Waters were more thoroughly
mixed in September, but slightly poorer water quality was present toward the
southeastern portion of the study area.  Similarly, patterns in zooplankton
(especially rotifers) abundance and distribution were most evident  in June and
exhibited statistically significant correlations with physicochemical
variables.  Patterns were not as distinct in August and September, but
consistent trends were discernible.
*Poorer water quality is defined here as having relatively high specific
 conductance, alkalinity, nutrient chemistry, and turbidity and low
 Secchi disc transparency.
                                      84

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     Total rotifers and predominant species (e.g., Kerate1la coch1earis,
^" crassa, Polyarthra vulgaris, and Conochilus unicornis)were distinctly most
abundant in nearshore waters of poorer water quality.  Statistically
significant correlations between high rotifer densities and high specific
conductance and alkalinity and low turbidity and Secchi disc transparency
often were observed.  Of predominant species, only Kellicottia longispina was
more abundant in higher quality waters.  This pattern may be related to
temperature as JC. longispina is most abundant in deeper, cool waters in summer
(Stemberger 1979).

     The predominant rotifers found in areas of poorer water quality are all
eurytopic species.  Eutrophic indicator species, such as Brachionus,
Euchlanis, and Trichocerca, were rare in southern Lake Michigan but were
confined or most prevalent nearest harbor mouths.  Because of the apparent
high rate of exchange between offshore and nearshore waters, it appears that
the major response to nutrient loading of the rotifer community is an increase
in density of predominant, eurytopic species rather than species shifts toward
more eutrophic forms (Tables 2 and 3).  Fuller et al. (1977) noted that the
initial response of rotifers to eutrophication is an increase in density of
indigenous species.  Williams (1966) made a sircilar observation in examining
rotifers in water intake samples from throughout the United States.  Only in
more persistently eutrophic waters, such as in lower Saginaw Bay of Lake Huron
(Stemberger _e_t _a_l. 1979), do eutrophic indicator species become numerically
important constituents of the rotifer community.

     The overwhelming abundance of rotifers in southern Lake Michigan simply
may be a response to the greater availability of food in more nutrient
enriched waters.  Rotifers have inherently high intrinsic rates of increase
under favorable environmental conditions and, therefore, appear to be more
sensitive indicators of water quality than crustacean plankters (Gannon and
Stemberger 1978).  However, size-selective predation by fishes also may be a
prominent factor in southern Lake Michigan.  Indeed, Webb (1973) reported that
alewives, Alog a pseudoh are ngus, were extremely abundant in southern Lake
Michigan during June and July,  They were less prevalent in August and
September as they moved offshore following spawning.  Crustacean zooplankters,
especially piacyclops thomasi, were the predominant food of alewives.
Alewives are well known to be size-selective in their food habits and to
considerably influence size structure and composition of crustacean
zooplankton populations (Brooks and Dodson 1965, Wells 1970, Gannon 1976).
The low relative abundance of crustacean zooplankters and the predominance of
smaller species suggest that size-selective predation by planktivorous fish,
principally alewives, influenced crustacean zooplankton density and
composition in southern Lake Michigan in 1977.

     Bo sim in a 1 ong i r o s t r i s was the predominant crustacean plankter in southern
Lake Michigan in 1977.  It is a eurytopic species but becomes overwhelmingly
abundant in eutrophic waters, such as Milwaukee Harbor and lower Green Bay
(Gannon 1972, 1974b).  Its abundance and distribution in southern Lake
Michigan were similar to predominant rotifers such as Kerate1la coch Learis.
It was the only crustacean plankter to exhibit consistent statistically
significant correlations with physicochemical variables.  Webb (1973) reported
that _B. longirostris was predominant food for alewives in southern Lake


                                      85

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Michigan.  Evidently, this parthenogenetic species reproduces rapidly in
nutrient-enriched nearshore waters and is able to offset potentially high
predation rates.

