Workshop Discussion

Sources of Contaminants
Map of the Chesapeake Bay
Transport and Fate of Contaminants
Bioloigical Effects of Contaminant Exposure

The workshop discussions centered on questions that were organized under three major topics: sources of contaminants, their transport and their ecosystem effects. While reference is made in this report to the Toxics of Concern List (fourteen "toxic substances which represent immediate or potential threat to the Chesapeake Bay System," as adopted by the Chesapeake Executive Council), the workshop deliberations used four classes of contaminants - heavy metals (copper and mercury), organic pesticides (Atrazine), products from fossil fuels (PAHs), and complex chlorinated hydrocarbons (PCBs) - as representative. Although not all compounds in a given class behave alike (copper and mercury can behave very differently, for example, and are therefore treated separately), these five chemicals serve as general examples of classes. This synthesis report also refers to arsenic, Dimilin (the Gypsy moth treatment) and other compounds.

The rationale here is that there are thousands of different species in the Chesapeake Bay and more than a thousand potential contaminants. Researchers cannot possibly test the risk of each species from each contaminant. What they can do is assess how representative species are affected by representative contaminants.

Sources of Contaminants in the Chesapeake Bay

Question 1: What are the sources of major classes of contaminants in the Chesapeake Bay?

Loading data are available for point and diffuse sources of chemical contaminants in the Chesapeake Bay, although a great deal of uncertainty remains, especially for diffuse sources such as stormwater runoff. According to the Chesapeake Bay Basinwide Toxics Reduction Reevaluation Report (1994), the highest estimated Toxics of Concern metal loading to the Bay basin comes from urban stormwater runoff, followed by point sources and atmospheric deposition. According to that report, metal loading is highest in the Potomac, followed by the Susquehanna, West Chesapeake, James, mainstem Bay, Patuxent, Eastern Shore, York and Rappahannock basins.

Also according to that report, the highest estimated loadings of Toxics of Concern organic chemical contaminants (PAHs and PCBs) are from atmospheric deposition, followed by urban stormwater runoff and point sources. The West Chesapeake has the highest organic chemical contaminant load, followed by the mainstem Bay, Susquehanna, Potomac, James, Eastern Shore, Patuxent, York and Rappahannock basins.

Atmospheric deposition is of relatively greater importance in the southern Chesapeake where riverine flow is less dominant than in the northern Bay. Some of these airborne materials may originate from sources hundreds of miles away, including the Ohio River Valley (cf. Airsheds and Watersheds - the Role of Atmospheric Deposition, 1995).

An additional source of chemical contaminants are Bay sediments, which have become reservoirs of toxic compounds that have settled there. In particular, compounds that do not break down easily and are now strictly controlled by regulations (e.g., Kepone and DDT) can be recycled from the sediment by biological and physical processes.

In summary, the basic pathways of representative chemical contaminants in the estuary, from source to transport to uptake and ultimate fate, are generally understood, with considerable background information available not only from the Bay but also from other ecosystems as well. While we can quantify, to a significant degree, loadings through inventories and monitoring, the effects on the ecosystem of those loadings remain unclear (as discussed in the final section of this report, under the heading of biological effects).

Over the past twenty years, discharges of contaminants from commercial and industrial areas have declined significantly, largely because of the Clean Water Act. At the same time, with increasing population in the Chesapeake Bay watershed, contaminants from diffuse sources in developed areas have been on the rise. An increasingly important area of investigation is the tracking of diffuse inputs, through small watersheds for example.

Question 2: How does the relative importance of the sources vary by different regions of the Bay?

Sources of contaminants vary widely throughout the Bay, even among classes - for example, while the highest loadings of PAHs are from atmospheric deposition, so are loadings of mercury. Generally, in the upper mainstem Bay, point sources - usually in urban and industrial areas - and riverine (fluvial) sources are the major sources of contaminants entering the estuary. In the lower mainstem Bay atmospheric deposition and other diffuse sources dominate. In the Bay's Regions of Concern (Baltimore harbor, Anacostia and Elizabeth rivers), the sources of toxic contaminants are in close proximity; in other areas, where "spikes" of contaminants have been detected, the sources are less clear.

