CHARACTERIZING CHESAPEAKE BAY -
A DOCUMENTATION OF ENVIRONMENTAL CHANGE
Gail B. Mackiernan
Willa Nehlsenl
David A. Flemer^
Virginia K. Tippie
U.S. Environmental Protection Agency
-Chesapeake Bay Program
839 Bestgate Road
Annapolis, MD 21401
U.S.A.
^present address:
CREST
P.O. Box 175
Astoria, OR 97103
USA
^present address:
ORD, DEFER (RD682)
U.S. EPA
401 M. St. SW
Washington, DC 20400
USA
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903R83103
CHARACTERIZING CHESAPEAKE BAY -
A DOCUMENTATION--OF ENVIRONMENTAL CHANGE
Gail B. Mackiernan
Willa Nehlsen1
David A. Flemer^
Virginia K. Tippie
U.S. Environmental Protection Agency
—Chesapeake Bay Program
839 Bestgate Road
Annapolis, MD 21401
U.S.A.
present address:
CREST
P.O. Box 175
Astoria, OR 97103
USA
present address:
ORD, DEPER (RD682)
U.S. EPA
401 M. St. SW
Washington, DC 20400
USA
CB 00180 I
-------
ABSTRACT
This paper is a summary of the U.S. EPA's report, Chesapeake Bay -
A Profile of Environmental Change (Flemer, et al. 1983). This
document provides an evaluation of the present environmental health
of the estuary and how it has changed in recent years. Historical
information from past Bay research and monitoring, as well as results
of EPA-supported studies, was collected in a comprehensive database.
These data were used to assess trends in a number of physical and
biological parameters. Potential relationships between observed
trends in environmental quality and resources were identified and
evaluated.
Major findings of the characterization effort include:
e Increase in concentrations of nitrogen and phosphorus,
particularly in the upper Bay and upper reaches of tributaries.
e Increase in volume of low dissolved oxygen water in summer.
• Elevated levels of toxicants in sediments and water,
particularly in the upper Bay and near urbanized areas.
o Loss of submerged aquatic vegetation (SAV) in areas of the
Bay characterized by increasing levels of nutrient enrichment.
e Significant declines in landings and spawning success of
freshwater spawning fish.
o Significant declines in oyster spat set, especially in the
upper Bay and western shore.
o Reduced diversity and abundance of benthos in areas impacted
by sediment toxicity or low dissolved oxygen.
e Ranking of Bay regions by environmental quality status shows
that the upper main Bay, upper reaches of tributaries, and
areas near urban sources show similar patterns of toxic and
nutrient enrichment.
Although unequivocal "cause-and-effect" between trends in
environmental quality and living resources could not be demonstrated,
there is a correspondence of observed patterns. Also, a number of
analyses support reasonable potential hypotheses as to effects of
environmental change. The inference is that at least some trends in
SAV, shellfish, and anadromous fish reflect the integrated response
to anthropogenic intervention in the Chesapeake Bay ecosystem.
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CHARACTERIZING CHESAPEAKE BAY - A DOCUMENTATION OF ENVIRONMENTAL CHANGE
INTRODUCTION
The Chesapeake Bay Program (CBP) characterization report,
Chesapeake Bay; A Profile of Environmental Change, describes trends
in water and sediment quality, and in the living resources of the
estuary (Flemer et al. 1983). Historical information from past
research and monitoring efforts was used to assess changes as well as
present status for a number of physical and biological parameters.
In addition, potential relationships between the observed trends in
water quality and resources were identified and evaluated. Findings
of the CBP's research program were integrated with historical trends
data for this assessment.
In part, CBP's characterization of Chesapeake Bay is based on
a ranking of specific areas, or segments, of the Bay in regard to
selected water quality variables, as well as the diversity and
abundance of their living components. The water quality parameters
evaluated include nutrients, dissolved oxygen, chlorophyll ^, organic
chemical compounds, and heavy metals. The living resources which
were assessed include phytoplankton, submerged aquatic vegetation,
benthic organisms (including shellfish), and finfish. Each segment's
relative status was established in accordance with CBP's principal
scientific findings, which are as follows:
o Levels of nutrients (primarily nitrogen and phosphorus) show
increasing trends in many areas of the Bay. Nutrient enrichment
is most severe in the northern and middle Bay and upper reaches
of tributaries. Only parts of the Potomac, James, and some
smaller areas currently exhibit improving water quality with
regard to nutrients. Concentrations of chlorophyll a_ are also
increasing in most regions where sufficient data is available
for assessment.
e The amount of Bay water showing low (or no) dissolved oxygen in
summer is estimated to have increased 15-fold in the last 30
years. Currently, much of the water deeper than 40 feet is anoxic
from early mid-May through September in an area reaching from
the Annapolis Bay Bridge to the Rappahannock River.
e Elevated levels of heavy metals and toxic organic compounds are
found in Bay water and sediments. Highest concentrations occur
near urban or industrialized areas, and in the upper Bay. Some
of these toxicants are being bioconcentrated by plankton,
shellfish, and finfish.
-------
• Oyster spat set has declined significantly in the past 10 years,
particularly in the upper Bay, western tributaries, and some
Eastern Shore areas such as the Chester River. Trends in oyster
harvest show a similar pattern.