     As in the rotifers, the primary response by the crustacean community to
water quality conditions was population increases of eurytopic, indigenous
species, such as B. longirpstris, in nutrient-enriched waters.  No significant
shifts in species composition were evident from the available historical
record.  Oligotrophic indicator species, such as Limnpcalanus and Senecella,
were rare in southern Lake Michigan and were confined to outermost stations.
The eutrophic indicator, Ac a n t ho eye lop s v e r n a 1i s, was rare but confined
nearest shore, especially off harbor mouths.

     Besides Bosmina longirostris, no consistent statistically significant
trends were noted between distribution of crustacean species and
physicochemical variables.  This indicates that their abundance and
distribution are not controlled principally by water quality conditions and
that biotic factors, such as size-selective predation, may play a more
prominent role.  Nevertheless, there were consistent and statistically
significant trends between physicochemical variables and the distribution and
abundance of crustacean plankton by major groups.  There still was a tendency
for calanoid copepods to be more prevalent in more oligotrophic offshore
waters in comparison with cyclopoid copepods and cladocerans as observed
elsewhere in Lake Michigan (Gannon 1974b, Gannon et al. 1976).

     Another distributional trend that often was evident in southern Lake
Michigan was low abundance of zooplankton nearest harbor mouths and highest
abundance slightly farther offshore.  Similar trends have been observed
elsewhere in Lake Michigan (Gannon 1972, 1974b; Stemberger 1974).  Perhaps
flushing times are sufficiently high near harbor mouths to provide a less
favorable environment for planktonic species, or toxic substances may be amply
concentrated nearest harbor mouths to inhibit zooplankters.  A combination of
lower flushing times, abundance of nutrients and suitable food, and adequately
diluted toxicants may allow high densities of zooplankton to develop nearby
(Gannon and Stemberger 1978).

     Stoermer and Tuchman (1980), in the concurrent phytoplankton study, noted
the increase of halophilic algae in southern Lake Michigan concomitant with
increased chloride concentrations.  A brackish water calanoid copepod,
Eurytemora affinis, is a comparatively recent addition to the Lake Michigan
zooplankton community.  It was first reported in Lake Michigan in 1964
(Robertson 1966) and apparently entered the Great Lakes in the bilge water of
ships passing through the St. Lawrence River or Erie Canal from the Atlantic
coast (Faber and Jermolajev 1966).  It was not a prevalent species in southern
Lake Michigan and the distribution of this euryhaline species elsewhere in the
Lake Michigan basin does not appear to be influenced by chloride
concentrations (Gannon 1972, 1974b).

     No discernible patterns (positive or negative) were observed in
correlation coefficients between phytoplankton and zooplankton species
abundances at particular stations in southern Lake Michigan.  Perhaps more
rigorous statistical scrutiny would have revealed more relationships between
                                      86

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the two data sets but any such patterns would undoubtedly be subtle.
Similarly, correlation coefficients between zooplankton and physicochemical
variables exhibit a few statistically significant and limnologically
interpretable correlations.  Consequently, it is not possible to
quantitatively define the trophic conditions (i.e., levels of physicochemical
variables and phytoplankton assemblages) maintaining the zooplankton
composition and spatial patterns observed in southern Lake Michigan.

     This lack of quantification does not distract from the utility of
zooplankton in water quality monitoring.  The most prominent and consistent
feature of the zooplankton comnunity in southern Lake Michigan during 1977 was
the overwhelming abundance of eurytopic species in contrast with the rarity of
eutrophic and oligotrophic indicator species.  This pattern has been observed
elsewhere in the Great Lakes (Gannon and Stemberger 1978) and in inland lakes
as well (Fuller jet _al. 1977).

