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.
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.
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.
The Chesapeake Bay may be
Lower Salinity Regions
Mostly silt & clay sediment
High influence of rivers & point sources
Moderate Salinity Regions
Mostly silt & clay sediment
High spring nutrients
High influence of atmospheric & nonpoint sources
Higher Salinity Regions
Mostly sandy sediment
High influence of atmospheric & nonpoint sources
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.
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.
Winds and tides,accoding to recent studies (Sanford et al. 1995), affect the scouring of bottom sediments and the movement of contaminants. This research shows that:
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.
Phytoplankton bloomscan concentrate metals and deposit them on the bottom or pass them directly up the food chain. In the Chesapeake Bay the predominant grazers appear to be very small zooplankton that release small fecal pellets. Those pellets will likely remain suspended for long periods of time, carrying any particle-bound contaminants that have attached to them. Systems dominated by larger zooplankton or benthic filter feeders (as the Bay once was) favor shorter suspension times for particulate contaminants.
Schaffner and Gammish (Virginia Institute of Marine Science, unreleased video) have visually documented the release of sediment into the water column by tube worms and other benthic organisms in the York River and lower Bay, where strong tidal currents spread the material and prevent rapid settlement.
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.
Research to date has gone a long way toward documenting the basic inputs, movement and final fate (in relative terms) of representative compounds.
Entry of contaminants in the food web is often determined by chemical speciation and metabolic conversion of contaminants. Studies in the Chespeake Bay have found differential toxicities of metabolities of Atrzine and HOCs (halogenated organic compounds), and it appears likely that speciation is a moajor factor not only in food web entry but also in trophic transfer and ecological effects (Jones et al. 1984, Cappone 1995). Chespeake this certainly appears to be the case for mercury, one of the most toxic chemicals on the Chesapeake Toxics of Concern list.
Some scientists have suggested that mercury concentration in fish is a direct consequence of the environmental factors determining the species of mercury (Mason and Morgan 1996). In the estuary, mercury is converted in part to methylmercury through sulfate-reducing bacteria. Uptake appears to be determined by the amount of mercury in uncharged lipid soluble forms, particularly forms combined with chloride. Both pH and salinity appear important in speciation and accumulation.
Studies elsewhere have also demonstrated that methylated forms of mercury were more toxic than inorganic mercury to certain marine diatoms, supporting the hypothesis that methylmercury is more easily taken up and is therefore more toxic to marine organisms. The critical step in accumulation of mercury occurs at the base of the food web (e.g., through phytoplankton).
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).
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.
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:
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.
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:
|Management actions in many parts of the watershed have led to improvements in controlling vast pulses of stormwater, though a significant problem remains throughout the Chesapeake watershed, especially in developed areas constructed before the advent of new stormwater retention technologies. Retrofitting of outdated systems poses a major engineering anf inancial challenge for the region's municipalities.|
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.
Carbon cycling in the Bay, clearly affected by the input of nutrients into the Bay from farm fields or waste treatment plants, has a significant impact on the transfer of contaminants. Overnutrification has led to an enhancement of phytoplankton, and perhaps a shunting from higher organisms to microbial production, which alters the flow of energy or carbon. This carbon flow in turn affects the way contaminants are cycled in the Bay, especially as food webs shift, varying pathways through which may travel.
Because of the importance of the Bay's food web to the cycling of both nutrients and toxic chemicals, ecosysem-based fisheries management also becomes important. The loss of the Bay's oyster population, for example, appears to have had a significant effect on the ecosystem's ability to filter large populations of phytoplankton from the water column. In this sense, oysters are not merely resources but habitat itself. Just what the large-scale effecrs of the loss of this habitat has meant for benthic populations and the cascading network of food web relationshipd is mostly a matter of speculation, though studies are now underway to sort through these effects. Currently, the relation between fisheries and natural resources management and the behavior of contaminants in the Bay remains unclear, since many specifics about interactions between toxic compounds and estuarine organisms are still not known.
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.
A number of conditions affect bioavailability of contaminants, though four in particular are extremely important:
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.
Complexation of contaminants by carbon compounds represents an important "sequestering" process in the estuary. In general, this complexation dramatically reduces the bioavailability of metals for uptake by organisms, and reduces many organic contaminants as well, although the data are not clear cut. For example, well over 90 percent of the copper and 60-70 percent of the cadium in the Chesapeake Bay (Donat 1995) are complexed by naturally occuring organics.
Some research (Wright 1996) has shown that mercury toxicity is reduced with increasing salinity, and other experiments (Reidel 1988) have found that chromium, which is highly toxic at low salinities, is less toxic at 5-10 ppt salinities of the middle Patuxent River. One mechanism for this mitigating effect of salinity is the presence of sulfate, a major component or saline waters, which often interferes with and slows the uptake of chromium by phytoplankton.
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.
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.
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.
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).
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.
Clearly there are regions of the Bay where we can predict that contaminants are likely to have biological effects:
More difficult to define are other target areas where contaminants could pose serious, if less obvious problems:
To apply predictive principles on a much broader basis, managers would need to know such factors as:
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 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:
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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