Chesapeake Bay Monitoring
"Monitoring for Management Actions"

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.
 
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.
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

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