     Increases in biomass of eurytopic species without shifts in species
composition appear to be the initial response by the zooplankton community to
advancing eutrophication.  Predominance of eurytopic zooplankton species is a
mesotrophic feature and, indeed, the eutrophic waters from rivers in this
portion of the lake dynamically mix with more oligotrophic offshore waters
resulting in the observed physicochemical and biotic mesotrophic character of
southern Lake Michigan waters in 1977.  Detecting future changes in the
abundance and distribution of eurytopic species relative to the rest of the
zooplankton conanunity and detecting shifts in composition, abundance, and
areal distribution of eutrophic and oligotrphic indicator species can be
useful in determining the biotic response to eutrophication control management
strategies in Lake Michigan.
                                      87

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Gannon, J. E., and R. S. Stemberger.  1978.  Zooplankton (especially crusta-
     ceans and rotifers) as indicators of water quality.  Trans. Ame r .
     Microsc. Soc. 97:16-35.
                                      89

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Gannon, J. E.,  K. S.  Bricker,  and T.  B.  Ladewski.   1976.   Crustacean zoo-
     plankton of the Straits of Mackinac and northern Lake Michigan.
     In: Schelske, C. L., Stoermer,  E.  F.,  Gannon,  J.  E.,  and  Simmons,  M.  S.,
     Biological, Chemical and  Physical  relationships in the Straits  of
     Mackinac.   Univ. Michigan, Great Lakes Res.  Div., Spec. Kept. No.  60.
     pp. 133-190.

Gannon, J. E.,  K. S.  Bricker,  and F.  J.  Bricker.   In press. Zooplankton
     community composition in  Green  Bay, Lake Michigan.   USEPA.

JBF Scientific Corporation.  1978.  In-place pollutants in Trail Creek  and
     Michigan City Harbor, Indiana.   USEPA Contract Kept.  No.  68-01-4336.
     86 pp.

Jennings, H. S.  1903.  Rotatoria of the United States,  II. A monograph of
     the Rattulidae.  Bull. U.. Fish.  Comm. 1902:272-352.

Johnson, D. L.    1972.  Zooplankton population dynamics in Indiana waters  of
     Lake Michigan in 1970.  Unpubl.  M.S. thesis,  Ball St. Univ., Muncie,  IN.
     129 pp.

Likens, G., and J. Gilbert.  1970.  Notes on quantitative sampling  of natu-
     ral populations of planktonic rotifers.  Limnol.  Oceanogr.  15:816-820.

Megard, R. 0.  1967.  Three new species of Alona (Cladocera, Chydoridae)
     from the United States.  Int. Rev, ges. Hydrobiol.  52:37-50.

Mortimer, C. H.  1975.  Environmental status of the Lake Michigan region.
     Physical characteristics  of Lake Michigan and its response  to  applied
     forces.  Argonne Nat. Lab., ANL/ES-40, Vol.  2, Part 1. pp. 1-102.

Robertson, A.  1966.  The distribution  of calanoid copepods in the  Great
     Lakes, pp. 129-139.  In:  Proc.  9th Conf. Great Lakes Res.,
     Univ. Michigan, Great Lakes Res. Div., Publ.  No.  15.

Roth, J. C., and J. A. Stewart.  1973.   Nearshore zooplankton  of south-
     eastern Lake Michigan, 1972, pp. 132-142.  In: Proc.  16th Conf. Great
     Lakes Res., Int. Assoc. Great Lakes Res.

Smirnov, N. N.   1971.  Fauna of the  U.S.S.R., Crustacea,  Chydoridae, Vol.  1,
     No. 2.  Acad. Nauk SSSR,  Zool.  Inst.,  New Ser. No. 101.   644 pp.

Snow, R. H.  1974.  Water pollution investigation:  Calumet area  of  Lake
     Michigan.   Vol. 1.  USEPA Rept.  No. EPA-905/9-74-011-A.   307 pp.

Stemberger, R.  S.  1973.  Temporal and  spatial distributions of  planktonic
     rotifers in Milwaukee Harbor and adjacent Lake Michigan.  Unpubl.  M.S.
     thesis, Univ. Wisconsin-Milwaukee.  57 pp.
                                     90

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Stemberger, R. S.  1974.  Temporal and spatial distributions of planktonic
     rotifers in Milwaukee Harbor and adjacent Lake Michigan, pp. 120-134.
     In; Proc. 17th Conf. Great Lakes Res., Internal. Assoc. Great Lakes Res.