Chesapeake Bay Salinity (Autumn)

The Chesapeake Bay may be a single ecosystem, but it com- prises regions that vary in salinity, temperature and tur- bidity. The head of the Bay is characterized by low salinity, the lower Bay by high salinity. The large middle Bay has a moderate (mesohaline) salinity. In the upper Bay sediments will be mostly silt and clay; in the lower Bay sediments will be sandy. Differingsediment types and degrees of salinity will affect how contaminants move through the estuary, and what chemical form they are likely to take.

Lower Salinity Regions Physical Characteristics Mostly silt & clay sediment Minuimum stratification Strong influence of freshwater inflow, wind & tides High particulate input from rivers High sedimentation & resuspension rates Ecosystem Characteristics High nutrients High turbidity Low primary production & phytoplankton density High bottom (benthic) abundance but low biological diversity Contaminant Characteristics High influence of rivers & point sources High metal burial Moderate cycling on the Bay floor (benthos) Documented biological effects	Moderate Salinity Regions Physical Characteristics Mostly silt & clay sediment High stratiification Strong influence of wind & tides High level of localized particulates Moderate sedimentation & resuspension rates Ecosystem Characteristics High spring nutrients Medium turbidity High primary production & phytoplankton density Intermediate benthic abundance & low biological diversity Contaminant Characteristics High influence of atmospheric & nonpoint sources Intermediate PCB movement (e.g., out of sedmients) Moderate metal burial Low cycling on the Bay floor (benthos) Biological effects uncertain	Higher Salinity Regions Physical Characteristics Mostly sandy sediment Low stratification Strong influence of wind & tides High level of localized particulates Low sedimentation but high resuspension rate Ecosystem Characteristics Moderate nutrients Medium turbidity Intermediate primary production & moderate phytoplankton density High benthic abundance & high biological diversity Contaminant Characteristics High influence of atmospheric & nonpoint sources High PCB movement (e.g., out of sediments) High metal burial Moderate cycling on the Bay floor (benthos) Biological effects uncertain

Sources of five representative contaminants:

• Copper. Loading from rivers are by far the most important, especially in the upper Bay. In the lower Bay, diffuse sources, such as atmospheric deposition and anti-fouling paint on boat hulls, are relatively more important.
  
  
• Mercury. Atmospheric deposition - largely from the burning of coal, but also from incineration, e.g., of batteries - appears to be the major source. Urban stormwater is also a significant source of this heavy metal.
  
  
• Atrazine. Runoff from farm fields is the most important source of this agricultural herbicide, used widely for control of annual broadleaf and grass weeds in corn, sorghum and other crops.
  
  
• PCBs (polychlorinated biphenyls). PCBs are persistent and appear to be widely spread throughout the Bay, although now banned from most uses. The current source for PCBs are the sediments where they are largely buried, as well as on-going recycling between air and water.
  
  
• PAHs (polycyclic aromatic hydrocarbons). Urban and industrial areas are the most significant source of these compounds. The most common vectors appear to be urban stormwater runoff and atmospheric deposition.

In short, understanding the relative importance of different sources depends upon an understanding of how varying classes of contaminants enter the estuary and how they behave.

Transport and Fate of Contaminants

Question 1: How do different processes influence the spatial distribution of chemical contaminants within the Bay?

To track contaminants, researchers generally use the concept of "partitioning" to contrast the amount of a material that is dissolved to that which is particle bound. Dissolved contaminants will stay within the water column, thus increasing the exposure to plankton and pelagic organisms. Contaminants bound to particles - either nonliving (sediment) or living (phytoplankton, bacteria, etc.) - are more likely to settle, where they may more directly impact bottom-dwelling organisms, such as oysters.

In other words, if compounds attach to particles, they will tend to follow the movement of sediments; if they do not, they will follow the water. Three major processes determine the movement of contaminants in the estuary: physical, chemical and biological.

Physical processes are the primary factors influencing spatial distribution of chemical contaminants in the Chesapeake Bay, with the movement of water and sediment providing the principal mechanism for transport. Winds, waves, currents, tidal actions and episodic events, such as storms and hurricanes, can cause major resuspension of bottom sediments and associated contaminants, and the frequency and intensity of these physical events will have a fundamental effect on residence time in any given area. Likewise, stratification and subsequent mixing will determine vertical, as well as horizontal, movement of toxicants, an important factor in a two-layered estuary like the Chesapeake Bay. The exact manner of sediment accumulation and contaminant flocculation will also affect how rapidly contaminants settle out and become buried.

