Energy Research
and Development
Administration
United States
Environmental Protection
Agency
Division of Biomedical
and Environmental Research
Germantown, Maryland 20767
Office of Research and Development
Office of Energy, Minerals and Industry
Washington, D. C. 20460
EPA-600/7-77-097
August 1977
PRODUCTION CYCLES IN
AQUATIC MICROCOSMS
Interagency
Energy-Environment
Research and Development
Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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LBL-5965
Production Cycles in Aquatic Microcosms
A. Jassby, M. Dudzik, J. Rees, E. Lapan, D. Levy, and J. Harte
Energy and Environment Division
Lawrence Berkeley Laboratory
University of California
Berkeley, California 94720
Research supported by the U.S. Energy Research and Development Administration
and the Environmental Protection Agency D5-E681 through contract #77BCC
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ABSTRACT
Four 700-liter cylindrical containers were filled with demineralized
water, enriched with nutrients, and inoculated with 3.5-liter lakewater
samples. The microcosms were maintained at a temperature of 18"°C under
a 12:12 L:D cycle for 6 months and several manipulations of their trophic
structure were carried out, including addition of snails (Physa sp.),
mosquitofish (Gambusia affinis), and catfish (Plaeostomas plaaostomas}.
Temporal variation of the phytoplankton resembled the bimodal patterns of
certain natural systems. Further analysis demonstrated a close analogy
with the predator-prey oscillations of temperate marine waters: an initial
bloom is terminated by zooplankton grazing; the resulting low phytoplankton
levels lead to gradual starvation of the zooplankton; and a second bloom
follows the final dieoff of zooplankton. Both decreasing the concentra-
tion of initial nutrients or stocking the microcosms with Gambusia decreases
the time between the "spring" and "fall" blooms. The problem of heavy
periphyton growth in microcosms was not solved with the introduction of
either Physa or Plaoostomas. Possible solutions to this and to other
problems peculiar to microcosm research are discussed, and modifications
are suggested for increasing the ability of microcosms to simulate
natural systems.
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INTRODUCTION
The word "microcosm", when used in an ecological context, refers
to a collection of chemicals and organisms within well-defined spatial
boundaries, generally under controlled physical conditions, and in a
volume convenient for laboratory study, i.e., much smaller than ecosys-
tems of interest in nature. Unispecific cultures usually are excluded
from this definition. A number of researchers view aquatic microcosms
as appropriate experimental objects for the investigation of systemic
properties of naturally occuring ecosystems and the delineation of
various details concerning trophic interactions, nutrient cycles, and
certain other topics (see reviews by Cooke, 1971, and Taub, 1974).
In particular, laboratory microcosms have the following desirable
properties: (i) the small size permits replication; (ii) the chemical
composition of the medium and the trophic structure can be manipulated,
so that analogs of qualitatively different ecosystems can be created;
(iii) the lack of complicated spatial heterogeneity allows more complete
definition of physical, chemical, and biological characteristics; (iv)
perturbations of different physical, chemical, or biological variables
can be carried out with little effort and expense; and (v) causal rela-
tionships often are more easy to deduce than in natural systems, where
uncontrolled environmental variability complicates interpretation.
These advantages are not necessarily compelling. A number of draw-
backs inherent in the use of aquatic microcosms, such as the high surface-
to- volume ratio of the containing structure, can be pinpointed on an
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a priopi basis. Although several analogies in biological patterns
between microcosms and natural systems do exist (e.g., diurnal gas
exchange, Beyers, 1963; long-term community production, Cooke, 1967;
nutrient cycling, Whittaker, 1961), it has not yet been demonstrated
that microcosms can be made to exhibit most of the features essential
to seasonally- varying natural systems of interest, such as the typical
succession of phytoplankton and zooplankton, the occurrence of both
spring and fall blooms of primary producers, etc. Part of the reason for
this lack of information has been an undue concern with creating
systems that exhibit steady-state characteristics, a situation almost
never observable in nature.
Considering the potential importance of microcosms in determining
the macroscopic properties of natural ecosystems, as well as the effect
of toxic contaminants on these properties, a need obviously exists for
more detailed investigation of the nature of small, synthetic aquatic
ecosystems. In this paper, results are presented from a study of 700-liter
freshwater microcosms in an attempt to clarify some of the analogies
between laboratory microcosms and natural water bodies, as well as to
further define the major problems associated with the use of micro-
cosms in environmental research. The microcosms differed in initial
chemical composition and in certain features of trophic structure, and
a variety of chemical and biological data were collected for periods of
up to 6 months. Particular attention is paid to those factors motivating
the detailed design of the microcosms.
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MICROCOSM DESIGN
Size. The size of freshwater microcosms employed in previous investi-
gations varies from less than 0.01 (Salt, 1971) to 200 liters (Whittaker,
1961), with the large majority less than 20 liters. Size is critical
in at least 3 respects: (i) smaller systems have larger container
surface-to-volume ratios, rendering surface effects more important
than in natural systems; (ii) smaller systems are disturbed to a greater
extent when samples are removed for analysis; and (iii) smaller systems
support a smaller number of trophic levels. Accordingly, we found it
desirable to maximize the size of our microcosms within the boundaries
of the available temperature-controlled space. The 4 microcosms are
cylinders, 60.9 cm in radius and 75.8 cm in height, constructed of
fiberglass with a non-toxic seal. When filled to a depth of 60.1 cm,
the water volume is 700 liters in each tank.
Physical conditions. The experiments reported here were oriented toward
an examination of microcosm behavior in the absence of seasonal changes
in temperature, light, and turbulence, i.e., toward patterns generated
solely as a result of the internal interactions between the various com-
ponents of the microcosms. The systems were maintained in a temperature-
controlled room at 19 ± 1 °C. Each tank was illuminated by a bank of 8
4-ft high-output fluorescent lights on a 12:12 light:dark cycle. The
water was agitated by air from a filtered (Dayton Electric Speedaire
2Z435) laboratory supply, passing through a capillary tube 30 cm below
the water surface, at a rate of 1.2 liter rnin" .
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No attempt was made to create a nonuniform vertical temperature
structure within the microcosms. Creation of a thermocline would not
be a difficult matter. However, we felt that the presence of the well-
illuminated hypolimnion of small volume that would result is so unrepre-
sentative of most naturally-occuring systems that the additional effort
was not merited. This scaling problem undoubtedly is one of the outstan-
ding drawbacks of aquatic microcosms. To a certain extent, the function
of the hypolimnion as a source of inorganic nutrients resulting from
decomposition processes, partially cut off from interactions with the
epilimnion, can be replaced by a porous benthic substrate that collects
sedimenting organic matter.
There were no inflows or outflows of water during this experi-
ment. Although the trophic state of an aquatic system is dependent
markedly on nutrient loading rates and hydraulic retention times
(Volenweider, 1975), the biological activity during summer stratifica-
tion of most temperate lakes appears to be determined by the concentra-
tion of dissolved nutrients already present at the onset of spring
overturn (Dillon and Rigler, 1974). Because our first concern is with
the period of high productivity including and subsequent to the spring
bloom, justification therefore exists for setting flowthrough rates to
zero. Water loss by evaporation (about 1 cm each week) was compensated
for by weekly addition of demineralized water.
Benthio substrate. The choice of substrate presented a difficult optimi-
zation problem. If the sediments were too fine in texture, the total
particle surface area would support adsorption rates and bacterial
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activities that might be unnaturally high relative to the volume of
overlying water. On the other hand, sediments of too coarse a texture
may preclude establishment of typical benthic macrofauna. Because of
the necessarily qualitative nature of these considerations, a compro-
mise between the 2 extremes had to be chosen without an explicit factual
basis. The benthic substrate consisted of a layer of river sand (sili-
cates) 4 cm in thickness, and with a particle size ranging from 0.3 to
3 mm. The mean particle size was approximately 1.5 nan.
