Principal Investigator(s):

David W. Evans and David W. Engel
NOAA/NMFS Southeast Fisheries Science Center
Beaufort Laboratory
September 1998

Health advisories are posted in northern Florida Bay warning of elevated levels of mercury in some upper trophic level fish. Spotted seatrout (Cynoscion nebulosus), crevalle jack (Caranx hippo), ladyfish (Elops saurus), bluefish (Pomatomus saltatrix) and gafftopsail catfish (Bagre marinus) have been found with mean mercury concentrations greater than the Florida advisory level of 0.5 ppm (wet weight). The origin of elevated mercury concentrations may lie in both external or internal sources of methyl mercury and in the food web dependent processes of bioaccumulation and biomagnification. We have sought to unravel the origin of these elevated mercury levels through 1) spatial patterns of mercury concentrations in biota, 2) temporal patterns associated with seasonal inputs of freshwater from the Everglades, and 3) coupling patterns of mercury accumulation in biota with food web structure as revealed by stable carbon, nitrogen, and sulfur isotope analyses.

We have sampled biota since December 1995 from throughout Florida Bay for mercury and stable isotope analysis. Initial efforts have focussed on three species of forage fish: bay anchovies (Anchoa mitchilli), rainwater killifish (Lucania parva), and mojarra (Eucinostomus gula). These three species are pelagic, seagrass canopy, and benthic feeders, respectively. They are important prey species of fish and avian predators, and they are important linkages of mercury transfer through the food web. During the past fiscal year, we have made six sampling trips for these and other fish species, common invertebrates, and primary producers such as seagrasses, macroalgae, and mangroves. Additional samples of top predator fish have been acquired through cooperators at Florida’s Marine Research Institute. We expect additional samples from Robert Brock, Everglades National Park and Geoff Scott, NOAA’s National Ocean Service.

Highest mercury concentrations are observed in fish from the eastern half of Florida Bay, the region most influenced by a potential methyl mercury source in freshwater runoff from the Everglades. This pattern is observed for two top predators, spotted seatrout and red drum (Sciaenops ocellatus), the most sought after recreational species in the bay, and for the three targeted forage fish species. Concentrations of mercury in spotted seatrout almost always exceed the 0.5 ppm advisory level in fish from the eastern bay (Figure 1). Because of biomagnification processes, predators are expected to have about five times the mercury concentration of their prey. Mercury concentrations in bay anchovies are consistent with this expectation, exceeding 0.1 ppm throughout most of eastern Florida Bay (Figure 1). Rarely are such concentrations exceeded in the western half of the bay.

The pattern of mercury concentrations among pelagic and benthic feeding species are expected to reflect the relative importance of the water column and sediments as proximate sources in mercury contamination. In this regard, Florida Bay contrasts with a Superfund site at Lavaca Bay, Texas where historic mercury contamination resides in bottom sediments. At the latter site, benthic feeding red drum exhibit higher mercury concentrations in comparison to more pelagic feeding spotted seatrout (Figure 2). Conversely, mercury concentrations in spotted seatrout in eastern Florida Bay exceed those in red drum. In addition, pelagic feeding anchovies in Florida Bay have higher mercury concentrations than benthic feeding mojarra or killifish feeding in the seagrass canopy (Figure 3). We conclude that in eastern Florida Bay, the pelagic food web is preferentially contaminated with mercury, thereby implicating a water column source.

We hypothesize that freshwater input from the Everglades (where mercury contamination in fish is well documented) carries methyl mercury into the eastern Florida Bay habitat. Temporal patterns in mercury concentrations in short-lived forage fish will be compared to seasonal freshwater inputs to the eastern bay. Covariance between mercury concentrations and salinity would implicate a mercury source from the Everglades. We continue to collect and analyze these species in order to establish this time series through several seasonal cycles of freshwater flow.

Food web and other ecological characteristics are also critical in determining mercury bioaccumulation in Florida Bay. Highly calcareous sediments in the bay, with relatively low organic content should enhance bioavailability of introduced methyl mercury by limiting binding to sediments. Restricted circulation should limit dilution. Oligotrophy in the eastern bay, including lower seagrass abundance, should allow entry of methyl mercury into the base of the base of food web with less dilution by newly produced biomass, hence greater concentrations.

In order to better define the food web in Florida Bay, we are using stable isotope ratios of C, N, and S as tracers of trophic transfers. We have collected a broad range of organisms from the bay for stable isotope analysis. These organisms include primary producers (seagrass, epiphytes, macroalgae, mangroves, and phytoplankton), top predator fish, and intermediate vertebrate and invertebrate consumers linking them trophically. We have coordinated our sampling with the Florida State University group of Jeff Chanton, our group emphasizing the eastern region of the bay, theirs emphasizing the western region. The two groups have provided each other with samples for analysis and shared data. We are employing the same analytical laboratory to assure comparability of isotope measurements.

By coupling mercury measurements with stable isotope patterns of food web structure, we expect to determine the regional trophic pathways leading to elevated mercury concentrations in upper trophic level predators. This should allow us to identify the species most vulnerable to accumulation of high mercury levels and to predict changes in intensity and spatial scale in response to altered freshwater flows from the Everglades to Florida Bay.

A poster describing our ongoing work was presented in May of this year at the Third Florida Bay Science Conference in Miami.