Research (Sanford 1995) shows that the depth of erosion resulting from tidal resuspension in the mid-Bay is on the order of 0.1 to 1 mm thick. These estimated erosion depths are very similar to the thickness of the "floc layer" - the thin layer of material which has gathered on the Bay bottom and essentially lies on top of the more permanent sediment until moved or incorporated into the bottom. The movement and resuspension of sediments lengthens the time it takes for sediments to become buried, and Sanford and his colleagues estimate a burial delay time of several days to several weeks in the Chesapeake.

Sediments tend to become buried faster in areas where mixing is minimal, for instance, small coves. Near the Bay mouth, greater wave action and currents stir up the bottom, resulting in a continual "slow release" of sediment particles. Contaminants are not "sealed" in the sediments until they become buried and settle below the layer reached by burrowing organisms that can stir them up again (Swift 1995). Whether sediments serve, on average, as either sources or sinks of potentially toxic trace elements will therefore depend on their location - and on the physical, chemical and biological conditions there.

Chemical processes related to oxidation and reduction potential (redox) affect the behavior of contaminants in the Bay, for example, the sulfidic conditions in bottom waters devoid of oxygen (especially in the middle Bay). Chemical factors can affect the processes through which particles will join together (flocculation) and the rate of adsorption or desorption of dissolved compounds to either organic or inorganic surfaces. Together with oxygen levels, temperature and salinity also have significant effects on the behavior of chemical contaminants. Other factors include photodegradation (the effect of light on PAHs, for example), hydrolysis (when a compound reacts with water), or pH (acidity/basic) or EH (oxidative reduction or redox potential). Experiments have shown that the metals most strongly influenced by anoxia are manganese, zinc, nickel and lead.

The importance of these changes is that the association of contaminants with sediments or ions greatly influences transport and biological availability and thus the exposure to different kinds of organisms. For example, the release or sequestration of metals in sediments by reduction/oxidation processes effectively determines whether those metals will be found in the water column or become tied up with sediments.

Research (e.g., Cornwell et al. 1995; Riedel et al. 1995) has shown that dissolved oxygen levels can mediate the release of metals from sediments. Rising levels of oxygen cause the release of such metals as copper and zinc, therefore causing greater contaminant exposure to organisms in the water column (i.e., plankton and fish). Decreasing levels of oxygen (hypoxia or anoxia) cause those same metals to be bound in sediments, thus increasing exposure to bottom-dwelling organisms.

Biological processes such as uptake, metabolism, biotransformation, bioaccumulation, and trophic transfer (passage of a contaminant through the food web) can all affect the movement and form of chemical compounds. Some contaminants may become more toxic once processed by microbes - for example, one experiment showed that the toxic metabolite of a chlorinated hydrocarbon proved more persistent in sediment than did its parent compound (Capone et al. 1995) Other contaminants may become less toxic, as occurs with the methylation of arsenic by phytoplankton.

Bottom-dwelling organisms can have a dramatic physical effect on the release of contaminants from sediments through such physical mechanics as burrowing, drilling, filtering, aggregating or ejecting. Contaminants can be transported and affected through microbial transformation, through uptake and transfer through the food web, and through the movement and migration of fish and other large organisms. Phytoplankton blooms, for example, have been shown to concentrate and deposit metals on the Bay floor. Eutrophication and high phytoplankton biomass could result in high uptake of chemical contaminants in the water column, but once phytoplankton settle they will eventually expose benthic environments to whatever contaminants they have taken up.

Whether contaminants remain longer in the water column or are quickly removed to the bottom sediments will obviously influence the kind of effects toxic compounds will have on the ecosystem. Some research suggests that while algae and other organic matter may fall to the Bay floor, the process of decay occurs so rapidly that any contaminants bound to them will return to the water column - leaving little of the original contaminant by the time burial finally occurs.