Sediments from natural ecosystems contain levels of organic matter
and inorganic nutrients partially determined by the productivity and depth
of the overlying water. When this material is removed into a laboratory
microcosm with a smaller depth of overlying water, the danger exists
that the sediment will exert a long-term effect on this water surpassing
its original effect on the parent system. That is, the material flowing
from sediments to water will be diluted far less than in the parent
system, and the resulting changes in water quality may proceed over long
time periods and to an extent not found in natural systems of interest.
Accordingly, we decided to acid-wash the sand thoroughly in concentrated
HC1 before use, permitting the biological and chemical characteristics
of the microcosm water to determine the ultimate organic and inorganic
chemical content of the sediments.
i
Miovooosm initiation. Two extremes can be recognized in the initiation
of aquatic microcosms in the laboratory. The first appraoch is exempli-
fied by the systems of Taub (1971), in which defined inorganic chemical
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media under controlled physical conditions were inoculated with organisms
from pure culture (an alga, a protozoan, and 2 bacteria). Because the
community composition is known completely, these microcosms are referred
to as gnotobiotic systems. The second extreme is represented by the
i f
work of Beyers (e.g., 1965), who removed portions of various naturally-
occurring aquatic ecosystems into the laboratory, where they were main-
tained under controlled conditions of light and temperature. Each
approach was chosen to facilitate the attainment of different experimen-
tal goals. The gnotobiotic approach was selected for analysis of physical
and chemical affects on steady-state community structure, and the confined
natural ecosystems for examination of diurnal community gas exchange.
Although both designs were suited eminently for the problems under-
taken by the respective investigators, both have certain deficiencies
as general tools for the study of macroscopic properties of aquatic
ecosystesm. The gnotobiotic systems differ from natural ones in that
the initial community lacks diversity and is synthesized by the investi-
gator, not by the natural selection of a diverse community from an even
more diverse initial assemblage. Those properties that depend on the
existence of a large number of species with subtle properties suitable
for their coexistence thus will be lacking.
The laboratory confinement of portions of naturally-occurring systems
entails a different set of difficulties. The chemical and biological
structure of natural systems depends, in many ways, upon their geometry.
When samples of these ecosystems suddenly are confined, the chemical
and biological parameters inevitably change in a way not totally repre-
sentative of the parent system. Although the qualitative behavior of the
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sample may resemble that of the parent for some time after confinement,
providing valuable information on specific phenomena such as chemical
transformations (e.g., Mortimer, 1941-42), the long-term behavior cannot
be viewed with any confidence as analogous to the parent unit or, perhaps,
to any natural system. As an example, we refer to the discussion in the
previous section on the possible consequences of decreasing the depth of
water overlying sediments collected from some natural aquatic system.
In parallel with certain other investigations (e.g., Maguire, 1971;
Neill, 1975), the method of initiation that we chose represents a compro-
mise between the above two extremes. The 4 tanks (designated I, II, III,
and IV) each were filled with demineralized water to a final water volume
of 700 liters. The water then was enriched from stock solutions of a
modification of a common freshwater algae medium (Woods Hole MBL;
Nichols, 1973). Enrichment was identical for each tank (Table 1),
except for concentrations of inorganic phosphorous and nitrogen. Systems
2
II, III, and IV were enriched with concentrations of 3.0 x 10 , 77, and
19 ymol liter" NaNO_, respectively. System I was enriched identically to III,
and all systems had molar N:P = 16 in the enrichment.
Vitamins were not added, as we preferred that the organisms in
each system establish levels of vitamin activity reflecting their own
metabolic rates and interactions. Tris buffer also was omitted. The
levels of tris normally used for buffering activity (equivalent to ca.10
mol liter organic carbon) far exceed the detrital carbon concentrations
of any freshwater system. Most unpolluted inland waters have organic
carbon concentrations of 0.2 to 3 mmol liter" (Wetzel, ,1975). The EDTA
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in our medium contributes 0.06 mmol liter" to the detrital carbon pool
and plays the essential role of a refractory humic-like chelating agent
for trace elements. The demineralized water contains an additional 0.10
mmol liter of organic carbon in unknown form. Biological activity
in the systems provides up to 0.32 mmol liter (see below). The organic
_ j
carbon in the microcosms thus ranges from 0.26 co 0.58 mmol liter ,
representative of many natural aquatic ecosystems.
On the day following chemical enrichment of each microcosm, the
systems each were inoculated with a 3,5-liter water sample collected
from the littoral zone of eutrophic Lake Anza in the Tilden Park area
of Berkeley, California. Manipulation of the trophic structure was
carried out at various times after inoculation. These manipulations,
summarized in Table 2, involved additions of juvenile mosquito fish
(Gambusia affin-is}, South American catfish (Plaoostomas plaoostomas'),
oligochaetes (Pptstina sp.), midge larvae (Tanytarsus sp.), and snails
(Physa sp.), and were designed to fill niches not necessarily represented
in the initial inoculum.
ANALYTICAL METHODS
The following parameters were measured on a weekly basis: tempera-
ture, 0_, pH, inorganic carbon (1C), organic carbon (OC), NH4, NO, + N02,
inorganic phosphorus (IP), total phosphorus (TP), phytoplankton species
and number, and zooplankton species and numbers. All chemical measure-
ments were duplicated. Chlorophyll a_ (Chi a) occasionally was measured
in the microcosm water and in periphyton scraped from the tank sides.
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Monitoring of phytoplankton and zooplankton continued until Day 198,
although all other analyses were terminated on Day 147.
Methods and special instrumentation for each parameter monitored
in the microcosms are summarized in Table 3. References to detailed
descriptions of the techniques are provided where necessary. Vertically
integrated samples were collected for analysis at 1100 h, 3h after the
beginning of the light period, by immersing a hollow glass tube to a
depth of 5 cm above the bottom of each system. Zooplankton were
collected by tows with a plankton bucket (Wildco) fitted with a 64 urn
straining net (Nitex).
Total phytoplankton, protozoa, and rotifer volumes were estimated
from microscopic measurements on representatives of each species. Crus-
tacean volumes were estimated from the numbers in various length classes
for each species, using the length-weight relationships of Pechen (1965)
for Daphni-a and Simooephalus and of Kelkowski and Shushkina (1966) for
copepods. The Daphnia relationship also was applied to Alona and
Cypridopsis.
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RESULTS
No additions of either Gambusia or Ptaoostomas were carried out
for system I, which thus contains the simplest structure of the 4
systems. It is important to note that the biomass of Gambusia intro-
duced into systems II, III, and IV is much higher than the level that
could develop within any natural system. The 5 fish in each microcosm
represent approximately 3 g or 4 mm liter" wet weight, compared to
3 -1
a maximum crustacean level of less than 2 mm liter . Although
crustacean production apparently was high enough to maintain the
Gambusia population (see below), the biomass of a planktivorous' trophic
level in natural systems normally is far less than the maximum biomass
of its food supply. Any phenomena attributable to the presence of
Gambusia thus can be viewed only in a qualitative manner; the effects
in natural systems would be far less dramatic. For this reason,
and in order to avoid unnecessary duplication in the presentation of
results for all 4 systems, we will concentrate on the details of system
I. Only those phenomena in II, III, and TV that differ significantly
from those in I are presented.
On the basis of phytoplankton and crustacean biomass, the experi-
ment with system I can be divided into 3 periods of differing biological
activity (Fig. 2):(i) an initial bloom that terminates by Day 56,
(ii) a period of low phytoplankton biomass whose end is marked by the
disappearance of the cladocerans on Day 161, and (iii) a secondary
bloom beginning on Day 161. A similar division may be applied to
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systems II and III (Figs. 4,5), although the second interval terminates
on Day 140 and 198, respectively. In IV, the distinction between an
initial bloom and a second interval of lower phytoplankton biomass cannot
be made (Fig. 6), although the disappearance of crustaceans on Day 77
may be taken as a natural division between the second and third period.