Stemberger, R. S.  1976.  Notholea laurentiae and N_. michiganensis,
     new rotifers from the Laurentian Great Lakes.  J_. Fish. Res. Board Can.
     33:2814-2818.

Stemberger, R. S.  1979.  A guide to rotifers of the Laurentian Great Lakes.
     USEPA, Rept. No. EPA-600/4-79-021.  185 pp.

Stemberger, R. S., and J. E. Gannon.  1977.  Multivariate analysis of rotifer
     distributions in Lake Huron.  Ergebn. Liumol. 8:38-42.

Stemberger, R. S., J. E. Gannon, and F. J. Bricker.  1979.  Spatial and sea-
     sonal structure of rotifer communities in Lake Huron.  USEPA, Rept. No.
     EPA-600/3-79-085.  159 pp.

Stoermer, E. F., and M. L. Tuchman.  1980.  Phytoplankton assemblages of the
     nearshore zone of southern Lake Michigan.  USEPA, Rept. No. EPA-905/
     3-79-001.  86 pp.

Torrey, M. S.  1976.  Environmental status of the Lake Michigan region.
     Chemistry of Lake Michigan.  Argonne Nat. Lab., ANL/ES-40, Vol. 3.
     418 pp.

Voigt, M.  1957.  Rotatoria: Die Radertiere Mitteleuropas, 2 vols.,
     Borntraeger, Berlin.  508 pp.

Webb, D. A.  1973,  Daily and seasonal movements and food habits of the ale-
     wife in Indiana waters of Lake Michigan near Michigan City, Indiana, in
     1971 and 1972.  Unpubl. M.S. thesis, Ball St. Univ., Muncie, IN.  104 pp.

Wells, L.  1960.  Seasonal abundance and vertical movements of planktonic
     Crustacea in Lake Michigan.  U.S. Fish Wildl. Serv., Fish. Bull.
     60:343-369.

Wells, L.  1970.  Effects of alewife predation on zooplankton populations
     in Lake Michigan.  Limnol. Oceanogr. 15:556-565.

Williams, L. G.   1966.  Dominant rotifers of major waterways in the
     United States.  Limnol. Oceanogr. 11:83-91.

Wilson, M. S.  1959.  Calanoida, pp. 738-794.   In; W. T.  Edmondson (Ed.),
     Freshwater Biology, 2nd Ed. Wiley,  New York.  1,248 pp.

Wilson, M. S., and H. C. Yeatman.  1959.   Harpacticoida,  pp.  815-861.
     In; W. T. Edmondson (Ed.).  Freshwater Biology, 2nd Ed.  Wiley,  New York.
     1,248 pp.

Yeatman, H. C.  1959.  Cyclopoida, pp. 795-814.   In: W. T. Edmondson
     (Ed.), Freshwater Biology. 2nd Ed.  Wiley, New York.   1,248 pp.


                                      91

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                               BURNS   HARBOR
Appendix Figure A-l.  Temperature contours, southern Lake Michigan; 11 June, 1977,

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>
                                    BURNS  B HARBOR
     Appendix Figure A-2.  Conductivity contours, southern Lake Michigan;  11  June, 1977.

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    87 20'
87 IO1
87 00'
          6.0
                               BURNS  I HARBOR
Appendix Figure A-3.  Secchi disc contours, southern Lake Michigan;  11  June, 1977.

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>
                                    BURNS  I HARBOR
     Appendix Figure A-4.  Contours for  pH, southern Lake Michigan; 11 June, 1977.

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Ui
                                   BURNS  I HARBOR
     Appendix Figure A-5.  Alkalinity contours, southern Lake Michigan;  11 June, 1977.

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>
                                    BURNS  I HARBOR
     Appendix Figure A-6.  N03~N contours, southern Lake Michigan; 11 June,  1977.

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>
                                    BURNS  I HARBOR
     Appendix Figure A-7.  Ammonia contours, southern Lake Michigan; 11 June,  1977,

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                               BURNS  I HARBOR
Appendix Figure A-8.  Silica contours,  southern Lake Michigan;  11 June,  1977.