The sequestering of contaminants by phytoplankton is intertwined with other management issues. For example, the Chesapeake Bay Program's 40-percent nutrient reduction goal may reduce phytoplankton to 1950s levels, but unless releases of chemical contaminants into the Bay are reduced in parallel, the relative toxic exposure will not lessen, and could even increase in some parts of the ecosystem.

In summary, research has made significant strides in understanding the influence of how each of these processes influences transport within the estuary. Detailed analyses of how these processes interact are essential if we are to make informed management decisions based on expectations of how the ecosystem will interact with various contaminants.

Question 2: Can scientists construct basic pathways of representative contaminants that include introduction, transport, uptake and their ultimate fate?

Research to date has gone a long way toward documenting the basic inputs, movement and final fate (in relative terms) of representative compounds.

The ways in which contaminants enter the food web will depend to a large extent on the level of the food web or the feeding mechanism. For example, experiments have shown that arsenic is more readily accumulated by an omnivore like grass shrimp feeding on plant material than by another omnivore like the mummichog feeding at higher levels in the food chain - the mummichog is apparently exposed to less readily available forms of arsenic (Sanders 1995).

That same research (Sanders 1995) also suggests that direct physical contact between fish and sediment during low tide apparently contributed to the uptake of contaminants (by mummichogs). Other recent studies have shown that the oyster is more likely to take up contaminants that are already bound in the phytoplankton on which it feeds, as opposed to contaminants bound to sediment particles, for example (Newell and Weston 1995).

Question 3: Are the biologically available chemical contaminants due to recent inputs or historical sources? What are the relative percentages of these sources?

Whether a contaminant threat is historical (e.g., compounds buried in the sediment) or recent will depend on the compound itself. For example, new inputs of lead have gone down dramatically, as a result of lead-free paints and lead-free gasoline. Similarly, with the significant decline of steel production and tighter controls on direct discharge over the last two decades, inputs of some heavy metals have also declined. Furthermore, DDT, a polychlorinated pesticide banned in the U.S. since 1972, primarily re-enters the Bay's ecosystem from sediments where it persists, though some atmospheric deposition also still occurs.

Other contaminants have become more prevalent since World War II, such as agricultural herbicides, pesticides and fertilizers, though in some cases their forms are less persistent. The early pesticides DDT and chlordane (as well as other compounds, such as the wood preservative penta-chlorophenol) proved to be resistant to degradation, with unanticipated effects on non-targeted organisms and ecosystems. Newer pesticides such as glyphosate degrade more rapidly with less effect on non-target species. Unfortunately, recent studies show that the chemical Dimilin, used to control Gypsy moth infestations, can have harmful effects on certain crustacea, such as copepods.

Though difficult to measure, household chemicals, from solvents and cleaners to garden pesticides and herbicides, have also increased dramatically along with rapid population growth in the region. Larger populations also mean more combustion products arrive in Bay waters from automobile and truck engines (even though cleaner burning) and from boats, lawnmowers and machines using two-cycle engines, which are relatively inefficient in the combustion of fuel.

At the ecosystem level, the sources and relative significance of historical versus recent contaminant inputs depend upon a number of factors, such as location (upper Bay versus lower Bay, urban versus rural areas) and mode of delivery (atmospheric versus riverine, point source versus diffuse source). Overall, it appears that in the Chesapeake Bay regulatory controls have reduced the importance of point sources relative to nonpoint sources. Ultimately, the significance to the Bay's ecosystem and food web is determined by the biological niche and the feeding mechanisms and behaviors of individual organisms and communities of organisms.

Question 4: What is the chemical contaminant memory - the "residence time" of toxicants - in the Bay?

How long will contaminants remain potentially active or available, once they have settled in the Bay? In other words, what is their residence time? The answer is that residence time depends on a number of factors, including:

• Location and removal rates from burial, degradation, dilution and mixing.
  
  
• Chemical structure of the toxic compound.
  
  
• Chemical characteristics of the surrounding environment (e.g., oxygenated or anoxic waters and sediments).
  
  
• Natural mechanisms such as storm events, hurricanes and strong currents, snow melt and precipitation.
  
  
• Exporting of contaminants into the Atlantic as a result of net movement of low-salinity river water; or importing of contaminants due to intrusion of dense, high-salinity ocean water.
  