Throughout the experiment, water temperatures remained at 18 ± 1 C and
02 concentrations between 0.29 and 0.31 mmol liter
(i) Initial bloom. As exemplified by system I, a number of distinc-
tive features characterize the chemical data during the first interval,
most of the parameters exhibiting a local extremum by Day 56(Fig.1) . On
Days 28 to 35, pH and OC attained a maximum, and 1C and NO + N02
a minimum for the period. The increase in OC of 0.18 ± 0.03 mmol
liter over the initial value corresponded in magnitude to the 1C
decrease of 0.21 ± 0.03 mmol liter' . The minimum NO, + NO (64 ± 1
o ^
yiriol liter below the initial level) was accompanied by the end of a
rapid decline in TP and IP from 3.8 to 0.5 ± 0.1 and 0.4 ± 0.1 ymol
liter , respectively. By the end of the first period, NH. had
risen to a peak of 23 ± 1 ymol liter from previous values of less
than 2 umol liter" . Systems II, III, and IV qualitatively were
similar in behavior, although IV exhibited no significant rise in
either NH or NO + NO- after the initial decline in NO + NO-.
A well-defined sequence of plankton pulses was observed during
the first interval. In all tanks, the phytoplankton bloom attaining
a maximum on Days 28 to 35 was preceded 1 or 2 weeks by a protozoa
peak, was more or less coincident with a rotifer peak, and was followed
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by a crustacean maximum within several weeks (Figs. 2, 4, 5, 6).
For systems II, III, and IV, the size of the phytoplankton, protozoa,
and rotifer maxima were in the same rank order as the initial N and P
concentrations. System I, which was treated in the same manner as III
until the addition of Gambusia on Day 34, exhibited unexpectedly large
deviations from III in levels of phytoplankton, protozoa, and rotifers
before Day 34.
The initial bloom was dominated by diatoms and green algae. In I,
Cyclotella meneghin-iana and Oooystis sp. accounted for 55% of the
total volume on Day 28, an unidentified LRGT and Oocyst-is for 84% on
Day 35, and Synedva ulna and the LRGT for 76% on Day 42. The first
protozoa peak was dominated by Pseudomierothorax sp. and the initial
rotifer peak consisted almost solely of D-Leranophorus sp. The crustacean
community up to Day 56 was formed primarily of the cladocerans Alona guttata
and Daphnia pulex, although levels of the only copepod present,
Cyclops vernalis, also had begun to rise (Fig. 3). As in I, the
local maximum of crustacean volume in II, III, and IV that occurred
during the first interval was dominated by Alona.
Only qualitative observations on periphytic growth and the associated
fauna were recorded. However, certain phenomena observed on the container
L
sides were of a dramatic nature, occurring within a well-defined time
interval, and offer some insights into the patterns developing in
the "pelagic" zone. A light periphytic covering in system I that
developed during the first 42 days evolved into a heavy growth by
/'•'•,-'*-
Day 49, the same day the pH levels began to rise a second time (Fig. la)
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The growth consisted primarily of Oso-lViatovla and Cladophora sp.,
the latter being covered by an epiphytic diatom community. Similar
growths appeared in II, III, and IV, although the taxonomic composition
was highly variable; the qualitative denseness of the periphytic
covering was in the same rank order as initial N and P concentrations.
Tanytarsus larvae were observed on the container sides of all systems
by Day 56. The midge larvae fed on periphytic algae, as could be deduced
from the presence of circular patches free of attached algal growth
surrounding individual larvae.
(ii) Period of low phytoplarikton biomass. The second interval in
system I was characterized by total phytoplankton volumes of less
than 0.12 mm liter" . The NH. peak on Day 56 was followed 1 week
later by a NO- + NO- maximum of 73 ± 4 umol liter" , close to the
O ^
intial value of 77 umol liter" (Fig 1). The subsequent decline in
NH. and NO, + N02 was accompanied by an increase in pH to a smaller
secondary maximum and a decrease of 1C to a smaller minimum. No
corresponding change in OC was observed. Secondary peaks in NH. and
N0_ + N02 occurred on Day 119 and 133, respectively. Systematic
problems with the carbon analyzer after Day 105 prevented collection
of accurate 1C and OC data for the remainder of the experiment. Both
IP and TP exhibited only erratic fluctuations at low levels after their
initial decline during the first interval. The chemical behavior of
systems II and III resembled that of I, except that no secondary
maximum for NH4 or NO- + N02 occurred in III. System IV exhibited
no significant changes in pH, C, N, or P after the first interval.
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Protozoa volume remained low during the second interval (fig. 2b);
3 small peaks dominated by Stpombidittm sp. were observed. Rotifer
4
levels also remained depressed, the small community consisting of
Keratella cochlearis and species of Lecane, Philodina, Tpiahotria, and
*
Voponkowia, until a final rise in rotifers was detected following
Day 140. This secondary rise consisted primarily of Polyarthra sp.,
accompanied by Keratella quadrata and Anupaeopsi-s sp. Among the
crustaceans, (Fig. 3), the major peaks for each group were segregated
in time in a clear manner. The first peak of Alona guttata was followed
successively by Daphnia pulex, Cyclops vernal-is, Simoeephalus vetulus,
and the sole ostracod Cypvidopsis sp. Smaller peaks for Dftphnia occurred
before and after the maximum for Simooephalus. The disappearance
of cladocerans on Day 161 coincided with the end of the second interval.
Protozoa and rotifer volumes in II, III, and IV also remained at rela-
tively small values during this second interval, but only in II was
there a suggestion of a secondary rotifer increase (Figs. 4,5,6). As
in I, the crustacean community was dominated by the cladocerans
Alona, Dapfmia and Simoeephalus (except for IV, in which Simoeephalus
was not detected). Cyclops and Cyppidopsis also were present, although
never in significant amounts. The maximum levels attained for the
various crustacean species in II, III, and IV were in the same rank
order as the initial N and P concentrations; the crustacean peaks in
I exceeded those in III.
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On Day 84, when the pH in system I had ceased to increase and
NO + NO levels had stabilized (Fig. 1), the 5 Physa individuals,
which had grown to adult size, were observed for the first time to
be exerting an effect on the periphytic community: snail movement
on the container sides was preserved clearly by a complex network of
swaths free of algae. On Day 105, a large number of tiny snails
_2
(less than 4 mm longest dimension, approximately 1 ind cm of side
surface), which had hatched from egg capsules adhering to the container
sides, were present on the sides; on this same day, NH. levels in the
water column began a secondary increase (Fig. Ib). By Day 119, the
sides essentially were free of algae, aside from scattered patches of
Cladophora, and, by Day 133, the snail numbers had dwindled and the
Cladophora again were present in dense amounts. In system II,
population increases and the subsequent effects on the periphyton
community resembled the corresponding phenomena in I. Although
Physa egg capsules occurred on the sides of III as early as Day 105,
no young snails were observed until Day 140 and the snail population
always remained 1 to 2 orders of magnitude less than in I and II. The
Plaeostomas fed on the side growth and maintained it at levels lower
than in I or II, although the periphyton levels remained high enough
to confer a distinct green color to the tank sides. The introduction
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of Plaeostomas resulted in a shift from unicellular to filamentous
periphytic forms. In IV, the Physa individuals added initially never
produced egg capsules, but periphytic growth was removed completely
9 days after the introduction of Plaoostomas on Day 59 . Side growth
never reappeared, even when 3 of the Plaoostomas were transferred to
system III.