• Landings of freshwater-spawning fish, such as shad, alewife, and
striped bass, have decreased in recent years. Spawning success
of these and other semi-anadromous or anadromous species has
also been fair to poor in most areas sampled. Harvests of marine-
spawning fish, such as menhaden, have generally remained stable
or increased.
e The recent loss of submerged aquatic vegetation (SAV) appears
related to increasing nutrient enrichment: enhanced phytoplankton
growth and epiphytic fouling of plants has reduced the light •
reaching SAV below critical levels. Increased turbidity from
sediment loads may also be contributing to light limitation in
some areas. Toxicants such as herbicides seem to be a problem
primarily in local areas close to sources.
« Reduced diversity and abundance of benthic organisms can be
related to toxic contamination of sediments in heavily impacted
areas. Low dissolved oxygen in summer is another major factor
limiting benthic populations, including shellfish, in the upper
and mid-Bay. Low dissolved oxygen also can be expected to reduce
available habitat for finfish, particularly demersal species.
e Declining water quality — nutrient enrichment and increased
levels of toxicants — is occurring in major spawning and nursery
areas for freshwater-spawning fish, as well as in areas experiencing
reduced oyster spat set.
e Ranking Bay regions based on water and sediment quality status
indicates that the upper and mid-Bay mainstem,tidal-fresh and
transition reaches of major tributaries, and many smaller
tributaries contain moderate to high levels of nutrients. The
pattern of toxic substances is generally similar, although high
contamination is also found near urban and industrialized areas
in the lower Bay.
CURRENT CONDITIONS AND TRENDS IN WATER AND SEDIMENT QUALITY
The quality of the Bay's water and sediments reflects both its
natural physical and chemical characteristics, and the impact of
man's activities. Over 150 tributaries - large and small - drain the
64,000 square mile watershed. Along with freshwater, the rivers
bring other materials into the Bay: nutrients, sediments, and toxic
substances. Although the Bay has the ability to assimilate some of
this material, we now know that most remains within the estuary (Bieri
et al. 1982, Smullen et al. 1982). Man's activities have greatly
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contributed to the inputs of nutrients and sediment into Chesapeake
Bay, in addition to the variety of synthetic chemicals, heavy metals,
and other potential toxicants from anthropogenic sources.
Chesapeake Bay has changed greatly since the time of the first
settlers (Figure 1). For example, in the last AGO years, human
population in the upper Bay watershed alone has increased to nearly
four million. Forests have been replaced by fields and developed
areas. Many of these observed changes, and their consequences, were
well underway by the 1850's (Brush and Davis 1982). Sedimentation
rate, due primarily to land clearing, has accelerated since the civil
war (Brush and Davis 1982). The estuary is also more "flashy" - that
is, peak runoff flow is higher and low flow is lower than in the past
(i.e., pre-1800) (Biggs 1981). This is due to loss of forest, which
tends to adsorb rainfall and release runoff water more slowly than
cleared fields, roads, and other modified areas. The delivery of
nutrients to the Bay has become greater, reflecting increases in
runoff containing suspended sediment and fertilizers, as well as the
addition of sewage effluents (Heinle et al. 1980, D'Elia 1982). The
amount of toxic materials - heavy metals and organic chemicals - has
similarly increased as industrialization has progressed (Helz et al.
1981). Many of these changes occurred before the first scientific
surveys of the Bay in the later 1930's. For that reason, it is
sometimes difficult to show strong recent trends, as the bulk of the
change in the Bay environment had already taken place before any
water quality data were collected.
Nutrient Enrichment
Nutrients, such as nitrogen and phosphorus, are essential for
plant growth, and thus for primary productivity in the estuary.
However, in excess these nutrients can cause problems, including
blooms of undesirable algae, reduction in dissolved oxygen, and
decreased water clarity. Based on data collected between 1950 and
1980, the CBP characterization effort determined that most areas of
Chesapeake Bay are experiencing increased nutrient concentrations.
Figure 2 summarizes the current nutrient status of the estuary: the
northern Bay and upper portions of tributaries presently have relatively
high nutrient concentrations, the mid-Bay, lower portions of tributaries,
and eastern embayments have moderate concentrations of nutrients, while
while the lower Bay (where sufficient data exists) appears not enriched.
When trends from 1950 to 1980 are analyzed, they indicate that in most
areas water quality is degrading - that is, nutrient levels are increasing.
Total nitrogen concentrations are declining in the Patapsco, lower
Potomac and upper James, while total phosphorus concentrations are
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Population
(millions)
Improved
Land. (%)
o
o
I
y
o
g
i
o
y
(D
Less than 5%
5%-50%
O
o
O
y yy y
Pollen
(oak/ragweed)
Sedimentation
Rate
(cm/yr)
Metal Load
(cadmium p.p.m.)
