Influence of taxonomy, ecology, and seasonality in river stage fish contamination risks in floodplain swamps of the lower Mississippi river

We compared contamination levels in fish from contaminated and uncontaminat ed floodplain swamps of the lower Mississippi River to assess differences i n contamination risks between swamps, across different taxonomic and ecolog ical groupings of fishes within and between swamps, and with seasonality in river stage. Fish tissue levels of inorganic contaminants were substantial ly lower than environmental levels in both swamps, suggesting either that f ish were not uptaking these contaminants, or they were effectively eliminat ing the contaminants from their bodies. Tissue levels of organic contaminan ts were high relative to environmental levels, suggesting that these contam inants were bioaccumulating. Organic contaminants were significantly higher in fish from the contaminated swamp (Devil's Swamp) than in fish from a re ference swamp up river (Tunica Swamp). Because the organic contaminants wer e largely confined to sediments, we expected bottom-oriented fishes to have higher concentrations than pelagic fishes. Assuming that uptake was primar ily through the food chain, we expected top predators to exhibit higher con centrations than low-level consumers. We also expected year-round swamp res idents to exhibit higher accumulations than more transitory users of backsw amp habitat. However, organic contaminant levels did not differ in the dire ctions expected for any of these groupings. We did observe differences in o rganic contaminant levels within and between swamps for different taxonomic groupings of fishes (species and genera). Some taxa occupying low to middl e positions in the food web (e.g., gizzard shad Lepomis spp.) exhibited hig her concentrations than taxa near the top of the food web. Within Devil's S wamp, organic contaminant levels were significantly higher at low river sta ge, when fish were confined to the swamp, than at high river stage, when fi sh were free to move between the river and the swamp. We caught more specie s and more fish per unit effort in Devil's Swamp than in Tunica Swamp, cont rary to expectations if contaminants in the former were negatively impactin g population and community structure. Species richness differences between swamps were a consequence of catch differences, with higher catch correspon ding to inclusion of more rare species. The lower catch in Tunica Swamp may have resulted from physical modifications of its waterways to support agri culture and hunting. The results of this study underscore the importance in factoring information on the taxonomy and ecology of organisms, and season al changes in environmental conditions, into assessments of contamination r isks.