Two Tanytarsus maxima were evident in I, the first on Day 56
when water column concentrations of emerging larvae reached 2.0 ind
liter , the second on Day 133 when levels of 0.7 ind liter were
recorded. Only the first maximum occurred in the remaining 3 systems.
In all systems, ostracod abundance always was far greater on and near the
sides and bottom than in the water column; a similar comment applies
to adult Cyclops vernalis, but not to the nauplius or early copepodid
stages. Daphnia pulex also occurred near the sides, but appeared
to prefer the open water. Daphn-ia ephippia consistently were present
on the sides near the water surface from Day 56 on.
(iii) Second bloom. The disappearance of Daphnia pulex on Day 161
marked the beginning of a smaller secondary bloom in system I, a peak
3 1
of 1.3 mm liter appearing on Day 175 (Figs. 2a, 3b). The bloom was
due almost entirely to the cryptophyte Cpyptochpysis sp., which
accounted for more than 85% of the volume on Days 168 to 189. Only
a small pulse of protozoa was noted, mostly individuals of a Parameoium
sp., but the rotifer community (mostly Polyarthva sp.) continued the
rise that began during the previous period. The crustacean
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community was absent completely after Cypvidopsis finished its decline
on Day 168. As in I, large increases in phytoplankton and rotifer
levels, with only minor changes in protozoa concentrations, took
place after the beginning of the third period in II and III although
the maximum volumes attained even exceeded those of the first period
(Figs. 4,5]. System IV offers a. slightly more complicated picture
(Fig. 6). Two distinct blooms occurred during the third period,
the beginning of the first on Day 77 coinciding with the disappearance
of the crustacean zooplankton, the second on Day 147 slightly preceding
the disappearance of rotifers. The small quantities of zooplankton
sporadically appearing in II, III, and IV during this third period
all were either Cyclops or Cypvidopsi,s individuals.
No dramatic neriphyton changes took place after the second interval:
I and II retained their heavy growths, III its lighter growth, and IV
remained free of visible attachment. All fish remained active and healthy
in appearance throughout the experiment. For II, III, and IV, the
Gombus-io. lengths increased from 11.8 ± 1.1 to 25.1 ± 4.8 mm. There
were no significant growth differences among systems.
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DISCUSSION
The production cycle of system I (Fig, 2a), less so that of II
and III, bears a striking resemblance to the bimodal patterns observed
in temperate marine waters (Gushing, 1959) and in certain inland
water bodies, such as the Laurentian Great Lakes and productive
lakes of the English Lake District (Hutchinson, 1967). Even the
interval length between the 2 peaks, approximately 5 months in the
case of I, is similar to the separation between peaks found in these
natural systems. Despite the lack of temperature and irradiance
variations, the similarity between microcosm behavior and that of
certain naturally-occurring water bodies is marked, and the analogy
will be explored further in what follows. As mentioned previously,
the presence of fish in II, III, and IV leads to a quantitatively
unrealistic trophic structure, so that our attention will be focused
primarily on system I.
(i) Initial period. Conditions of Day 0 are similar to those at the
beginning of the spring bloom — a large reserve of inorganic
nutrients and low levels of plankton biomass. The resulting well-
defined sequence of plankton pulses is suited particularly well for
deducing the trophic relationships that characterize this initial
period. In large natural systems, where a given trophic process
may start at slightly different times at different locations, hori-
zontal mixing tends to obscure the trophic relationships that can
be deduced from sampling at a fixed point. It must be noted, of
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course, that this very mixing may influence the subsequent biological
development of natural systems in a manner that cannot be captured
with the use of isolated microcosms.
The initial increase in phytoplankton productivity is retarded
by protozoan grazing, as indicated by the larger standing crop of
protozoa during the first 2 or 3 weeks (Figs. 2,4,5,6). The subse-
quent decline in protozoa appears to be due to grazing by rotifers
and not to a decrease in food supply, which continues to rise for 1
or 2 weeks. On the other hand, the decline in rotifers clearly is
related to the decreasing phytoplankton levels, although it is not
obvious whether the decline in the rotifer community occurs solely as
a result of overgrazing by rotifers or is aided by competition with
the increasing crustacean community for a common food supply. The
fact that the phytoplankton continue their rapid increase between
Days 28 and 35, when the rotifers already have attained maximum
levels, does suggest that the rotifer downturn is accelerated by
competition with Alona. Similar conclusions may be deduced for the
remaining 3 microcosms.
Whether by rotifers or by crustaceans, grazing must be the
dominant factor behind the termination of the phytoplankton bloom;
inorganic N and P levels observed on Day 35 (15 ± 1 and 0.4 ±0.1 umol
liter" , respectively; Figs. Ib, Ic) clearly are sufficient to support
*
further growth (e.g., Carpenter and Guillard,1971; Perry, 1976),
while all other required nutrients were added in excess (Table 1).
The fact that the second major bloom in II, III, and IV, when crustaceans
are absent, exceeds the first lends additional weight tofthis viewpoint.
-------
-21-
Although the "spring" bloom may have been halted by grazing,
the size of the bloom must be determined, at least in part, by the
<
quantity of nutrients initially present. The peak phytoplankton
levels in II, III, and IV during the first period are 5.7, 0.76,
and 0.13 mm liter , respectively. The protozoa and rotifer peaks
in this interval also fall in the appropriate rank order, reflecting
the relative levels of their food supply.
The discrepancy: in magnitude between the phytoplankton peak of I
(2.4 mm liter ) and III is disconcerting, in view of the fact that
both systems were treated in an identical manner until the addition of
Gambusia on Day 34. The protozoa attained a maximum of 2.0 mm liter"
3 -1
in III, only 0.52 mm liter in I, suggesting that the lower phytoplank-
ton levels in III reflect a more intense grazing pressure from protozoa
during the first few weeks. In turn, the discrepancy in protozoa may
result from a different protozoan composition in the initial inoculum;
although the inoculum volume was large (3.5 liter), protozoa were
sparse and a significant random variation in the initial protozoa
levels may have resulted. The rotifer peaks in I and III (0.12 and
0.082 mm liter" , respectively) show less of a discrepancy than either
the phytoplankton or protozoa concentrations, probably a result of
the fact that a smaller amount of phytoplankton is compensated for by
a larger amount of protozoa in the food supply of the rotifers in III.
-------
-22-
The correspondence between the magnitudes of the 1C decrease
and the OC increase (Fig. la) must be viewed as somewhat coincidental.
Although the 1C decrease should equal the OC increase in a closed
homogeneous system, aeration and sinking of particulate organic
matter will lower the absolute magnitude of each peak, respectively,
and not necessarily by the same amount. Further evidence for signi-
ficant sinking losses (whether before or after being processed by
grazers) is offered by the TP (Fig. Ic) and NO, + NO- concentrations
(Fig. Ib).
In the case of TP, a drop of 4.3 umol liter in the water
column takes place by Day 35, although some of these phosphorus
losses may be due to adsorption by the sand and tank sides and periphyton
uptake, not necessarily to the sinking out of phytoplankton. The rate
constant for TP loss estimated for the first 35 days in system I is
-1 2
22 a (r = 0.81); using Vollenweider's (1975) empirical approxima-
tion of 10/z a for the rate constant, where z" = mean lake' depth (m),
_ j
a value of 18 a is obtained for the microcosm. The agreement between
the 2 numbers provides some evidence that the loss processes for TP
in the microcosms resembles those of natural systems.