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>
                                    BURNS I HARBOR
      Appendix Figure A-9.   Anaerobic heterotroph contours,  southern Lake Michigan;  11  June,  1977,

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                               BURNS  V HARBOR
Appendix Figure  A-10.  Turbidity contours,  southern Lake Michigan; 11 June, 1977.

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    87*20'
87! O1
87 00'
                               BURNS  I HARBOR
Appendix Figure A-ll.  Fecal coliform contours, southern Lake Michigan;  11 June, 1977,

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08
                                    BURNS   HARBOR
     Appendix Figure  B-l.  Temperature contours,  southern Lake Michigan; 20 August, 1977.

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w
NJ
         87 20'
87 10'
8 7 00'
                                                                                         4I40'
                                    BURNS   HARBOR
     Appendix Figure B-2.   Conductivity contours, southern Lake Michigan;  20 August, 1977.

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08
to
                                    BURNS I HARBOR
     Appendix Figure B-3.   Secchi disc contours,  southern Lake Michigan;  20 August,  1977.

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w
C-
                                     BURNS  I HARBOR
     Appendix Figure B-4.   Contours  for  pH, southern Lake Michigan;  20 August,  1977.

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Ul
                                     BURNS   HARBOR
     Appendix Figure B-5.   Alkalinity contours, southern Lake Michigan;  20 August, 1977.

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                              BURNS  I HARBOR
Appendix Figure B-6.  N03~N contours, southern Lake Michigan; 20 August, 1977.

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w
i
         87 20'
87 10'
87 00'
                                                                                          41*40'
                                    BURNS  V HARBOR
      Appendix Figure B-7.  Ammonia contours,  southern Lake Michigan; 20 August,  1977.

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w
 I
oo
                                    BURNS  I  HARBOR
     Appendix Figure B-8.  Silica contours, southern Lake Michigan;  20 August,  1977.

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i
VO
         87" 20'
87" 10'
8700f
                                    BURNS  I HARBOR
     Appendix Figure B-9.  Anaerobic heterotroph  contours, southern Lake Michigan;  20 August,  1977,

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td

(-
O
                                    BURNS  I  HARBOR
     Appendix Figure B-10.   Turbidity contours, southern Lake Michigan;  20 August,  1977.

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OS
 I
         87 20'
87*10'
8700f
                                    BURNS  I HARBOR
     Appendix Figure B-ll.   Fecal  coliform contours, southern Lake Michigan; 20 August, 1977.

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o
                                    BURNS  I HARBOR
     Appendix Figure C-l.  Temperature  contours, southern Lake Michigan; 24 September, 1977,

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o
                                    BURNS  K HARBOR
      Appendix Figure C-2.  Conductivity contours,  southern Lake Michigan;  24 September,  1977.

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n
 I
OJ
                                    BURNS  I HARBOR
      Appendix Figure C-3.  Secchi disc contours,  southern Lake Michigan; 24 September,  1977,

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n
 i
                                    BURNS  I HARBOR
      Appendix Figure C-4.  Contours for pH,  southern Lake Michigan; 24 September,  1977,

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n
 i
Ul
                                    BURNS  I HARBOR
      Appendix Figure C-5.  Alkalinity contours,  southern Lake Michigan;  24 September, 1977,

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o
 I
                                    BURNS  I HARBOR
      Appendix Figure C~6.  N03~N  contours, southern Lake Michigan; 24 September,  1977.

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o
 I
                                    BURNS I HARBOR
      Appendix  Figure C-7.  Ammonia contours, southern Lake Michigan; 24 September, 1977,

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n
 i
00
                                    BURNS  I HARBOR
     Appendix Figure C-8.   Silica  contours, southern Lake Michigan;  24 September, 1977.

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o
 I
VO
      Appendix Figure C-9.   Anaerobic heterotroph contours, southern Lake Michigan;  24 September,  1977,

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n
 i
                                    BURNS  V HARBOR
     Appendix Figure C-10.  Turbidity  contours, southern Lake Michigan;  24 September, 1977.