SAV'S
Diatoms
Chesapeake Bay at Tolchester
Chesapeake Bay at Tolchester
Waterweed. pondweed
wild celery abundant
Waterweed, pondweed
sporadic, celery abundant
Wild celery, few others
Wild celery. All SAV
pondweed Milfoil gone
Epiphytes and clear water forms
Decrease in all species
Decreased epiphytes
Increased eutrophic forms
Eutrophic
& Planktonlc
forms
dominant
Fishery
Landings
(x 103 igs)
20.000
10:000
0
600.000
300.000
0
F
F
Shad
Menhaden
FIGURE 1. TIME HISTORY OF NORTHERN CHESAPEAKE BAY, 1600 to 1980. An Important aspect of understanding how Chesapeake Bay will respond
to pollution Is to examine the Bay's past. In the northern Bay, human activity, beginning at the top of the chart with population growth,
has been changing water quality since the time line began (see Appendix A for further discussion).
-------
Limited
data
FIGURE 2.
Rank of Chesapeake Bay segments
according to nutrient status.
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declining in the upper Potomac and throughout the James. Elsewhere,
trends are increasing (or stable) for most forms of nutrients,
particularly in the mid and upper Bay mainstem and larger tributaries.
The improvement in the Potomac is probably due primarily to phosphorus
control efforts at the Blue Plains Sewage Treatment Plant. Control
efforts appear to be making a difference, but it is apparent from the
effects discussed below that additional Bay-wide nutrient controls
are needed.
Dissolved Oxygen and Chlorophyll a Trends
In order to assess the resource management implications of these
nutrient trends, it is valuable to examine a related parameter -
dissolved oxygen. As nutrient levels increase, phytoplankton (algal)
growth is encouraged and more organic matter is produced. Bacterial
decomposition of this organic matter consumes oxygen. If more oxygen
is used than supplied by reaeration or photosynthesis, as often occurs
in deep water, the water becomes near or fully anoxic, and devoid of
most forms of life except anaerobic bacteria. This process occurs
naturally in some Bay areas during the summertime; however, high
nutrient loads can increase its severity.
Both the chlorophyll a. trends, as an indication of algal biomass,
and the dissolved oxygen trends suggest that the duration and extent
of anoxia has been accelerated in the Bay in recent years. As
indicated in Figure 3, there were no anoxic waters and only limited
areas of low dissolved oxygen in the main stem of the Bay during July
of 1950. A general trend towards increased volume of low dissolved
oxygen water occurred throughout the 1960's and 1970's. By 1980 a
large area of the main stem of the Bay was experiencing anoxic
conditions in July, at depths greater than 12-15 meters (Figure 3).
It is estimated that the volume of water with dissolved oxygen
concentrations equal to or less than 0.7 mg L"1 (0.5 mg I/"') was 15
times greater in 1980 than in 1950. The duration of oxygen depletion
also increased. It was sporadic during the mid-1950's; occurred from
mid-June to mid-August during the 1960's; and, in 1980, began during
the first week in May and continued into September. This increase in
the spatial and temporal extent of low dissolved oxygen levels reduces
the area of the Bay that can support normal firifish and shellfish
populations.
Organic Compounds in Water and Sediments
Organic compounds can occur naturally; although the ones of major
concern as pollutants are synthetically produced. The distribution
of organic compounds, such as hydrocarbons, pesticides, and herbicides,
in the bottom sediments and the water column of the main Bay (Figure 4)
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1950 ,
1980 ,
FIGURE 3. Extent of anoxlc bottom water In the main stem of Chesapeake Bay In 1950 (left) and 1980 (right).
-------
16'
•1O
Concentrations of Organic
Compounds in Sediment
(parts per billion)
10
102
103
10"
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
4
3
2
A
1 1 1
*i
*
, -
-
•
' , , .
> ••
ft
« •*•
-
> •
Negligible
-
- ^
Negligible
Negligible
* ,,--.•
Negligible
r - ' . ' * '
Negligible
FIGURE 4. Station locations and bar graphs representing concentration sums (ppb) of all
recognizable peaks for organic compounds after normalizing for silt and clay content.
(From Bieri and Huggett, 1982).
-------
and an analysis of limited tributary data suggest that organic
compounds concentrate near sources, at river mouths and in maximum
turbidity areas (Bieri et al. 1982a). The highest concentrations of
organic chemicals in the sediments were found in the Patapsco and
Elizabeth Rivers, exceeding 100 parts per million (ppm) at several
locations (Bieri et al. 1982b). In the main Bay, highest concentrations
of organic substances occur in the northern half. Most observed
sediment concentrations range from 0 to 10 ppm; however, in the upper
Bay some stations had levels over 50 ppm total organics (Bieri et al.
1982c).
These general trends suggest that many of the problem organic
compounds in the Bay tend to adsorb to suspended sediments, and then
accumulate in areas dominated by fine-grained sediments. Benthic
organisms located in such areas tend to accumulate the organic
compounds in their tissues. Studies of the pesticide Kepone, which
was discharged into the James River during the 1970's, have further
substantiated these conclusions (Nichols and Cutshall 1981). A major
mechanism for accumulation of this persistant pesticide appears to be
bioconcentration byplankton; this has implications for transfer of
this and similar toxicants through the food web. Some toxic organic
compounds, such as the herbicides atrazine and linuron, appear to
undergo fairly rapid chemical and physical degradation once they
enter the estuary and, therefore, they probably do not pose as serious
a problem Bay-wide (Kemp et al. 1982).