As no total nitrogen determinations were carried out, the exact
fate of the NO, + NO, losses is more difficult to pinpoint. If all
of the NO, + N09 decrease up to Day 35 (64 umol liter ) represented
•J &
a transformation to organic form remaining in the water column, the
OC increase of 0.18 mmol liter would imply a C:N molar ratio of 3
in this organic matter. Although C:N ratios of this magnitude have
-------
-23-
been observed for marine deep water ditritus (e.g., Duursma, 1960"),
the detritus of lakes more generally is characterized by C:N ratios
exceeding 10 (Birge and Juday, 1934). In fact, assuming a value of
0.1 for the ratio of phytoplankton carbon to wet weight and a
value of 3 to 6 for the molar C:N ratio in phytoplankton (e.g.,
Antia et al., 1963), only 3 to 7 ymol liter' N is required to
account for the nitrogen contained in the phytoplankton peak, a small
proportion of the actual decrease in NO + NCL. Accordingly, we can
conclude that a significant proportion of the NO, + N0_ decrease
results from sinking of biological material (including that which may
have been grazed upon) onto the sediments or from uptake by periphyton.
It is interesting to note that the molar N:P ratio for the lost
nutrients by Day 35 is 15, close to the mean Value traditionally
applied to phytoplankton (e.g., Antia &t al.3 1963), and it is tempting
to conclude that the decrease in NO + N0? and TP is due primarily to
uptake by phytoplankton and periphyton, and subsequent sinking of
living and grazed phytoplankton. However, the lack of a correspondence
between N and P changes in the intervals preceding Day 35 precludes
such a simple explanation. Whatever the exact mechanism behind the
decrease in NO, + N0_, bacterial and zooplankton processing of nitro-
O «
genous organic matter leads to a rapid building-up of NH. as the OC
decreases after Day 35; nitrification of the NH. results in the
reappearance of almost all the NO, + N02 shortly after the start of
the second period.
-------
-24-
The large loss in N and P and the OC increase of 0.18 mmol liter" ,
when compared to the phytoplankton standing crop of approximately 20 ymol
liter on Day 35 (using the C:wet weight ratio of 0.1), all suggest
that the maximum standing crop is only a small fraction of the actual
phytoplankton production during the first half of the bloom. Assuming
extracellular production of photosynthate is negligible compared to
particulate phytoplankton production, the increase in OC represents
a minimum estimate of the quantity of phytoplankton produced up to Day 35
(uncorrected for respiration and sinking losses). A minimum estimate
for zooplankton grazing thus is obtained by subtracting the standing
stock of phytoplankton, and it then appears that at least 90% of the
particulate primary production was processed by zooplankton.
A crude estimate of the loss rate in each interval can be obtained
by assuming that phytoplankton generation rates and loss rates are
constant within any given interval, and by choosing a reasonable value
for generation rates. Because
*p = C°r - Cl)Xp ' (1)
where x = phytoplankton biomass (mm liter ), c = generation rate
(d ) , and GI = loss rate (d ) , c can be determined from
i rwi
i = cr - ct, - 1 ) ln xVr
^ 1 o L P ° -I
where t and tj (d) mark the endpoints of the interval. A reasonable
estimate of the generation rate cr can be determined from the maximum
rate of increase of phytoplankton standing crop, usually at times when
the zooplankton are at lowest levels. This maximum rate corresponds to a
-------
-25-
doubling time of 1.0 to 2.9 d in the various systems; accordingly,
we chose c^ = 0.7 d~ , equivalent to a doubling time of 1d . The
resulting values of cl for the first period in system I range from
0.1 to 1 d . The correlation between c and x , where x = zooplankton
volume (mm liter" ), is 0.75 (d.f. = 8, p < 0.05), suggesting that
the losses are due primarily to zooplankton grazing, although the
grazed material subsequently may sink out. Note that the correlation
is independent of the value chosen for c , providing that c remains
constant for all intervals; the fact that irradiance and temperature
is constant may be construed as partial evidence for this assumption.
(ii) Period of low phytoplankton levels. After the termination of
the spring bloom by zooplankton grazing, the phytoplankton remain at
low levels due to continued grazing'pressure. The continuing importance
of the crustaceans is illustrated in Fig. 7, which depicts the results
of removing part of the zooplankton community with a 64-um mesh size
plankton bucket from a 4-liter sample of system I. After a delay of
several days, a rapid increase in fluorescence takes place with respect
to a control beaker in which the zooplankton are left undisturbed.
When the zooplankton are removed similarly from the control beaker
on the ninth day and transferred to the beaker'containing no zooplank-
ton, fluorescence rises in the control beaker and decreases in the
other. Apparently, the generation rate of phytoplankton and the
grazing pressure of zooplankton remain more or less in balance during
the second period.
-------
-26-
Further evidence that the production losses after the bloom
reasonably may be attributed to zooplankton grazing is obtained by
-1-1
consideration of the quantity c,/x' (liter ind d ), where x^ =
_L Z Z
zooplankton number (ind liter~ ). This quantity is the filtering
rate that would be required to explain production losses in terms of
crustacean grazing pressure alone. The mean value for Days 56 to 154,
that portion of the second period when crustaceans dominate the
zooplankton (Fig. 2b), is 0.01 ± 0.01 liter ind~ d~ . Lowest values
of 0.002 liter ind d~ are obtained when Cyclops vernalis is at its
maximum; intermediate values of 0.007 ± 0.004 liter ind d when
Alona guttata and Daphnia pulex are dominant; and highest values of
0.02 ± 0.02 liter ind d when Simooephalus vetulus is the dominant
species. For the low food concentrations during this period (<10
cell liter ), these values correspond to the results of various
zooplankton feeding studies (e.g., Wetzel, 1975, Table 16-8, for
copepods; Infante, 1973, for D. pulex; Sushtchenia, 1958, for 5.
vetulus}.
Because the magnitude of the peaks for all 5 zooplankton species
in systems II, III, and IV fall in the same rank order as initial N
and P levels, the peak sizes clearly are dependent upon food supply
in the microcosms. The limiting nature of the food supply is suggested
also by the presence of cladoceran ephippia in all 4 microcosms after
the initial bloom. The larger zooplankton peak sizes in I compared
with III imply that fish planktivory also is a factor in determining
-------
-27-
the maximum development of zooplankton species in these systems.
Simooephalus, a large zooplankter and presumably one of the most
f
susceptible to Gambusia feeding, was absent completely from IV, the
least productive microcosm. In addition, a second Tanytapsus maximum
occurred only in I, where fish were absent.
The clear temporal separation between the zooplankton peaks in
system I (Fig. 3) is probably not just a reflection of life cycle
timing. In particular, the manner in which the Daphnia peaks are ,
interspersed between those for the remaining species suggests signi- /
ficarit competition between members of the zooplankton community.
However, it is difficult to deduce the exact causal mechanisms that
lead to the temporal segregation of zooplankton peaks in Fig. 3.
Damped oscillatory populations often are observed in unispecific
Daphnia cultures (Pratt, 1943; Slobodkin, 1954), and frequently the
period of oscillation is approximately 40 d. One way to view the D.
pulex data is to hypothesize a series of 3 damped oscillations
generated as a result of interactions between Daphnia and its food
supply, with peaks that would have occurred around Days 50, 90, and
130 in the absence of other crustaceans. The presence of Alona
suppresses the first peak, and that of Simoaephalus the second; Cyclops
and Cyppidopsis then take advantage of those times when Alona and
Daphnia are suppressed. The exact cause-effect relationships undoubtedly
are more complicated. For example, the raptorial food habits of Cyclops
may initiate or accelerate the depression of Daphnia after Day 63. In
-------
-28-
any case, the value of this microcosm design for competition studies is
clear; many of the details of competition phenomena probably would
reveal themselves by using an inoculum for the microcosms in which the
crustacean composition was manipulated artifically.
The rise in pH and the decrease of 1C and NO, + NCL during the
second period (Fig. 1) appear to result from periphyton photosynthesis,
as evidenced by an increase in periphyton density from Days 49 to
84. Periphyton scraped with a razor blade off small areas of the
_2
tank sides revealed densities as high as 60 mg m Chi a_ by Day 84,
equivalent to 38 ug liter Chi a_ if dispersed throughout the water.
