Among the numerous organic compounds of potential concern in the Chesapeake Bay are Atrazine and Dimilin, each engineered for a very different purpose, and each an interesting case of how toxicants can enter the estuary and how they behave.

Atrazine. Atrazine is a broad spectrum herbicide used widely in the cultivation of corn and sorghum. It has been estimated that about 1% of Atrazine applied to the Chesapeake Bay watershed reaches the aquatic environment (Kemp et al. 1981; Wu, 1980) and although Atrazine levels sufficient to inhibit various species of submerged aquatic vegetation do occur in localized areas, it does not appear that Atrazine has been the major cause of SAV decline in the Chesapeake. In fact, Solomon et al. (1996) conclude in an extensive review that Atrazine does not pose a signigicant risk to the aquatic environment, although transient inhibitory effects may occur in phytoplankton and aquatic vegetation in areas suseptible to agricultural runoff.

There are, however, reasons to be concerned. For example, Atrazine has a differential effect on varying species of submerged grasses, and could perhaps shift species composition in areas where it reaches the Bay or its tributaries in significant quantities. Research has shown, for example, that water milfoil (Myriophyllum spicatum)� a less desirable grass than, say, wild celery, which is important food for diving ducks � proved more resistant to Atrazine than several other species of Bay grass (specifically, Potomageton perfoliatus, Ruppia maritima, and Zannichellia palustris) (Jones et al. 1984).

Atrazine also has the potential to enter the Bay in pulses, arriving in spring, when applied to farm fields, and when Bay grasses are beginning their spring growth. Atrazine may also reach the Bay in rural, otherwise pristine areas that could be important habitat, for molting blue crabs, for example.

Finally, there is the potential for interaction with environmental variables, resulting, for example, in the formation of various metabolites (such as deethylated, deisopropylated and hydroxyatrazine metabolites). These metabolites, which result from biological breakdown and conversion occurring after application of the herbicide, may have increased or reduced toxicity, depending on their chemical structure. We know that Atrazine and its metabolites, if they enter the water in sufficient quantities, can have a deleterious effect on underwater grasses, and could therefore be important components of the chronic toxics burden in the Chesapeake � whose combined effects we are just beginning to understand.

Dimilin. Dimilin is a pesticide (a growth inhibitor) used in parts of the Chesapeake region to control the gypsy moth. Dimilin's mechanism of action is to inhibit the insect molting process through impairment of chitin incorporation. Because of its specificity, Dimilin is not a concern to organisms that do not have a chitinous exoskeleton. Crustaceans, however, also have such exoskeletons, and Dimilin appears to affect them in the same way it affects insects. Studies conducted by Savitz et al., (1994), demonstrate that Dimilin is toxic to the copepod Eurytemora affinis at low, environmentally realistic concentrations. Crustaceans are important and abundant components of the Chesapeake food web including, in addition to copepod species, amphipods, shrimp and crabs -- including the commercially important blue crab.

Dimilin also offers an example of how an organic contaminant might interact with environmental variables. Wright and Dawson (1995) show that survival of E. affinis exposed to Dimilin concentrations up to 1 ug/ liter was enhanced at higher (15 ppt) salinities. These observations suggest that Dimilin toxicity to crustaceans is of greater concern in the upper, low-salinity portions of the Bay and the low-salinity portions of the tributaries of the lower Chesapeake, which includes major finfish nursery areas where copepods are importand food for juvenile fish. As with other organic contaminants, Dimilin is also converted into various metabolites through biological, physical or chemical activity. Dimilin's breakdown products include 4-chlorophenylurea, 2,6-difluorobenzoic acid, and 4-chloroaniline.

Dimilin use appears most likely to occur later in the season than Atrazine, which is applied early in spring. But like Atrazine, Dimilin, which is released in dissolved or particulate form from forests treated for gypsy moth infestation, has the potential to be released in strong pulses in response to rainfall.

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