Trace Metal Contamination
Metals are chemical elements which occur naturally in the
environment, but which, in excess can become toxic to organisms.
Many areas of the Bay show trace metal concentrations in bottom
sediments that are significantly higher than natural background
levels. Figure 5 shows the degree of metal contamination (i.e.,
enrichment over background) in the surface sediments of Chesapeake
Bay. The contamination index (Cj) was developed by comparing present
concentrations of Cd, Cu, Cr, Ni, Pb, and Zn in the Bay's surface
sediment to predicted natural levels from the weathering of rock in
the Bay watershed and from measured pre-colonial levels from sediment
cores. If the present concentration of a given metal exceeded these
natural Chesapeake Bay background levels it was considered to be
anthropogenically enriched. The most contaminated sediments are
located in the Patapsco and Elizabeth Rivers, both heavily
industrialized tributaries. Metal concentrations up to 100 times
greater than expected natural background levels were found in
these areas. High levels of metal contamination (Cj^, 14) were
also found in the upper Potomac, upper James, upper mid-Bay
and small sections of the Rappahannock and York. Moderate
contamination occurs in the Susquehanna flats and off the mouth of
the Potomac River. These trends suggest that higher concentrations
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(C.)
FIGURE 5.
Degree of Metal Contamination in the Bay
based on the Contamination Index (Q).
No data exist near shore, and large local
increases should be expected close to
outfalls.
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are found near industrial sources and in areas where fine sediments
accumulate, such as in the deep shipping channel of the upper Bay.
In general, there is little movement of metals out of the most
contaminated areas, except when physically transported, as might
occur through movement or disposal of contaminated dredge material.
Significant levels of particulate and dissolved metals occur in
the water column. Concentrations of particulate Co, Cr, Cu, Ni, and
Zn are greatest in the upper Bay and near the turbidity maximum;
actual values vary greatly with salinity, tidal cycle, and amount of
suspended sediment (Nichols et al. 1981, Kingston et al- 1982). High
dissolved values, some exceeding EPA water quality criteria, have
been observed, particularly for cadmium, copper, zinc, and nickel.
These are most frequent in areas near industrial sources, and upper
reaches of the main Bay and western shore tributaries.
CURRENT CONDITIONS AND TRENDS IN LIVING RESOURCES
Major changes in Bay resources can be identified, which include
shifts in relative abundance of species or the types of biological
communities found in various areas. The CBP focussed on individual
living resource groups (e.g., submerged aquatic vegetation and
finfish), described documented trends, and compared present conditions
with past or potential status.
Phytoplankton in Two Well-Documented Areas
The upper Bay (above the Annapolis Bay Bridge) and upper Potomac
River (tidal-fresh reach) have shown increased dominance by a single
species of phytoplankton and increased algal biomass. Such changes
are considered to be indicative of eutrophicatiqn, and in fact have
paralleled changes in nutrient enrichment in these areas. These two
areas are those for which best data are available; similar changes
may be occurring elsewhere, or could be expected to occur if nutrient
enrichment continues.
The Potomac River's tidal-fresh reach was characterized in the
1960's and 1970's by massive blue-green algal blooms, an indicator of
excess nutrients (Pheiffer et al. 1972, Clark et al. 1980). Increased
phosphorus control in the watershed in recent years appears to have
been beneficial (Champ et al. 1981). In 1979 algal' populations were
diverse, and blue-greens made up only 25 percent of the total
phytoplankton population. Total cell counts for 1979 and 1980 were
also considerably lower than previously (Boulukos and Stoelzel 1980).
Trends in nutrient enrichment of the upper Bay tributaries have
closely paralleled those of the upper Potomac during the 1960's.
Massive algal blooms are frequently reported in the upper main Bay
-------
(above the Annapolis Bay Bridge), with elevated chlorophyll levels
due to increasing numbers of blue-green algae (Clark et al. 1973).
By comparison, observations of this area in 1965 to 1966 reported
only occasional occurrence of blue-green algae (D. A. Flemer, pers.
comm.). It is estimated that cell numbers in this area have increased
approximately 250-fold since a 1955-1956 study by Whaley and Taylor
(1968) , even taking into account the proportions of nannoplankton in
most recent studies by Allison (1980).
Submerged Aquatic Vegetation (SAV) Declines
Since the late 1960's, a dramatic, Bay-wide decline has occurred
in the distribution and abundance of submerged aquatic vegetation
(Orth and Moore 1982). Loss has moved progressively down-estuary
affecting the upper and mid-main Bay, most major tributaries, and
upstream reaches of many smaller tributaries (Figure 6). SAV now '
occupies a significantly more restricted habitat in Chesapeake Bay
than at any time during the past, according to CBP studies (Brush and
Davis 1982, Orth et al. 1983). SAV's role in the Bay ecosystem has
been reduced, and its ability to recover from this current status is
uncertain (Boynton and Heck 1982). Changes in distribution and
abundance of Bay waterfowl, which feed on SAV, have paralleled these
vegetation changes (Munro and Perry 1981).
