Abstract
Total mercury (Hg) and stable isotopes of nitrogen (8 15N) and carbon (8 13C) were measured in samples from 58 species of invertebrates (18 species) and finfish (40 species) from the coastal waters off Southwest Florida to evaluate the biomagnification of mercury in the food web and to examine patterns of trophic transfer as a function of species, size, diet, and life history. Tissue Hg concentrations were highly variable both within and among species. All samples (n=320) had measureable amounts of Hg, with mean Hg concentrations for species ranging from 0.004 mg/kg in an unidentified species of brittlestar (Class: Ophiuroidea) to 2.839 ± 1.39 mg/kg in king mackerel (Scomberomorus caval/a). In general, fish were more enriched in 8 15N (%c) than invertebrates; exceptions to this general pattern included an unidentified species of isopod (Isopoda), the calico crab (Hepatus epheliticus), and stone crab (Menippe spp.), which had higher 8 15N values than several species of the smaller fish. The values of 813C exhibited a relatively narrow range from -21.78 to -12.42%o and were consistent with an offshore food web. Inter-species variability in Hg was explained by increases in both size and trophic level of species. For most species, intra-species variability in Hg was best explained by size; however, there were some significant relationships with increasing 815N consistent with an ontogenetic shift in diet in pinfish (Lagodon rhomboides) and red snapper (Lutjanus campechanus). Log transformed Hg concentrations among species means was positively correlated with 815N (p<O.OOl, r2 = 0.66) and had a slope of 0.21. A food web magnification factor (FWMF) of 5.05 was calculated from the relationship between log transformed mean Hg concentrations and trophic level (calculated from 815N), indicating that Hg increased at a factor of 5 with each increase in trophic level. The 815N-derived slope and FWMF were comparable to the values reported from other regions and ecosystems, but possibly slightly higher than other marine systems. The basal Hg value, calculated as 0.00064 mg/kg, was also higher than other marine systems, which was expected because of the known high rate of Hg methylation in South Florida.
