1. benthic organisms
The Chesapeake Bay is home to an active community of organisms which
live in association with bottom sediments. This assemblage, collectively
known as the benthos, includes familiar organisms such as oysters, clams
and crabs, as well as less familiar forms, including segmented and
unsegmented worms, small crustaceans, snails, and anemones. A large portion
of the living and dead organic material in the water, including the
plankton discussed in the previous chapter
and plant material washed in from the watershed, settles to the sediment
surface and decays. This decaying material is a food source for benthic
organisms. As benthic organisms burrow through the sediments and digest
this food, a portion of the nutrients and other chemicals buried in the |
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sediments are returned to the overlying water. These recycled nutrients
frequently contribute to excess phytoplankton production and eutrophication.
The Chesapeake Bay is a nursery ground for many commercially and
recreationally important fish. While on their nursery grounds many of these
fish feed on the benthos. Benthic organisms thus form important links
between primary producers and higher levels of the Bay food web.
The benthic component of the OEP water quality monitoring program is
designed around the concept that the composition of benthic communities is
determined by ambient sediment and water quality. Therefore, the variety
and abundance of organisms composing these communities are likely to
respond to improvement in water and sediment quality resulting from
pollution abatement programs. Because many benthic organisms live for 1-2
years, changes in their populations are an integration of changes in
environmental conditions occurring over their life span. In addition,
because benthic organisms are relatively immobile, they complete their life
cycle within the Bay and often within specific regions of the Bay. Thus,
benthic responses to changes in water quality resulting from Bay-wide
cleanup programs are likely to be region specific and thus more easily
interpreted. Finally, as important intermediate links in the Bay food web,
benthic responses to cleanup actions are likely to be representative of the
responses of other living resources. The benthos are, therefore,
potentially good indicators of the effectiveness of cleanup efforts.
 DESIGN
CONSIDERATIONS
The benthic program component is jointly sponsored by OEP and the Maryland
Power Plant Research Program (PPRP) and includes historical PPRP long-term
benthic monitoring stations. PPRP stations along the mainstem of the Bay
have been sampled regularly since 1971. PPRP stations in the Potomac and
Patuxent Rivers have been sampled regularly since 1979-1980. Building the
current OEP benthic monitoring element around the PPRP long-term benthic
monitoring program provides a data record that can be used to interpret
responses due to cleanup programs in the context of long-term fluctuations
associated with natural phenomena. This separation of changes caused by
natural phenomena from improvements brought about by cleanup programs will
continue to be an important consideration in future analyses of the benthic
program.
Benthic sampling is conducted ten times per year throughout the mainstem
Bay and in all the major Bay tributaries. Station locations (Map 9)
encompass the range of salinity and sediment types that occur in the
Maryland Bay. Physical and chemical properties of the water (e.g.,
salinity, temperature, and dissolved oxygen concentration) and sediments
(e.g., particle size, and carbon content) that are known to affect benthic
organisms are measured at each sampling location.
Map 9
RESULTS
Salinity is the major natural environmental factor controlling regional
distributional patterns for the Bay benthos. Differences in sediment
characteristics and in the levels of bottom dissolved oxygen concentrations
that occur from shallow to deep habitats control local benthic
distributions. Five major assemblages of benthic populations occur along
the salinity and sediment gradients (Map 9). These are: (1) a tidal
freshwater assemblage, (2) a trace salinity assemblage, (3) a low salinity
estuarine assemblage, (4) a high salinity estuarine sand assemblage, and
(5) a high salinity estuarine mud assemblage. Salinity increases with
bottom depth throughout the Bay. Thus, high salinity assemblages located in
deep habitats can be adjacent to low salinity shallow water assemblages.
The tidal freshwater assemblage is limited to the upstream portions of Bay
tributaries. Aquatic earthworms, called oligochaetes, and larval insects
are numerically dominant in this habitat. The trace salinity assemblage
occurs in the transition zone between tidal freshwater and estuarine
habitats. Its greatest extent occurs in the upper portions of the mainstem
Bay and the Potomac River, and is of limited extent in smaller tributaries.
A mix of freshwater organisms which tolerate exposure to low salinity, and
estuarine species which tolerate exposure to freshwater are abundant in the
trace salinity habitat. The low salinity estuarine assemblage is dominated
by estuarine species. A few marine species that tolerate exposure to low
salinity also occur in this assemblage. The high salinity estuarine sand
and mud assemblages are distinct benthic groups, each dominated by marine
species that tolerate exposure to low salinity. These assemblages are
distinguished from each other by groupings of species that associate with
particular types of bottom sediments. It is apparent from
Map 9 that
most of the Maryland portion of the Bay is inhabited by estuarine
assemblages.
The spatial distribution of benthic organism biomass is summarized in
Map 10. Biomass is the total dry weight of all benthic animals
collected. The height of the bars represents the average annual amount of
benthic biomass per square meter of bottom area. The deep central portion
of the Bay and lower half of the Potomac River support the lowest benthic
biomass. Low benthic biomass also occurs in the deeper regions near the
mouths of smaller tributaries. In these habitats, annual abundance and
biomass of benthic organisms is depressed because of adverse effects
associated with oxygen-depleted (i.e., anoxic) bottom waters that occur
during warmer months. The effects of anoxia on the benthos are most
apparent just downstream of the Bay Bridge where anoxia is generally most
severe and of greatest duration. Benthic organisms occurring in habitats
that experience anoxia are small, rapidly-growing forms that can reproduce
in any season.