Annual surveys of SAV conducted by the Maryland Department of
Natural Resources and the U.S. Fish and Wildlife Service Migratory
Bird and Habitat Research Laboratory have shown that the number of
vegetated stations in Maryland dropped from 28.5 percent in 1971 to
4.5 percent in 1982. Species diversity also declined significantly.
Comparison of the habitat filled in 1978 with the "expected habitat"
(based on depth, substrate, and other criteria) shows that areas of
greatest loss (upper Bay, western shore tributaries, and upper Eastern
shore tributaries) correspond with areas of greatest nutrient enrichment
(Figure 7).
Changes in Benthic Invertebrates
Benthic animals are considered useful indicators of pollution
because most are relatively immobile and cannot readily escape
unfavorable conditions. Changes in benthic biomass, community
structure, and diversity can indicate a variety of stressful conditions
(Boesch 1977). Where sufficient data on benthic communities existed
comparisons were made of current conditions, particularly in the main
Bay and certain tributaries. Trends in diversity, and relative
abundance of pollution-tolerant species such as annelids, were
investigated and compared to various environmental criteria. In the
main Bay, benthic abundance and diversity seems most related to
physical aspects of the environment - i.e. , salinity and sediment
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FIGURE 6. General area of SAV distribution in 1965 (left) and 1980 (right).
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Lost before 1970
0-Z5%
2.6-6.3%
jjjjSjll 6.4-15.8%
15.9-39.8%
39.9-100%
ND No Data
FIGURE 7.
Percent of expected submerged aquatic
vegetation habitat occupied in 1978 tor
aggregated sampling areas.
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type, - as well as predation and other biological interactions (Boesch
1977a). Highest community diversity occurs in the lower Bay polyhaline
zone. In some polluted tributaries, especially the Patapsco and
Elizabeth Rivers, significant declines in species diversity and
enhancement of pollution-tolerant annelids, relative to molluscs or
Crustacea, are observed (Reinharz 1981, Schaffner and Diaz 1982).
These changes are characteristic of stressed communities, and have
been observed in other impacted areas such as the New York Bight
(Wolfe et al. 1982).
Trends in Commercial Shellfish
The density of annual oyster (Crassostrea virginica) spat set is
a measure of the success of oyster reproduction and recruitment, and
is a reasonable predictor of oyster harvest (Ulanowicz et al. 1980).
Comparison of the average oyster spat set for the past ten years
with the previous ten to thirty years shows significant declines
in the upper main Chesapeake Bay and the Chester, James, Nanticoke,
Patuxent, Pocomoke, Potomac, Rappahannock, and Wicomico Rivers,
Eastern Bay, Fishing Bay, and Pocomoke Sound. In general, 1980 was
a good year for spat fall, particularly in Eastern Shore tributaries;
this fact is related to high salinities during the spawning period
(Davis et al. 1981). However, spat set in the upper Chesapeake
and its western tributaries was generally .light even in this good
year. The trend toward light spat set in upstream reaches has
been documented in detail for the Potomac River; while spat set in
the lower river has continued to vary in response to salinity, set
in the middle and upper Potomac has been suppressed since the late
1960's (Krantz and Carpenter 1981). Changes in water quality and
increased sediment loads are implicated (Krantz and Carpenter 1981).
Harvest of oysters for Cheaspeake Bay has declined since 1880,
but since 1960-1965 has remained relatively stable (Figure 8). This
is in part due to more intensive fishery management practices, such
as shell and seed planting. Harvest for the western shore has decreased
significantly during the period 1962 to 1980, while harvest for the
Eastern Shore has increased significantly. This is consistent with
the Eastern Shore's maintaining better spat set. For the Chesapeake
Bay as a whole, declines in oyster harvest have been somewhat offset
by increased harvest of blue crabs (Callinectes sapidus). As a
result, the Bay-wide landings of shellfish (in pounds) have not
changed greatly from 1962-1970 to 1970-1980. However, overall shellfish
harvest for the western shore has decreased significantly during this
period.
Shifts in Finfish Harvest
The CBP examined trends in harvest and other indicators (e.g.,
young-of-the-year surveys) for the major commercial species historically
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12,000
o 9,000
Q_
T5
o 6,000
3,000
1880 1900 1920 1940 1960 1981
• *
Year
FIGURE 8. Historical pounds of shucked oyster meat for Chesapeake Bay, 1980 to 1981.
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landed in Chesapeake Bay. These include freshwater spawners such as
striped bass (Morone Saxatilis), white perch (II. americana), yellow
perch (Perca flavescens), catfish (Ictalurus spp), shad (Alosa
sapidissima), and alewife (Alosa pseudo-harengus), marine spawners
such as menhaden (Bevoortia tyrannus), croaker (Micropogonias
undulatus), spot (Leistostomus xanthuros), bluefish (Pomatomus
saltatrix), and weakfish (Cynoscion regalis) as well as three .forage
fish, Bay anchovy (Anchoa mitchilli), mummichog (Fundulus heteroclitus),
and Atlantic silverside (Menidia menidia).
The Maryland juvenile index provides consistent data since 1958
for the upper Bay, Nanticoke, Choptank, and Potomac Rivers (Boone
1980). Juvenile indices of most anadromous and freshwater species
show declines in recent years, with the exception of the Potomac
River where white perch and yellow perch have increased. Information
for Virginia waters is not directly comparable, due to differences of
methodology and target species (sciaenids) (Wojcik and Austin 1982).