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o
I
                                    BURNS  I HARBOR
     Appendix Figure  C-ll.  Fecal coliform contours,  southern Lake Michigan; 24 September,  1977.

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        TABLE D-l.  SPECIES COMPOSITION AND MEAN AND MAXIMUM ABUNDANCE
                              (NUMBERS x 103/M3)
         OF ROTIFERS IN THE INDIANA WATEES OF SOUTHERN LAKE MICHIGAN.
          SUMMARY IS BASED ON POOLED NEAR SURFACE AND BOTTOM SAMPLES
                FROM ALL STATIONS AND SAMPLING DATES COMBINED.
               PRESENCE  OF  A  SPECIES  IN NUMBERS  LESS  THAN  100/M3
                        IS  INDICATED  BY A PLUS  SIGN  (+)
Class Monogonata
Order Ploima
  Subfamily Colurinae
  Lepadella patella (0.  F.  Muller)

  Family Lecanidae
  Lecane mir a (Murray)
  Monostyla closterocerca (Schmarda)

  Family Trichocercidae
  Trichocerca cylindrica (imhof)
  Z-  roult icr in is (Kellicott)
  Z*  porcellus (Gosse)
  i*  Pusi-Ha (Jennings)
  T-  rousseleti (Voigt)
Mean
                                                        0.2
                                                        2.4
                                                         +
                                                        0.4
                                                                  Maximum
Family Brachionidae
Subfamily Brachioninae
Brachionus angularis Gosse
B. caudatus Barrois and Daday
Euchlanis dilatata Ehrbg.
Kellicottia longispina (Kellicott)
Keratella cochlearis cochlearis (Gosse)
K. cochlearis f. hispida (Lauterborn)
K. cochlearis f. robusta (Lauterborn)
K. cochlearis f. tect'a (Gosse)
K. crassa Ahlstrom
K. earlinae Ahlstrom
K. quadrata (0. F. Muller)
K. valga f. brevispina
Lophocaris salpina (Ehrbg.)
Notholca foliacea (Ehrbg.)
N. labis Gosse
N. laurentiae Stemberger
Is. _s quamu la (0. F. Muller)
Platyias patulus (0. F; Muller)
Trichotria tectractis (Ehrbg.)
6.4
77.2
1.7
0.1
10.0
0.3
0.4
0.1
0.4
0.1
0.4
1.0
35.8
384.4
0.4
7.1
0.8
45.0
2.0
2.0
0.4
0.4
3.4
2.7
              0.4
              2.9
             10.4
               +
              2.5
(continued).
                                     D-l

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TABLE D-l.  (continued).
Class Monogonata
Order Ploima                                           Mean       Maximum
  Family Gastropidae
  Ascomorpha ecaudis Perty                               +            +
  A s c omorpha ovalis (Bergendal)                         0.7           3.5
  Castr opus st y 1 if e r Iir.hof                              2.9          15.1

  Family Tylotrochidae
  Tylotrocha monopus (Jennings)                         0.4           3.7

  Family Asplanchnidae
  Asplanchna priodonta Gosse                            2.0          17.2

  Family Synchaetidae
  Ploesoma hudsoni (Imhof)                              0.1           0.7
  P. lenticulare Herrick                                0.6          11.9
  E.' truncatum (Levander)                               1.3           7.9
  Polyarthra dolichoptera Idelson                       0.1           2.6
               Wierzejski                                +            0.2
  JP. roajor Burckhardt                                   2.1          12.1
  P_. remata Skorikov                       .            29.0         164.2
  L' yulgg:^5 Carlin    '                               37.4         113.7
  Synchaeta kitina Rousselet                            0.3          2.0
  S. la_ko_witziana Lucks                                  +            +
  ]L [oblonga"hrbg.                                      +           0.2
  S. pectinata Ehrbg.             ,                       +           0.5
  .' sty,!3.^5 Wierzejski                                 4.8          34.6