Chi a_ measurements occasionally collected in the water column during
the second period never exceeded 4 ug liter" , so that the phytoplankton
biomass was far less than the periphyton biomass by Day 84. It is of
some interest to estimate the maximum density that can be attained
by periphyton packed on the tank sides. Assuming that the thickness
of the periphyton layer cannot exceed the distance for which incident
irradiance would be reduced to compensation levels of irradiance, the
following relationship must be satisfied:
Ie - V , (3)
_2
where I = compensation irradiance (W m PAR) ; I = incident irra-
d- O
_2
diance (W m PAR) ; k = specific extinction coefficient for Chi a
C """"
2-1 -3
(m mg ) ; C = Chi a. concentration on the sides (mg m ) ; and d = thick-
ness of periphyton layer (m) . The mean of I incident on the sides is
-2 2 -1
approximately 7 W m PAR. Using a value of k = 0.02 m mg and
I*
_2
I = 1 W m PAR (Platt and Jassby, 1976), an areal Chi a. concentration
-------
-29-
_2
(=Cd) of 100 mg m is obtained, close to the value observed. The
periphyton thus appear to have built up to their maximum light-limited
density. The same argument does not apply to forms capable of developing
filaments, which avoid light limitation by extending into the water
column where individual filaments can avoid significant shading effects.
The consequences for studies of nutrient cycles and trophic
dynamics in microcosms are overwhelming. Once the periphyton are
established, temporal changes in nutrient concentrations (such as
the second NCL decrease illustrated in Fig. Ib), no longer can be
attributed to the behavior of planktonic organisms alone. In addi-
tion, the explanation of population fluctuations in organisms that
partition their time between the "littoral" and "pelagic" zones (such
as observed for Cypridosis, adult Cyolops, and Daphnia) requires
more complicated considerations of food supply. Although the littoral
zone fulfils a similar complicating role in natural systems, the produc-
tivity of non-phytoplanktonic vegetation exceeds phytoplankton produc-
tivity by an order of magnitude or more only in small, shallow lakes
(e.g., Wetzel, 1975, Table 15-15). The periphyton problem thus
limits severely the use of microcosms as more general analogs of inland
water bodies or for investigation of purely planktonic relationships.
The introduction of Physa and Plaaostomas into the microcosms
represents an attempt to decrease the periphyton abundance by herbivory.
In the case of Physa, the initial individuals grew continuously but
did not make their presence felt until Day 84, when visible effects of
their grazing coincided with the second decrease in pH (Fig. la). The
resulting decrease in periphyton production apparently permitted aeration
-------
-30-
to return the pH to its initial level. The appearance of large
populations of young snails on Day 105 coincided with the second NH.
increase, presumably because of snail excretion of grazed periphyton
nitrogen. The snails subsequently died from overgrazing after Day 119
and the periphyton quickly reappeared. The increase in pH accompanying
the reappearance of periphyton is suppressed possibly because of CO™
production from the decomposition of starved Physa. The lower concen-
trations of young Physa in III, compared with I and II, can be
attributed to Plaeostomas feeding on snail egg capsules attached to
the sides, and no secondary NH peak from snail excretion was observed.
In I, productivity was not sufficient to support Physa reproduction.
Because of the time lag between periphyton growth and snail reproduc-
tion, and because of overgrazing, the snails generally proved incapable
of maintaining sides consistently free of periphyton. Accordingly,
we do not feel that introduction of snails offers even a partial
solution of the periphyton problem. In fact, their short-term effects
on the pelagic N concentrations make interpretation of nutrient
cycles even more complicated.
The presence of Plaoostomas in IV proved to be completely
effective in removing periphyton from the sides and preventing recoloni-
zation. However, system IV had the lowest level of nutrients and the
least dense periphyton growth. In III, where initial N and P levels
were 4 times higher, the 3 Plaoostomas never succeeded in eliminating
periphyton growth, even with the aid of the snails present. Although
-------
-31-
growth was always less dense than in I or II except when the snails
temporarily had eliminated the periphyton in these latter 2 systems,
the color suggested that Chi a_ densities were not an order-of-magnitude
lower than in I or II and probably still exceeded phytoplankton biomass.
The Placostomas appeared to be particularly ineffective against the
long Cladophora strands that developed in III during the third period.
Accordingly, neither the snails nor catfish turned out to be an
adequate method for dealing with dense periphyton communities. This
difficulty still remains unsolved and is the most serious hindrance
to long-term studies in microcosms.
1
(iii) Second bloom. In all 4 microcosms, the beginning of the second
bloom coincides precisely with the disappearance of crustaceans from
the water column. The release from grazing pressure permits the phyto-
plankton community to take full advantage of the nutrients available
and increase to levels approaching or exceeding those of the initial
bloom. The timing of the crustacean dieoff reflects the effects of
both food supply and predation. In systems II, III, and IV, the
dieo'ff is postponed more when the initial N and P levels are increased;
in I, the lack of a planktivorous fish allows the intermediate
period between blooms to continue for the longest time of all 4 systems,
The bloom starting on Day 77 in IV appears to be prematurely destroyed
by the rotifer community, and it is only when the rotifers disappear
between Days 147 and 154 that a major phytoplankton rise takes place
(Fig. 6).
-------
-32-
The termination of the "spring" bloom by grazing, and the onset
of the "fall" bloom after starvation of the zooplankton, both suggest
that the production cycles in these microcosms reflect a predator-
prey oscillation (although not necessarily of the Lotka-Volterra
type). The oscillations are forced by the initial conditions favoring
high phytoplankton growth rates with little interference from crustacean
grazers. At a later time, the crustaceans in the inoculum spawn and
the grazing capacity of the zooplankton community increases. Subsequent
grazing mortality among the phytoplankton results in primary produc-
tion levels too low to support the zooplankton, whose levels gradually
decrease through starvation. Once the cladocerans, in particular, have
disappeared, conditions resemble those at the start and a new sequence
of production begins.
The course of events is similar, in many ways, to that observed in
certain temperate marine waters where production cycles essentially
are a reflection of predator-prey relationships, and nutrient levels
respond to, rather than cause, the cycle (see discussion by Gushing,
1975). As in the microcosms, the quantity of phytoplankton produced
is many times the maximum standing crop, and about 1 month is required
before crustacean grazing becomes effective enough to terminate algal
increases in the spring. Although tropical and polar production cycles
also can be viewed as resulting from predator-prey interactions (Gushing,
1959), latitudinal differences in the annual temperature pattern result
in differences in the annual production pattern. In arctic waters, the
-------
-33-
\ , -
colder temperatures result in a longer delay time between the onset of
, j
spring phytoplankton increases and the onset of effective grazing, and
only a single phytoplankton peak of high amplitude occurs before produc-
tion is halted by winter conditions. In tropical waters, the short
delay time results in low amplitude phytoplankton changes without the
discontinuity characteristic of higher latitudes. In general, the
longer the delay time, the greater the phytoplankton peak (up to the
onset of nutrient limitation) and the greater the proportion of phyto-
plankton biomass lost to sinking and respiration without being grazed.
Thus, the delay time between phytoplankton increases and the beginning
of effective grazing is an extremely important factor governing the
amplitude of production cycles and the ratio of secondary to primary
production. The use of microcosms offers a unique opportunity for
exploring the precise conditions under which different production
patterns result; irradiance, temperature, initial nutrient levels,
and initial (i.e., "overwintering") levels of crustacean females or
resting eggs all can be manipulated to analyze effects on delay times
and subsequent levels of primary and secondary production.