However, trends in marine-spawning fish were similar in both data
sets. Marine spawners show general overall increases in all basins,
although some species show declines in the most recent surveys. In
Maryland, mummichog shows an increasing pattern similar to that of
marine spawners, while the Bay anchovy and Atlantic silverside show
declines. However, the anchovy has been increasing in Virginia
tributaries surveyed during the same period. This may reflect differences
in water quality or habitat (particularly availability of SAV, used
as shelter by this species) between the two states.
Fishery statistics maintained by the National Marine Fishery
Service indicate that harvests of anadromous and other freshwater-
spawning finfish have been declining in Chesapeake Bay (Figure 9).
The downward trend in American shad has been continuous since 1900,
while declines in river herring, yellow perch, and striped bass
landings have been more recent. Landings of alewife, shad, and yellow
perch are now at unprecedented low levels. Harvests of marine
spawners, on the other hand, have increased in'most areas. Menhaden
landings have risen steadily since 1955, while the increase in bluefish
landings has been more recent. The increased yield of marine spawners
and decreased yield of freshwater spawners represent a major shift in
the proportion of the finfishery accounted for by each group: during
1881-1890 marine spawners accounted for about 75 percent of the
fishery, while during 1971-1980 they accounted for 96 percent.
Similarly, an assessment of individual basins for the two periods
1962-1970 and 1971-1980, shows significant declines in freshwater
spawners while landings of marine spawners in most basins increased
significantly.
The large relative increase in marine spawners and actual decline
in freshwater spawners illustrates gradual reduction in the diversity
of Chesapeake Bay fisheries. Diversity is not used here in the sense
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600,000
g 400,000
o.
M—
O
o 200,000
o
0
1880
Marine
Estuarine
Anadromous
1900 1920 1940 1960
Year
1981
FIGURE 9. Historical landings for the commercial fish species investigated by the Chesapeake Bay Program,
separated into freshwater spawners, estuarine spawners (shellfish), and marine spawners.
-------
of number of species alone, but in the sense of number of species and
the relative eveness of the contribution of these species towards the
total harvest. Such a loss of diversity could be viewed as potentially
undesirable because harvests are more vulnerable to year—to—year
fluctuations in population size of major commercial and recreational
species. There is less resiliency (both economic and ecological) in
single-species fisheries. The economic impacts of the failure of the
California sardine, the Peruvian anchoveta, or the Delaware Bay
menhaden fisheries are prime examples (e.g., Price 1973).
Because freshwater-spawning fish and estuarine-spawning shellfish
spend all or most of their sensitive life stages in the Bay, their
well-being may be considered as an indication of the health of the
estuary. Thus, the declines in most of these species simultaneously
is reason for concern.
RELATIONSHIPS BETWEEN WATER AND SEDIMENT QUALITY AND LIVING RESOURCES
Organisms respond directly to changes in their habitat and changes
in food supply, competitors, or predators. Major factors which affect
the Bay's living resources include natural variables such as freshwater
inflow, temperature, or other organisms, as well as man-induced stress
such as nutrient and toxicant enrichment. Distinguishing between
effects triggered by anthropogenic, as opposed to natural causes, is
often difficult due to the natural variability of organism distribution
and abundance. Although the GBP was unable to pinpoint exact causes
for specific resource changes, the similarity in patterns and the
overlap in the distribution of water or sediment quality and living
resource trends in the Bay should be considered as more than a striking
coincidence.
Submerged Aquatic Vegetation
The GBP supported a major research effort to identify the causes
of the recent SAV decline. Investigators focussed on two main
hypotheses: 1) In recent years .use of toxic agricultural materials,
particularly herbicides, has increased within the Chesapeake watershed.
Runoff of these materials from agricultural areas may be reducing or
eliminating aquatic vegetation within the estuary; 2) Reduction in
the light available to the plants due to an increase in water column
turbidity or increased growth of epiphytes (or both) may be causing
the decline. Nutrient enrichment was considered a major factor
affecting both turbidity (through increase in phytoplankton biomass)
and epiphyte growth. Research supported by the GBP implicated light
limitation as the most important factor regulating the SAV loss Bay-
wide (Kemp et al. 1983). Herbicides could be important locally,
close to sources (although areas affected may represent significant
habitat) (Kemp et al. 1983).
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The GBP's research conclusions are supported by field observations.
Comparison of a map of current SAV status to Bay nutrient conditions
reveals that vegetation now occurs primarily in areas that are not
enriched or only moderately enriched. Statistical analysis (rank
correlation) shows significant correlation between declines in SAV
and increased nutrient concentrations in many areas. The major
nutrient which appears to correlate with SAV abundance is nitrogen
(Fig. 10). A negative response to maximum chlorophyll ji values, an
analog of both nutrient loading and turbidity, was also found. These
analyses support experimental results linking the recent loss of Bay
vegetation to increases in nutrient loadings and ultimately to light
stress caused by increasing phytoplankton biomass and epiphytic
growth (Twilley et al. 1982, Kemp et al. 1983).