  
• Anthropogenic activities, including dredging (where timing and controlling depth will be important variables), and natural activities of benthic organisms burrowing in sediments, which can contribute to the resuspension and regeneration of contaminants. Only beneath levels affected by these physical forces will contaminants be considered "permanently" buried.

Contaminants will obviously "disappear" from the ecosystem more quickly if they degrade relatively rapidly. Some contaminants - PCBs are notorious examples - persist, while others such as Atrazine are designed to degrade once in the open environment. Compounds can also be considered "removed" when they join to form more stable complexes. There are diagenic reactions that can tie up inorganics; and there are similar reactions that occur with organic compounds as well.

Question 5: What have been the effects of management actions on chemical contaminant loadings and transport processes in the Chesapeake Bay?

Federal and state laws have served as the basis for reducing point-source discharges of numerous chemical contaminants and metals, among them, PCBs, DDT and lead. Tributyltin (TBT) concentrations in the Bay, for example, have dropped dramatically since TBT's use as an antifoulant was restricted in the late 1980s. The annual reporting by industry of direct discharges and the publication of Toxic Reduction Inventories (TRIs) have helped to document these declines. Of course nutrients have likewise been targeted in the Bay: phosphorus discharges from waste treatment plants, for example, have decreased as a result of banning phosphorus-based detergents, while nitrogen discharges have essentially been held constant because of targeted facilities upgrades.

Even with regulation, however, reducing contaminant loading (and nutrients as well) from diffuse sources continues to have mixed success. On the one hand, traces of lead from automobile combustion have decreased significantly, as monitored in sediments; on the other hand, hydrocarbons such as PAHs have increased because of growing population and inputs of hydrocarbons from fossil fuel combustion - from automobile exhaust, for example.

Management actions have been addressing the wide-ranging effects of increased human population in the watershed and airshed, with their concomitant deforestation, dredging, sewage effluent and stormwater runoff - all of which increase contaminant inputs to the estuary. Each of these factors has a strong role in the transport of toxics:

• Deforestation increases the flow of sediments into the estuary and leads to more pronounced stratification as stronger pulses of freshwater pour over heavier saltier waters.
  
  
• Dramatic declines in underwater grasses and the reduction of natural wetlands have had profound effects on the way the ecosystem traps and processes toxic compounds.
  
  
• Dredging of channels resuspends potential contaminants but also changes physical circulation patterns in the Bay, often with unknown effects.
  
  
• Impervious surfaces, from roofs to roads to parking lots, gather and propel rain water and snow melt into gutters and storm drains. This storm water carries contaminants - such as hydrocarbons, pesticides and metals - directly to the Bay's tributaries.

Agricultural practices can play a significant role in the transport of soil and contaminants into the estuary. While soil conservation and Best Management Practices (BMPs) are helping to decrease the movement of animal wastes and soil - which becomes sediment - into the Bay, different practices affect not only rates of input of contaminants bound to sediment particles, but also rates of burial (by new sediment) on the Bay bottom. For instance, farming practices that are beneficial for one purpose may have negative tradeoffs - as an example, no-till farming may decrease surface runoff in one area, but could also lead to more contaminants seeping into ground water, since no-till farming depends on the use of such herbicides as Atrazine and Simazine to control weed growth. BMPs may need to be assessed on a region by region basis to optimize their benefits.

To understand such relationships between land use and impacts to the Bay, research will need to continue to uncover the ways that living organisms mediate toxic contaminants. With increasing knowledge in this area, managers will know more precisely what actions could potentially modify the current pathways that contaminants follow through the ecosystem. This issue presents a series of difficult scientific challenges, as described in the following section.

Biological Effects of Contaminant Exposure

Question 1: What factors influence the bioavailability of chemical contaminants?

A number of conditions affect bioavailability of contaminants, though four in particular are extremely important:

• chemical species or form (often affected by environment)
• environment (e.g., salinity, pH, dissolved oxygen concentration)
• physiology of the organism
• ecological structure (e.g., whether benthic or pelagic)

Chemical species of a compound will determine whether it is more or less available for uptake by living organisms. For example, copper used in antifouling paint is Cu2+; it is very reactive and highly toxic to many living organisms, hence its effectiveness against fouling communities; other forms of copper may be less toxic.