The analogy between the microcosms and inland water bodies is less
general. Evidence for predator-prey oscillations dominating production
cycles is clear only in the simplified associations of extreme habitats,
such as highly saline or alkaline lakes (Anderson et at., 1955; Anderson,
1958). In most well-studied cases, nutrient depletion is a more signifi-
cant factor than grazing in causing the spring decline of phytoplankton;
-------
-34-
for example, silicate depletion plays this role in parts of the English
Lake District (Lund et al-, 1963), and phosphate depletion in certain
Laurentian Great Lakes (Schelske et al. , 1972). In addition, upwelling
of subthermocline nutrients or algae (Fee, 1976) during the fall overturn,
rather than zooplankton disappearance, appears to be the main cause of
the fall bloom.
The possibility of a more realistic simulation of inland waters
with microcosms cannot be ruled out. As pointed out previously, protozoa
grazing is a major factor in suppressing initial phytoplankton increases
in the microcosms. In natural systems, protozoa usually constitute
only a minor portion of the zooplankton and achieve maximum levels
after the spring bloom. (e.g., Schonborn, 1962; Sorokin and Paveljeva,
1972). Initial microcosm conditions apparently allowed the inoculated
protozoa to rapidly outgrow predators and exert an undue effect on the
spring bloom. It is possible that, if protozoa could be excluded from
the inoculum, phytoplankton increases would proceed at a rate sufficient
to exhaust nutrients before crustacean grazing became effective enough
to terminate the bloom. In addition, certain aspects of the fall
overturn perhaps could be simulated by vigorous mixing of the benthic
substrate into the water column for a short time, releasing the accumu-
lation of nutrients from decomposed organic matter and returning viable
algae that have settled out. Neither modification was attempted in the
present set of experiments, but both bear serious consideration for
/
any future work of this nature.
-------
-35-
CONCLUSIONS
A number of important drawbacks deserving further discussion have
been made clear to us in the course of this work. Some of these draw-
backs are inherent difficulties in size scaling. For example, the
shallow depth results in higher sinking losses, smaller vertical
irradiance changes, smaller migration distances for zooplankton, etc.,
as compared to natural systems. Little can be done about such scaling
problems and their possible interference must be interpreted within
the context of each individual experimental aim.
Other major problems, however, do have potential solutions:
(i) It is not possible to stock planktivorous fish in realistic concentrations,
and their presence results in premature disappearance of the cladocerans.
The obvious answer to this difficulty is the complete exclusion of fish
from the microcosms. Their exclusion also permits the use of smaller
systems, although proper zooplankton sampling sets a lower limit on
2
microcosm volumes somewhere between 10 and 10 liters.
(ii) Poor replication will result unless all inoculated organisms that
can proliferate in the microcosms are present in sufficiently high
numbers in the inoculum; also, protozoa appear to play an unnaturally
important role. Both of these problems suggest the use of gnotobiotic
systems, although experiments then are subject to the lack of realism
inherent in the gnotobiotic approach (see section on microcosm design).
Perhaps a compromise is suitable, in which the inoculum is prepared
from several unialgal (although not axenic) cultures, along with, say,
1 rotifer species and 1 crustacean species (with large numbers of indivi-
duals in each case). Greater ability to exclude contamination by other
-------
-36-
zooplankton will be obtained in the smaller systems without fish.
(iii) Periphyton growth is too high and can dominate events in the
microcosms. The periphyton problem can be dealt with in part by limiting
experiments to less than about 1.5 months, the time when the consequences
of side growth first become apparent. This limited time still permits
a detailed examination of the circumstances surrounding the spring bloom.
An alternative method is mechanical removal of periphytic attachment,
although daily manual scraping of the tank sides is too tedious for
routine work and an automated method is desirable. In any case, the
solution of the periphyton problem deserves the most serious considera-
•
tion in any further work of this nature. The dominance of biomass by
side growth precludes the simplified study of nutrient cycles and
energy flow that microcosms potentially can offer.
Despite these drawbacks, this study has demonstrated certain resem-
blances between microcosms and natural water bodies where production
cycles are determined by predator-prey relationships* Also, modifica-
tions have been suggested to increase the generality with which these
microcosms can be made to simulate natural systems. Because of the
resemblance to certain natural production cycles, and because of the
clear sequence of plankton pulses in the absense of spatial heterogeneity,
we feel that the microcosm method described here offers a powerful
tool for assessing environmental effects (whether of human origin or
not) on production patterns, trophic relationships, and competition
phenomena.
-------
-37-
REFERENCES
Anderson, G.C. 1958, Seasonal characteristics of two saline lakes
in Washington. Limnol. Oceanogr. 3_:51-68.
Anderson, G.C., G.W. Comita, and V. Engstrora - Heg. 1955. A note on
the phytoplankton-zooplankton relationship in two lakes in Washing-
ton. Ecology 36_:757-759.
Antia, N.J., C.D. McAllister, T.R. Parsons, K. Stephens, and J.D.H.
Strickland. 1963. Further measurements of primary production using
a large-volume plastic sphere. Limnol. Oceanogr. 8^:166-183.
APHA. 1971. Standard methods for the examination of water and waste-
water. American Public Health Assciation.
Beyers, R.J. 1963. A characteristic diurnal metabolic pattern in
balanced microcosms. Publ. Inst. Mar. Sci., Texas 9^:19-27.
Beyers, R.J. 1965. The pattern of photosynthesis and respiration
in laboratory microecosystems. Mem. 1st. Ital. Idrobiol. 18
suppl.:61-74.
Birge, E.A., and C. Juday. 1934. Particulate and dissolved organic
matter in inland lakes. Ecol. Monogr. 4_:440-474.
Carpenter, E.J., and R.R.L. Guillard. 1971. Intraspecific differences
in nitrate half-saturation constants for three species of marine
phytoplankton. Ecology 52:185-185.
Cooke, G.D. 1967. The pattern of autotrophic succession in labora-
tory microcosms. Bioscience 17:717-721.
Cooke, G.D. 1971. Aquatic laboratory microsystems and communities,
p.48-85. In J. Cairns (ed.), The structure and function of freshwater
microbial~cbmmunities. Virginia Polytechnic Institute and State
University.
Gushing, D.H. 1959. On the nature of production in the sea. Fish.
Invest. Lond. (Ser. 2) 22; 40 pp.
Gushing, D.H. 1975. Marine ecology and fisheries. Cambridge Univ.
Dillon, P.J., and F.H. Rigler. 1974. The phosphorus-chlorophyll
relationship in lakes. Limnol. Oceanogr. 19:767-773.
-------
-38-
Duursma, E.K. 1960. Dissolved organic carbon, nitrogen, and phosphorus
in the sea. J,B. Wolters, Groningen,
Fee, E.J. 1976. The vertical and seasonal distribution of chlorophyll
in lakes of the Experimental Lakes Area, northwestern Ontario:
implications for primary production estimates. Limnol. Oceanogr.
2^:767-783.
Golterman, H.L. 1969. Methods for chemical analysis of fresh waters.
Blackwell Scientific.
Guillard, R.R.L. 1973. Division rates, p. 289-311. In J;R. Stein (ed.)
Phycological Methods. Cambridge Univ.
Hutchinson, G.E. 1967. A treatise on limnology, vol. 2. Wiley.
Infante, A. 1973. Untersuchungen uber die ausnutzbarkeit verschiedener
Algen durch das Zooplankton. Arch. Hydrobiol. Suppl. 42:540-405.
Klekowski, R.Z., and E.A. Shushkina. 1966. Ernahrung, Atmung, Wachstrum
und Energie-Umformung in Macrooyclops al'b'idus Jurine. Verh. Int.
Verein. Limnol. 16:399-418.
