Biological Effects

The ecological impact of chemical contaminants in the Chesapeake remains the most challenging area for scientists and managers. Demonstrating the connection between even well documented inputs of contaminants and ecosystem responses to those contaminants relies on the science of aquatic toxicology, an evolving discipline that evaluates responses at hierarchical levels, from molecules to ecosystems. Although in such ecological research there is always the question of extrapolating observations at one level (say, the level of the cell) to higher levels (such as the whole organism, or a community of organisms), scientists have nevertheless been steadily detailing relationships between specific contaminants and biological effects. We now know that:

• Contaminants can compromise the immune systems of Bay organisms. Laboratory studies of the oyster's immune system, for example, have shown that exposure of oyster cells to TBT (tributyltin) (Anderson et al. 1996), to PAHs (polycyclic aromatic hydrocarbons) (Chu and Hale 1995) or to the chemical carcinogen DEN (n-nitrosodiethylamine) (Winstead and Couch 1988) interferes with hemocytic response to the Dermo parasite, thus enabling Dermo to progress more rapidly and with more lethal results than observed in cells of unexposed oysters. Further, research on oyster feeding (Newell and Weston 1996) has shown that the oyster is more likely to take up contaminants already bound in the phytoplankton on which it feeds, in contrast to contaminants attached to sediment particles, for example.
  
  
• Chemical preservatives on wooden structures - on bulkheads for example - can harm marine life. A study on the ecological impacts of bulkheads treated with CCA (copper, chromium and arsenic) has shown that aquatic organisms grazing on green algae attached to the bulkheads ingested enough contaminants to cause death. This research also found that organisms, populations and communities in the vicinity of CCA-treated materials showed reductions in biomass and limited species composition, leading to lower biological diversity and less productivity in the area.
  
  
• Contaminants can affect phytoplankton at the base of the Bay's food web. Research in the Patuxent River, for example, has shown that phytoplankton blooms can concentrate trace metals, which are then passed to the zooplankton that graze on them.
  
  
• Unconsumed algae and other organic matter that fall to the Bay floor can decay so rapidly that the contaminants bound to that matter can remain in the water column. Whether contaminants remain longer in the water column or more quickly join bottom sediments depends on physical processes and obviously influences the kind of effects toxic compounds will have on an ecosystem like the Chesapeake Bay.
  
  

  The cellular link between contaminants and biological responses allows researchers to anticipate and predict effects at higher levels, from the individual to entire ecosystems. Scientists point out that: Biochemical and cellular alterations are usually the first detectable, qualifiable responses to environmental stresses. Chemically induced changes in a biochemical system or cellular function can serve as markers of both exposure and effect. Alterations in these systems are often more sensitive indicators than those at higher levels of biological organization, and will underlie effects seen in whole populations.

  
  
• In aquatic organisms generally, the most confirmed linkage between toxic contaminants and physiological endpoints is as follows: carcinogens (e.g., PAHs) lead to a direct response in an organism (such as an increase in the enzyme P450) involved in the organism's attempt to detoxify environmental contaminants; this response then leads to DNA alterations and mutations, and ultimately to cancer. While the links in this chain of cause and effect can be quantified and interrelated, sublethal effects have not been quantified under varying ecological and climatic conditions. Therefore, long-term ramifications in the Bay remain uncertain.

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