Some organics such as PCBs have molecular compositions and structures that remain stable and resistant to enzymatic action under a range of environmental conditions. These compounds are therefore extremely persistent and will accumulate in fatty tissues of organisms, with possible genetic and carcinogenic effects.

Furthermore, the source of a chemical contaminant - for example, whether it is "pyrogenic," resulting from the combustion of fossil fuels, or "petrogenic," resulting from unburned petroleum products - will affect its form and therefore its toxicity. Generally PAHs from combustion are bound within soot or fly ash particles and are, in effect, biologically less available. Petrogenic PAHs, on the other hand, are more likely found on the surface of particles and are therefore potentially more available.

Environmental conditions such as salinity, pH, redox potential and other ambient factors, will also affect the bioavailability of toxic compounds, but in ways that cannot easily be generalized. Salinity, for example, reacts with contaminants in several different ways. First, it acts to complex metals, making them less available for uptake by Bay organisms - this is why copper becomes less available in the water column as one moves from the head of the Bay toward its mouth. Second, salinity can affect various physiological mechanisms in an organism (movement across cell membranes, for example); and it can affect an organism's biological functioning, influencing how it may respond to the presence of contaminants.

In oysters, higher salinities "improve" physiological functioning, such as increased feeding rates, thus affecting the rate of metal uptake. Some studies have indicated that oysters can take in and then expel contaminants - in pseudofeces, for example, by gathering chemicals in the gametes and then expelling them during spawning. Precisely how oysters in any particular location will be affected by contaminants under varying environmental conditions will depend on an interaction of factors, such as those described above.

Physiological differences of specific organisms can amplify environmental effects. For example, mussels and oysters will take up metals at different rates, even after accounting for differing filtration capacity. In addition to feeding rates, the rate at which organisms metabolize chemical contaminants also affects toxicity, as does the distribution of a metal or toxicant within the body of an organism. PCBs are sequestered in the fatty tissue of fish, for example, potentially making the fish unsafe to eat, but not necessarily causing direct illness or mortality in the fish itself. Likewise, the binding of metals to metal-binding proteins or within granules in the tissues of organisms can serve as an effective defense against a contaminant's toxicity.

Ecological structure affects how contaminants move through the food web remains key to understanding how some compounds behave in particular parts of the carbon cycle. As mentioned earlier, dissolved contaminants tend to stay in the water column, thus increasing the exposure to plankton and pelagic organisms. Contaminants bound to particles are more likely to settle, where they can more directly impact bottom-dwelling organisms.

Question 2: Can researchers quantify transfers of chemical contaminants through the food web - both in the water column (planktonic) and along the Bay bottom (benthic)?

Scientists have been developing the ability to quantify the transfer of toxic compounds through Bay food webs. Such measurements are important because they can demonstrate how contaminants accumulate at different trophic levels in the food chain. Studies of phytoplankton blooms in the Patuxent River, for example, have indicated rates at which copper uptake by phytoplankton is transferred to copepods that graze on them (Sanders and Sellner 1995).

Grazing by copepods and other zooplankton is extremely important in the Chesapeake for several reasons: at certain times of the year the zooplankton community removes the bulk of phytoplankton production; at the same time, zooplankton serve as prey for other organisms, including anchovies, which are prey to a variety of food fish, among them, striped bass. Knowing the transfer rates of contaminants as they are processed through food webs and in recycling processes could make possible more effective risk-based predictions of the toxicity of contaminants in the Chesapeake.

Contaminants transfers through the food web have been documented by a number of recent studies.

• Rates at which copper appears to be transferred fromtoh phytoplankton to the copepods that graze on them have been determined (Sanders ans Sellers 1995). These studies also suggest that some elements such as cadmium are more likely to be absorbed by grazers directly from the water.
• Oysters accumulate metals in varying amounts, according to changes in environmental conditions (Roesijadi 1996). Concentrations of metals are influenced by such environmental and biological factors as temperature, salinity and changes in tissue composition as a function of season and stage of reproductive cycle.
  
  
• Mercury uptake, transfer and bioaccumulation occur in the water column, with uptake by phytoplankton and transfer to copepods a key step in entering the food web (Wright 1996).