  Family Testudinellidae
  F i1i n i a longiseta (Ehrbg.)                             +            +
   terminalis (Plate)                                 0.1          1.2
  Pompholyx sulcata Hudson                               +           0.4

  Family Conochilidae
  Conochilus unicornis (Rousselet)                     36.2         322.1

  Family Collothecidae
  Co11o t hec a mu t ab i1i s (Hudson)                         3.8          11.8
  C. pelagica (Rousselet)                                +           1.2
  Stephanocercos firobriatus (Goldfuss)                   +            +

  lotal Rotifers                                      221.4
                                     D-2

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  TABLE D-2.  SPECIES COMPOSITION AND MEAN AND MAXIMUM ABUNDANCE (NUMBER/M3)
    OF CRUSTACEAN PLANKTON IN THE INDIANA WATERS OF SOUTHERN LAKE MICHIGAN
                       DURING THE 1977 SAMPLING PERIOD.
          SUMMARY  IS  BASED ON  STANDARDIZED NET  TOWS FROM ALL  STATIONS
                       AND ALL SAMPLING DATES COMBINED.
               PRESENCE OF A SPECIES IN NUMBERS LESS  THAN 10/M3
                        IS  INDICATED BY A PLUS  SIGN (+)
                                                       Mean       Maximum
Subclass Copepoda
  Order Calanoida                                     2,740        7,580
    Senecella calanoides Juday                            +            +
    Limnocalanus macrurus Sars                            +           30
    Eurytemora affinis (Poppe)                           40          410
    Epischura lacustris Forbes                          180        1,480
    Leptodiaptomus sicilis Forbes                         +           50
    L_. ashlandi Marsh                                   240          870
    L_- minutus Lilljeborg                               250        1,430
    Skistodiaptomus oregonensis Lilljeborg               60          340
    Diaptomid copepodids                              1,980        6,950

  Order Cyclopoida                                      800        7,170
    Acanthocyclops vernalis Fisher         "              20          140
    Diacyclops thomasi Forbes                           760        7,170
    Mesocyclops edax (Forbes)                             +            +
    Tropocyclops prasinus mexicanus Kiefer                +           20
    Eucyclops afiilis (Koch)                               +            +
    Cyclopoid copepodids                                 20          100
  Order Harpacticoida
    Can t ho c amp t u s s t a phy 1 ino id e s Pearse                   -t-            +
Subclass Branchipoda
  Order Cladocera                                     4,250       15,750
  Family Leptodoridae
    Leptodora kindtil (Focke)                            20          100

  Family Polyphemidae
    Polyphemus jediculus (L.)                            10          200
  Family Sididae
    Diaphanosoma spp.                                   220        1,330
  Family Macrothricidae
    Ilyocryptus spinifer Herrick                          +            +
(continued).
                                      D-3

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TABLE D-2.  (continued).
                                                        Mean       Maximum
  Family Holopedidae
    Holopedium gibberum Zaddach                           10          110

  Family Daphriidae
    Ceriodaphnia lacustris Birge                          10          110
    C_. quadrangula Miiller                                  +           40
    Daphnia galeata mendotae Birge                       140          920
     retrocurva Forbes                                 690        2,910
    D_. ambigua Scourfield                                  +            +

  Family Bosminidae
    Bosmina longirostris (Miiller)                      3,000       15,610
    Eubosmina coregoni (laird)                           140          940

  Family Chydoridae
    Alona affinis (Leydig)                                 +            +
    A. quadrangular is (Miiller)                             +            +
    A. setulosa Megard                                     +            +
    Camptocercus rectirostris Sch^dler                     +            +
    Chydorus sphaericus (Miiller)          .                 +            +
    Eurycercus lamellatus (Miiller)                         +           10
    Leydigia _quadr angular is (Leydig)                       +            -f
    Pleuroxus procurvus Birge                              +            +

    Total Crustacea                                    7,800       23,980
      U.S. GOVERNMENT PRINTING OFFICE: 1983-655-121.

                                      D-4

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