Benthic Organisms
Major anthropogenic factors which could adversely affect benthic
organisms in Chesapeake Bay are toxic materials, either in bed
sediments or in the overlying water column, and nutrients. Toxicants
can produce either acute (e.g., elimination of susceptible species)
or sublethal (e.g., accumulation in body tissues) effects. Nutrient
enrichment can alter the Bay's benthic community structure by
stimulating phytoplankton production, and thus increasing organic
matter loads to the sediments (Bascom 1982). Excessive production of
organic material has been linked to the increased duration and extent
of low dissolved oxygen in Chesapeake Bay, which is decreasing
available benthic habitat (Taft et al. 1980).
Episodes of low dissolved oxygen have been cited as the
major factor limiting benthic distribution in deeper waters of the
upper and mid-main Bay (Mountford et al. 1977). The documented
increase in extent of anoxic water in the mid-Bay can be related to
complete loss of benthic habitat or replacement with emphemeral
assemblages. This may have secondary impacts on bottom-feeding
predators such as crabs or demersal fish, which can be stressed by
food limitation as well as reduction of habitat. Recent changes in
the mid-Bay blue crab fishery, especially the necessity to set crab
pots in shallower water, may be a direct result of these anoxic
episodes.
Changes in benthic diversity, abundance, and community structure
could be related to toxic contamination of sediments only in areas
recognized as "impacted" (e.g., the Patapsco River and the Elizabeth
River)(Figure 11). These areas are characterized by low benthic
diversity and abundance, and dominance by pollution-tolerant annelids,
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A ET-4
D Spring
A Summer
EE-3 A A ET-5 CB_
.25
.50
.75 1.00 1.25 1.50
Seasonal total nitrogen of previous year (1977)
(mg/L)
1.75
2.00
2.25
FIGURE 10. Correlation between percent vegetated stations and annual total nitrogen of previous year.
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Baltimore
Ft. %iilk Harbor
Low diversity
Moderate diversity
High diversity
FIGURE 11. j Comparison of benthic community diversity to sediment toxicity. (Toxicity Index. T|) in the
Patapsco River, Maryland.
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in comparison to nonpolluted reference areas (Reinharz 1981', Schaffner
and Diaz 1982). Elsewhere, other factors, primarily physical or
biological, are apparently controlling benthic distributions. However,
bioaccumulation of certain metals in tissues of shellfish could be
correlated with enrichment of those metals in the bed sediments, even
in the main Bay.
Oysters
Oysters (and other shellfish) are benthic organisms but, because
of their commercial importance oysters are treated separately here.
Factors affecting benthic communities in general (i.e., low dissolved
oxygen water, and toxicants in sediments or water column) will impact
oysters as well. In addition, oysters are potentially vulnerable to
shifts in phytoplankton species brought about by nutrient enrichment
(Ryther 1954). Phytoplankton species usable as food can be replaced
by undesirable or inedible forms such as blue-greens or non-motile
green algae (Ryther and Officer 1981). Comparison of EPA water
quality criteria to measured and estimated concentrations of toxicants
in the water column- revealed a number of violations in areas of oyster
habitat; these were chiefly for heavy metals. Although the duration
or extent of high toxicant concentrations is presently unknown, the
observations may be significant. Populations stressed by a variety
of factors, may be more vulnerable to diseases, such as MSX (Minchinia
nelsoni) or "Dermo" (Perkinus marinus). Impact of these protozoan
parasites has increased in recent years due to higher salinities
resulting from drought (Krantz and Davis 1983).
In addition, oyster habitat is adversely impacted by the increased
rate of sedimentation in Chesapeake Bay. Beds,may be buried, or spat
set impeded, by deposition of sediment. Loss of once productive
oyster bars in the upper Bay is probably due in part to sedimentation
over the past 100 years (Alford 1968).
Fishery Landings and Juvenile Index
Although total fishery landings in Chesapeake Bay have increased
since 1920, the distribution of landings among species has changed
significantly. Anadromous and other freshwater-spawning fish such as
shad and striped bass have declined greatly in importance, while
marine spawners have remained stable or increased. Finfish juvenile
index - a measure of recruitment success - reflects these trends as
well.
Several causes of this change in distribution have been suggested:
1) nutrient enrichment may lead to food web shifts (i.e., changes in
phytoplankton and zooplankton species) primarily affecting early life
stages; 2) the level of toxicants, particularly heavy metals,
pesticides, and chlorine, in some major spawning areas are elevated
-------
and, in-fact, have exceeded EPA criteria in some spawning or nursery
areas used by anadromous fish; 3) habitat is being lost because of
increased area of low DO water; 4) adverse climatic conditions
(freshwater inflow, temperature, etc.) have reduced the spawning
success of anadromous species; 5) overfishing is affecting stock
sizes; 6) construction of dams represents a physical obstruction that
impedes spawning success of anadromous and semianadromous species;
and 7) modifications of upstream spawning and nursery habitat, such
as wetlands destruction and stream channelization further stresses
fishery stocks. It is possible that all of these factors are
contributing to the changes observed in Chesapeake Bay.resources.