Quantifying the movement of contaminants through the food web thus depends on models that can enumerate important components of the Bay's ecosystem. For example, the Chesapeake Bay is dominated by small zooplankton, which excrete dissolved or very fine particles. These waste products can remain suspended for long periods of time, and easily re-enter the food web. In systems dominated by large zooplankton copepods, waste products may take the form of heavier fecal pellets that settle out of the water column. Likewise, systems dominated by large filter feeders such as oysters also concentrate waste products that are more likely to settle out of suspension.

Quantification of contaminants in the bottom (benthic) community likewise depends on understanding transfer mechanisms. Research on phytoplankton uptake of trace elements in the Patuxent River found increased levels of some elements on the river bottom that had apparently settled out of blooms (Sanders and Sellner 1995). With studies by Harding and others (1992) that have documented the timing and location of phytoplankton blooms and of certain hydrologic events (e.g., the spring freshet and the fall "turnover" of the Bay) we can begin to quantify the delivery of phytoplankton potentially ladened with contaminants to the benthos.

Question 3: What evidence is there that chemical contaminants at concentrations observed in the designated "Regions of Concern" impact living resources?

Acute and sublethal biological effects of contaminants have been observed in the field and in laboratory experiments. In the Anacostia and Elizabeth rivers and Baltimore harbor, for instance, fish and other organisms have exhibited lesions and cancers due to toxic concentrations of contaminants. In the Patapsco River, shifts have occurred in populations and in community structures of organisms there.

For some time we have known that oysters can serve as sensitive indicators of metal contamination in a region, and more than thirty years ago Galtsoff (1964) observed higher concentrations of copper, lead and iron in oysters located near areas of industrialization. Researchers have now shown that contaminants can impact oysters in subtle ways. Laboratory studies on Crassostrea virginica's vulnerability to Dermo disease (Perkisus marinus) give strong indications that the oyster's immune system is impaired by chemical contaminants. In the Elizabeth River, for example, scientists (Chu et al. 1995) demonstrated that PAHs (polycyclic aromatic hydrocarbons) rendered oysters more vulnerable to Dermo. Scientists studying the effects of chemicals such as TBT (tributyltin) (Anderson et al. 1995) and the chemical carcinogen DEN (n-nitrosodiethylamine) (Winstead and Couch 1988) have found similar reactions. Although these findings are generally based on laboratory studies, contaminant concentrations employed in these experiments are environmentally realistic, especially within the Regions of Concern, but also elsewhere in the Bay (see below).

Question 4: What evidence is there that chemical contaminants at low concentrations impact living resources?

A number of reports have documented the biological effects of ambient toxicity in waters of the Chesapeake Bay; they include shellfish (Bender and Huggett 1987), striped bass (Hall et al. 1985 and 1989), blue back herring and the American shad (Klauda et al. 1988). Studies conducted by Lenwood Hall et al. (1995) used larval sheepshead minnow, larval grass shrimp, a copepod and the coot clam to test water from nine ambient stations in the Chesapeake over a two-year period. Results showed toxic effects at most stations, including reduced growth or mortality of sheepshead minnow or copepods during one year. Minimal effects were observed during the subsequent year of testing, suggesting that ambient effects are subject to considerable annual as well as spatial variation.

A number of research studies have uncovered measurable biological reaction to low concentrations of contaminants - as previously discussed, trace levels of arsenic can cause a shift toward species of smaller phytoplankton, along with uptake and transfer into the food web. While the trophic effects of these transfers remain to be determined, clearly the potential exists for accumulation through the food chain.

Changes in phytoplankton community structure and changes in size of dominant phytoplankton species can have a profound effect on aquatic ecosystems. One classic study, for example, in Great South Bay (Ryther et al. 1954) documented a shift in phytoplankton size toward smaller cells (nannoplankton) as a result of eutrophication. These small plankton were not suitable food for filter feeders and resulted in a decline of the oyster populations there.

Sublethal effects, then, do not necessarily reveal themselves to the eye or even to the microscope - a contaminant or multiple contaminants can affect organisms at the cellular level, can lead to molecular deterioration of a species' immune system or, with other factors, can affect gene mutation in another species' reproductive system. By the time sublethal effects do become evident, it is because their impacts have already cascaded through the food web.