Map 10
Shallow habitats along the margins of the mainstem Bay and the lower half
of the Potomac River do not experience summer anoxia. These regions are
characterized by much greater benthic biomass than the adjacent deeper
habitats that do experience summer anoxia (Map 10). A variety of
benthic organisms are abundant in shallow habitats including small,
rapid-growing polychaetes and larger, slower-growing crustaceans and
mollusks.
The greatest biomass of benthos, represented by the tallest bars in
Map
10, occurs in trace salinity and low salinity estuarine habitats. Much
of the suspended sediment and organic inputs to the Bay are deposited in
this habitat, the zone of maximum turbidity, and become an available food
source for the benthos. The Macoma clam, Macoma balthica, and the brackish
water clam, Rangia cuneata, comprise most of the benthic biomass in the
zone of maximum turbidity. These clams are particularly well adapted to
feeding on microorganisms associated with organically rich, frequently
resuspended sediments.
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The biomass of benthic organisms at any one place in the Bay fluctuates as
much or more over an annual cycle as it does from place to place.
Figure
16 summarized month-to-month variation for the benthos of typical Bay
habitats. In all habitats, peak benthic biomass occurs in the spring.
Factors influencing within-year variation in benthic biomass vary among
habitats. Essentially no benthic organisms survive anoxic conditions that
occur in deep habitats during summer (Figure 16, A). When anoxic
conditions dissipate in early fall, deep habitats are repopulated within
weeks by small, rapidly-growing polychaetes. Benthic biomass is also low
during summer in shallow habitats along the margins of the Bay and its
tributaries (Figure 16, B). Summer low biomass values in shallow
habitats are, however, larger than peak biomass values in deep habitats
that experience anoxia (Figure 16, A and B). Annual biomass cycles
for shallow habitats appear to be associated with the annual phytoplankton
cycle suggesting a direct linkage between shallow water benthic biomass and
phytoplankton. |
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Figure 16. Annual cycle of benthic
organism biomass in three representative habitats found in Chesapeake
Bay. These three habitats are high salinity, deep mud (A), high
salinity, shallow sand (B) and high trace salinity (C). (Note:
Biomass scale is different on each graph.) |
A variety of species contributes to biomass peaks in shallow habitats,
including polychaetes, crustaceans, and mollusks. Seasonal
variation in benthic biomass is reduced in the trace salinity habitat (Figure
16, C); however, biomass levels in this habitat are always an order of
magnitude higher than those in other habitats.
In the Patuxent River, the abundance of adult Macoma clams peaked in
1978-1980 near the zone of maximum turbidity at the same time that
suspended sediment and sewage loadings were at the highest levels recorded
for this system (Figure 17). As discussed above, Macoma biomass is
closely linked to the amount of organic material that is produced within or
input to the system. Patuxent Macoma populations have declined since 1980
as suspended sediment loadings have declined and as sewage treatment
facilities have been upgraded. Declining Macoma biomass may indicate that
the amount of organic material accumulating in Patuxent sediments is
decreasing and water quality is improving. These data suggest that
pollution abatement and cleanup programs for the Patuxent River are
improving water and sediment quality by limiting inputs and production of
organic material. The benthos appears to be responding in a measurable and
interpretable way and therefore this biological community may be an early
indicator of system-wide improvements.
Figure 17. Historical abundance
of adult Macoma clams, Macoma balthica, in the turbidity maximum region of
the Patuxent River. Gaps between years indicate periods during which
no data were collected.
Natural effects of salinity fluctuations on long-term benthic abundance
trends are shown in Figure 18 for the low salinity estuarine
assemblage in the middle reaches of the Potomac River. This figure suggests
that year-to-year fluctuation in salinity during the reproductive periods
is a major factor influencing long-term trends for benthic organisms.
Salinity exerts the most influence over benthic distributions during early
life stages, shortly after reproduction, because these life stages
generally have narrower salinity tolerance ranges than do adults. The
long-term distributional pattern shown in Figure 18 is
representative of most of the Chesapeake Bay. Long-term benthic responses
to salinity and other sources of natural variation can and must be
determined before responses to Bay-wide management actions can be assessed.
Figure 18. Historical abundance
of the small crustacean, Leptocheirus plumulosus, in the low salinity
estuarine habitat of the Potomac River. River salinity during the
spring of each year is plotted to show its relationship with the annual
spring reproductive peak in abundance.
CONCLUSIONS
- Benthic organisms are an important component of the Bay ecosystem,
serving as food for fish and crabs and mediating exchange processes
between bottom sediments and the overlying water column.
- Benthic organisms provide a sensitive indicator of water quality that
integrates within the food web, over time, and over a number of
environmental variables.
- The impact of low dissolved oxygen waters on bottom habitats is
difficult to measure directly but is clearly evident in benthic
communities.
- The long-term response of benthic organisms to reductions in organic
inputs and initial pollution abatement programs in the Patuxent River has
been documented and appears to be favorable.
- Benthic responses to pollution controls can be accurately tracked
because natural sources of variation are known and can be partitioned
from responses associated with pollution abatement and cleanup programs.
Contents
a.
Preface
b.
Acknowledgements
1. Introduction
2. Understanding The Bay's Problems
3. Program Description
4. Chemical and Physical Properties
5. Plankton
6. Benthic Organisms
7. Ecosystem Processes
8. Pollutant Inputs
9. Management Strategies and the Role
of Monitoring
10. Glossary |