Concentrations of toxicants exceeding EPA water quality criteria
were discussed earlier. Many of these criteria violations occurred
in spawning or nursery areas used by anadromous fish. If such high
toxicant concentrations prove to be wide-spread or frequent, adverse
impacts would be expected, particularly to sensitive larval stages.
The large increase in duration and extent of anoxia in the mid-
Bay is certainly reducing available habitat for a number of species,
particularly bottom feeders such as spot or croaker. Food limitation
could result in reduced growth or other "quality changes" in the
condition of fish, even if populations per se are not affected.
Statistical analysis of juvenile indices shows that, although
natural variables such as temperature and flow exert a major influence
on recruitment success of most freshwater spawning species, factors
such as nutrients and dissolved oxygen can also affect year-to-year
success (Flemer et al. in press). Major influencing factors vary
from river to river, suggesting that management strategies also will
have to differ between each of these systems.
Effects of harvest, particularly on already declining populations,
may potentially be significant. Unfortunately, information on fishing
effort is lacking for most Chesapeake Bay fisheries, so that attempts
to relate commercial landings to actual stock abundance have not been
successful (Rothschild et al. 1981, Bartone 1982). There is also
evidence that recreational catch for some species is significant
(Williams et al. 1982).
Finally, impacts of habitat modification on freshwater spawning
finfish, or species which use low salinity nursery areas, could be
important in many areas of the Bay. Many tributary streams have man-
made structures, such as dams, which impede the migration of anadroraous
fish (O'Dell et al. 1980). Loss of wetlands in the Bay region was
extensive prior to passage of protective legislation in the early
1970's; much of this loss was in areas important as finfish nursery
habitat (Settle 1969, Metzgar 1973).
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STATE OF THE BAY
CBP developed a numerical ranking system to assess the status of
certain water quality and living resource variables to provide a
snapshot of the "state of the Bay". Nutrients, trace metals, and
submerged aquatic vegetation, were used as the primary classification
variables because of their importance and the completeness of data.
Variables were ranked numerically by Bay segment.
Ranking of nutrients was based on mean concentrations of total
nitrogen and total phosphorus observed in each segment during the
period 1977-1980. The potential dissolved oxygen demand of organic
matter containing these given nitrogen and phosphorus concentrations
was also taken into account, because of the link to biological
resources (Table 1). Class 1 and 2 are characteristic of areas with
low nutrient enrichment (e.g., lower Bay, Eastern Bay, and lower York
River) while class 5 and 6 represent the most impacted systems (e.g.,
upper reaches of major tributaries, Baltimore Harbor, and the Back
River). Rankings of trace metals were based on Cj., the Contamination
Index. Eastern shore ajid lower Bay areas show lowest GI values,
while regions close to urbanized or industrial areas have highest Ci
values. When nutrient and toxicant ranks are aggregated, the following
assessment can be made: poorest water and sediment quality now occurs
in the upper Bay, upper reaches of the Potomac and James Rivers, the
Patuxent River and some smaller areas such as the Patapsco River, the
lower Potomac, James, mid-Bay, and most of the York and Rappahannock,
and eastern shore tributaries are moderate in water and sediment
quality, the lower Bay mainstem, Mobjack Bay, Pocomoke Sound, lower
Choptank, and Eastern Bay currently have the best water and sediment
quality (Figure 12).
TABLE 1. CLASSIFICATION OF CHESAPEAKE BAY WATER USING TOTAL NITROGEN
(TN) AND TOTAL PHOSPHORUS (TP), AS RELATED TO POTENTIAL
OXYGEN DEMAND
Class
TN
mg L'1
Potential DO
TP . Demand
mg L"1 mg
1 0-0.40 0 - 0.056 8
2 0.41 - 0.60 0.057 - 0.084 12
3 0.61 - 0.80 0.085 - 0.112 16
4 0.81 - 1.00 0.113 - 0.140 20
5 1.01 - 1.75 0.141 - 0.245 35
6 1.76 + 0.246 + 36+
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FIGURE 12. |
Environmental quality of Chesapeake Bay
based on the environmental quality
classification scheme.
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Submerged aquatic vegetation (SAV) was ranked according to the
utilization of potential habitat by the plants. Most favorable are
the lower Bay, Eastern Bay, and lower Choptank River, while most
other segments were poor in respect to SAV. Oyster spat set
(recruitment) and anadromous fish harvest were also assessed, although
not ranked numerically because of lack of bay-wide data. When this
asessment is combined with SAV ranking, the following evaluation can
be made: the upper Bay and western shore shows a general pattern of
decline in the three resources, the upper Patuxent, James, and Patapsco
and Middle Rivers showed the poorest resource quality, Tangier Sound
and the lower and mid-Bay display moderate resource quality, the
Eastern Shore generally appears to be the most productive region of
the Bay system (this conclusion is supported by the regional comparison
where Eastern Bay, and the Choptank and Chester Rivers ranked most
favorably).
When the ranks of water quality are compared to the ranks of
living resources, it is evident that areas with the best resource
quality correspond to areas with the best water and sediment quality
(e.g., eastern embayments). This suggests that, to improve the
quality of resources in the Chesapeake Bay, water and sediment
pollution must be reduced. Achieving this goal will require the
concerted effort of both government and the private sector.
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