Origin of Elevated Mercury Concentrations in Fish from Eastern Florida Bay

Topical Area: Toxins and Contaminants

David W. Evans, David W. Engel, and Peter H. Crumley, NOAA, National Marine Fisheries Service, Beaufort, NC

Advisories are posted in Florida Bay warning of elevated levels of mercury in upper trophic level fish. The Florida Department of Environmental Protection (FDEP) observed mean mercury concentrations exceeding 0.5 ppm in crevalle jack (Caranx hippos) and spotted seatrout (Cynosicon nebulosos) from the eastern part of the bay, but much lower concentration west of Madeira Bay. These fish, sampled near the mainland Everglades suggest that freshwater draining through Taylor Slough and C111 canal contributes to the elevated concentrations of mercury accumulated in these fish. Our research is attempting to determine if the Everglades is an important mercury source in three ways. First, we employ forage fish as sentinels of spatial and temporal patterns of mercury bioaccumulation indicative of an Everglades= source. Second, we measure both mercury and stable isotopes of carbon, nitrogen, and sulfur in an array of food web organisms supporting upper trophic level fish to determine trophic pathways of mercury accumulation. Third, we plan to determine the stable carbon isotope signature of methyl mercury extracted from Florida Bay and Everglades fish to test possible common origins.

We have established seven sites (Shell Key, Little Madeira Bay, Nest Key, Russell Key, West Key, Roscoe Key, and Palm Key Basin) where the forage fish Anchoa mitchilli (bay anchovy), Lucania parva (rainwater killifish), and Eucinostomus gula (silver jenny) are sampled quarterly for mercury analysis. The sites in eastern, central and western Florida Bay cover a range of distances from presumptive mercury sources in the eastern area of the Northern [Everglades] Transition Zone. Additional sites were sampled through collaboration with J. Colvocoresses of FDEP and NMFS colleagues at Beaufort (Hoss, Thayer, Powell, Peters, and Hettler). We hypothesize that, given an Everglades source, a gradient in mercury concentrations in forage fish should emanate outward from this source. In addition, mercury concentrations should show seasonal cycles associated with freshwater inflows. Finally, we predict that the most pelagic feeding forage fish, Anchoa mitchilli, should most clearly reflect these patterns since it depends most on the water column food web into which imported waterborne Everglades= mercury would be introduced.

In more than 300 analyses of individual fish beginning in September 1995, we have found that mercury concentrations are substantially higher in forage fish from the eastern bay compared to the western bay:











Anchoa mitchilli



Lucania parva



Eucinostomus gula



While seasonal and spatial pattern details await additional analyses, we already observe a significant inverse relationship between salinity and mercury concentrations in Anchoa mitchilli indicative of a possible freshwater source of mercury in northeastern Florida Bay. As hypothesized, anchovies have the highest mercury concentrations among the three forage fish species and the strongest inverse relationship to salinity.

A broad range of fish, invertebrate, and plant species were collected in June 1996 and September 1997 for both mercury and stable C, N, and S analyses. Additional sampling is planned for April/May 1998. We expect to obtain additional critical samples of upper trophic level fish, especially spotted seatrout, from Robert Brock of Everglades National Park. Isotope analyses will be used to better define pathways leading to upper trophic level species and to characterize nutritional sources in terms of benthic/pelagic, terrestrial/estuarine, and trophic level effects. These isotope data will complement the work of Koenig, Chanton, Coleman, and Grimes, APast and Present Trophic Structure of Florida Seagrass Communities and the Relationship to Associated Coastal Fisheries: Diet and Stable Isotope Analyses@ with whom we are cooperating in sample collection and analysis. By measuring mercury concentrations as well, we hope to better identify the sources and trophic pathways by which mercury is biomagnified to concentrations of concern in upper trophic level species. Bioenergetics, mercury assimilation and excretion kinetics, and food web pathways will be integrated in a predictive model of mercury bioaccumulation.

Finally, we expect to make a definitive test of the Everglades mercury source hypothesis by compound specific stable carbon isotope analysis of methyl mercury extracted from upper trophic level fish from eastern Florida Bay and from freshwater fish collected from the Taylor Slough and C111 canal. If methyl mercury is conserved in transit from the Everglades into Florida Bay and bioaccumulated into fish, then the stable carbon isotope ratios of the methyl carbon of methyl mercury should be similar in both groups of fish.