Using molecular biological methods, researchers (Kramer et al. 1995) are now exploring new ways to identify subtle sublethal effects. By developing DNA probes, these scientists hope to find genetic signals that will reveal whether or not an organism (such as phytoplankton) has been exposed to metals. They will do this by tracking genes responsible for triggering the production of metalothienin - a clear signal that an organism's genetic mechanism is mounting a response to metals in the environment.

Question 5: Given the observed spatial distribution of chemical contaminants and our understanding of the influence of processes on contaminant distribution and fate, are there regions of the Bay where (1) we can predict that effects are likely to occur and (2) we should focus research efforts?

Clearly there are regions of the Bay where we can predict that contaminants are likely to have biological effects:

• Most obvious are the identified Regions of Concern (Baltimore Harbor, Anacostia River and Elizabeth River). As previously emphasized in this report, organisms in these regions have shown demonstrable effects, such as lesions or tumors, and more indirect effects, such as shifts in benthic community composition.
  
  
• Resource managers already target certain areas as being at risk, given some knowledge of the vulnerability of particular organisms to particular toxicants. One example is the management of chlorine discharges from waste treatment plants in fish spawning areas.

More difficult to define are other target areas where contaminants could pose serious, if less obvious problems:

• Studies on areas outside the Regions of Concern - specifically as reflected in the Ambient Toxicity Study supported by the Chesapeake Bay Program - have revealed unexpected levels of ambient toxicity. While highly suggestive of potential problems, these results require additional analysis to determine their precise replicability and the full significance of detected levels.
  
  
• If areas and times of predictable "pulses" (e.g., resulting from the seasonal application of herbicides or pesticides) can be identified, then it may be possible to describe "danger zones" and "danger times" for some species and some contaminants. Dimilin, shown to affect survival and reproduction of crustaceans, especially at lower salinities, is an example - its application could potentially be better managed to reduce its impact on species that are especially vulnerable.

To apply predictive principles on a much broader basis, managers would need to know such factors as:

• Depositional rates, especially of fine-grained sediments, and to identify areas where there are high flux rates. Considerable information is already available for some regions of the Bay.

The effects of multiple contaminants are difficult to track. Much of what we know about the ecological impacts of toxic compounds is based on observing responses or organisms to acute exposure of sinlge contaminant. Tracking sublethal responses of an organism to mulitple contaminants is more complicated because of the network of physical, chemical and biological interactions; impacts at higher trophis levels may be obscured by the passge of a contaminant (or range of contaminants) through hierarchal levels of the ecosystem. The most difficult scientific challenge will therefore be to understand the extent and nature of interactions between contaminants -- singly and combined -- and the various physical and chemical components of the environment.

• The relation between the effects of single contaminants (which are more easily tracked) and the effects of contaminants in possible combinations - where synergies could result in toxic impacts on organisms.
  
  
• Other broad interactions, such as those at the air/water interface.

The next steps for determining which regions (or zones, such as depositional areas or areas of high productivity) include learning how low-level contaminants may have system-wide effects on living organisms in various parts of the Bay at hierarchical levels. Specifically, future research must:

• Continue to focus on learning to predict where effects are likely to occur, given our understanding of the distribution of chemical contaminants and their behavior in the estuary. Such prediction can be difficult, especially in light of counter-intuitive data, for instance, the unexpected sediment contamination spikes in some relatively clean Eastern Shore tributaries.
  
  
• Build on research that has shown direct effects on benthic communities from chronic, low-level toxicants, such as wood preservatives.
  
  
• Develop methodologies to apply findings from other estuarine systems, especially in the mid-Atlantic - for example, in the Hudson River, Raritan Bay, Delaware Bay, and Pamlico Sound - to local situations.

It will be important to evaluate a range of distinct ecosystems in the Bay to determine if specific keystone species are missing or impacted. These keystone species play a central role in the healthy functioning of a food web. Already, research has documented changes in phytoplankton communities that could profoundly influence organisms higher up the food chain. Further research should help clarify just how significant these changes, brought about by metals and other contaminants, relate to the ecological functioning of the Chesapeake Bay.

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