Lund, J.W.G., F.J.H. Mackereth, and C.H. Mortimer. 1963. Changes in
depth and time of certain chemical and physical conditions and of
the standing crop of Aster-ionella fovmosa Hass. in the North Basin
of Windermere in 1947. Phil. Trans. Roy, Soc. London (Ser. B)
246:255-290.
Maguire, B. 1971. Community structure of protozoans and algae with
particular emphasis on recently colonized bodies of water, p.121-149-
In J. Cairns (ed.), The structure and function of freshwater
microbial communities. Virginia Polytechnic Institute and State
University. t
Mortimer, C.H. 1941-42. The exchange of dissolved substances between
mud and water in lakes. J. Ecol. 2£: 280-329; 30:147-201.
Neill, W.E. 1975. Experimental studies of microcrustacean competition,
community composition and efficiency of resource utilization.
Ecology 56_:809-826.
Nichols, H.W. 1973. Growth media-freshwater, p.7-24. In_ J.R. Stein (ed.).
Phycological methods. Cambridge University.
Pechen, G.A. 1965. Produktsiya vetvistousykh rakoobraznykh ozernogo
zooplankton. Gidrobiol. Zh. \\19-26
-------
^39-
Perry, M.J. 1976. Phosphate utilization by an oceanic diatom in phos-
phorus-limited chemostat culture and in the oligotrophic waters of
the central North Pacific. Limnol. Oceanogr. 21:88-107.
Pratt, D.M. 1943. Analysis of population development in DapTmia at
different temperatures. Biol. Bull. Mar. Biol. Lab., Woods Hole,
85:116-140.
Salt, G.W. 1971. The role of laboratory experimentation in ecological
research, p.87-100. In J. Cairns (ed.), The structure and function
of freshwater microbial communities. Virginia Polytechnic Institute
and State University.
Schelske, C.L., and E.F. Stoermer. 1972. Phosphorus, silica, and
eutrophication of Lake Michigan, p.157-176. In_ G.E. Lidens (ed.),
Nutrients and eutrophication. Special Symposium, Amer. Soc.
Limnol. Oceanogr.
Schonborn, W. 1962. Uber Planktismus und Zyklomorphose bei Difflugia
lirmetiea (Levander) Penard. Limnologies. 1^:21-34
Slobodkin, L.B. 1954. Population dynamics in Daphnia dbtusa Kurz.
Ecol. Monogr. 24:69-88.
Solorzano, L. 1969. Determination of ammonia in natural waters by the
phenol-hypochlorite method, Limnol. Oceanogr. 14:799-801.
Sorokin, Ju, I., and E.B. Paveljeva. 1972. On the quantitative
characteristics of the pelagic ecosystem of Dalnee Lake (Kamchatka).
Hydrobiologia 40:519-552.
Strickland, J.D.H., and T.R. Parsons. 1968. A practical handbook of
seawater analysis. Fish. Res. Bd. Canada, Bulletin 167.
Sushtchenia, L. 1958. Kolichestvennye dannye o_fil'tratsionnom
pitaniiplanktonnykh rachkov. Nauch. Dokl. vyssh. shk,, Biol.
Nauki 1:241-260.
Taub, F.B. 1971. A continuous gnotobiotic (species defined) ecosystem,
p. 101-120. In J. Cairns (ed.), The structure and function of
freshwater infcrobial communities. Virginia Polytechnic Institute
and State University.
Tuab, F.B. 1974. Closed ecological systems. Ann. Rev. Ecol. System. 5_;139-160.
Vollenwieder, R.A. 1975. Input-output models, with special reference to
the phosphorus loading concept in limnology. Schweiz. Z. Hydrol. 37:53-84.
Wetzel, R.G. 1975. Limnology. Saunders.
Whittaker, R.H. 1961. Experiments with radio-phosphorus tracer in
aquarium microcosms. Ecol. Monogr. 31:157-188.
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TABLE HEADINGS
Table 1. Composition of the inorganic enrichment medium for the 4 700-
liter microcosms. The stock solution is diluted 1:700 in the
microcosms.
Table 2. Manipulation of the biological structure in the 4 700-liter
microcosms.
Table 3. Methods of analysis for the parameters monitored.
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Table I.
Nutrient
"'
CaCl.«2H_0
2 2
MgS04-7H20
NaHC03
Na0HPO -7H.01
242
NaN031
Na0SiO'9H00
232
KC1
Na2EDTA
FeSO.-7H00
4 2
CuSO.-SH-O
4 2
ZnSO.-7H00
4 2
CoCl0-6H00
2 2
MnCl -4H.O
2 2
Na0MoO.'2H00
242
Stock solution
(g liter'1)
37
37
37
--
30
10
1.5
1.0
0.010
0.020
0.010
0.20
0.010
Microcosm concentrations
(Vimol liter- 1)
3.6 x 102
2.1 x io2
6.3 x io2
—
1.4 x IO2
2.0 x IO2
5.7
5.3
0.057
0.10
0.060
1.4
0.060
see text
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Date Day Manipulation
05-06-76 - 0 Inoculation of each of I, II, III, IV'with 3.5-liter sample of lakewater
05-21-76 15 Addition to each of I, II, III, IV of 5 Pristina and 5 Tanytarsus larvae
06-09-76 34 Addition to each of II, III, IV of 5 Gambusia of length 1.2 cm
06-24-76 49 Addition to each of I, II, III, IV of 5 Physa of length 2.5-5.Omm
07-04-76 59 Addition to IV of 4 Placostomas of length 1.0 cm
08-15-76 101 Transfer of 3 Placostomas from IV to III.
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Table III.
Parameter
Method
Special equipment
Reference
°2 ..
pH
1C
oc
NH4
N03 + N02
IP
TP
Chi a
phytoplankton
zooplankton
polarography
electrometry
infrared absorbance
combustion to 1C
blue indophenol reaction
reduction, diazotization
ascorbic acid reduction
persulfate digestion to IP
fluorometry
Sedgewick-Rafter cell
r64-urn tow
02 meter (YSI 57)
pH meter (Orion 601)
IR analyzer (Beckman 865)
TOC analyzer (Beckman 915A)
spectrophotometer (Zeiss PM2 DL)
fluorometer (Turner 111)
phase microscope (Reichert Zetopan)
dissecting microscope (AO 570)
Solorzano, 1969
Golterman, 1969
APHA, 1971
APHA, 1971
Strickland and Parsons, 1968
Guillard, 1973
i
-o
u>
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-44-
FICURE LEGENDS
Figure 1. Chemical measurements from System I. Vertical lines indicate
the standard error for duplicate measurements. (a) pH, inor-
ganic carbon (1C), and organic carbon (OC); (b) NH. and
NOg + NC>2; (c) inorganic phosphorus (IP) and total phosphorus
(TP).
Figure 2. (a) Total volume of phytoplankton in system I; (b) total
volume of protozoa, rotifera, and Crustacea in system I.
Figure 3. (a) Resolution of the Crustacea in Fig. 2b into cladocera,
copepoda, and ostracoda; (b) resolution of the cladocera in
Fig. 3a into component species.
Figure 4. (a) total volume of phytoplankton in system II; (b) total
volume of protozoa, rotifera, and Crustacea in system II.
Figure 5. (a) Total volume of phytoplankton in system III; (b) total
volume of protozoa, rotifera, and Crustacea in system III.
Figure 6. (a) Total volume of phytoplankton in system IV; (b) total
volume of protozoa, rotifera, and Crustacea in system IV.
Figure 7. Importance of zooplankton grazing in 4-liter samples collected
from system I on Day 63. Removal of zooplankton with 64 um
net (solid line) results in increased fluorescence with respect
to a control beaker (dashed line). When zooplankton are
transferred from the control beaker to the beaker that is
zooplankton-free, fluorescence trends reverse.
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Figure I
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Figure II
Phytoplankton
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Figure IV
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Figure VI
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