Using Nitrogen And Carbon Isotopes To Explain Mercury Variability In Largemouth Bass

Using Nitrogen and Carbon Isotopes to Explain Mercury Variability in Largemouth Bass

Bryan E. Bemis, and Carol Kendall U.S. Geological Survey, Menlo Park, CA

Ted Lange Florida Fish and Wildlife Conservation Commission, Eustis, FL

Linda Campbell National Water Research Institute, Environment Canada, Burlington, ONT, Canada

We investigate whether variations in the nitrogen (15N) and carbon (13C) isotopic composition of largemouth bass (Micropterus salmoides) tissue can explain a significant amount of the variation in mercury concentrations in this top predator fish. The 15N and 13C values of tissues are integrated measures of diet assimilated over time, with consumers enriched in the heavier isotope relative to their diet. This stepwise isotopic increase toward higher trophic positions has been used for decades to reconstruct relative food web structure in a variety of ecosystems, with a focus on intercomparison of northern temperate lakes. Relatively few studies have applied this technique to wetlands.

One of the goals of restoration efforts in the Everglades is to better understand the sources and distribution of mercury in the food web, in order to develop management strategies to minimize mercury contamination of the ecosystem. Because bioavailable methyl mercury is retained in tissues of Everglades biota, successive trophic levels within the food web accumulate higher mercury levels. Mercury concentrations in top predators such as the sport fish largemouth bass typically exceed EPA safety limits. Isotopic estimates of diet and energy flow within the food web are expected to provide potential information about the sources and pathways of mercury bioaccumulation in Everglades marshes.

Stepwise multiple regression is used to identify parameters that explain a significant amount of variance in largemouth bass total mercury (THg) at 12 marsh and canal sites throughout the Everglades representing a variety of hydropatterns (extremely short to long). The five predictor variables include: latitude of collection site (i.e., spatial influence), collection year (1996-1998; i.e., temporal influence), total fish length (a size estimate), 15N, and 13C. In addition, we repeat the regressions for individual sites to identify locally important influences. Of the twelve marsh and canal sites we investigate, seven have complete data for the years 1996-1998. When data for these seven sites are combined, spatial differences (i.e., latitude) explain the most THg variance in largemouth bass tissue (~38%), which is consistent with locally differing influences (Kendall et al., 2001). Fish length explains an additional ~15% of THg variance, with relatively minor contributions by 15N (~5%) and interannual differences (~3%). 13C is not a significant influence at this landscape scale. The spatial differences may reflect differences in hydropattern, available methyl mercury concentrations (Gilmour et al., 1998), or other environmental variables such as dissolved organic carbon (DOC) concentration or water pH. Future addition of these data to the regression models may help identify the sources of the intersite differences.

When the data are analyzed separately for the twelve collection sites, fish length explains the most THg variance (approximately 34-77%) for ten of the twelve sites (fig. 1). Length is the second most important predictor in another site, explaining ~11% of THg variations. The importance of fish size on THg found here is consistent with the results of the site-grouped analysis in this study, as well as previous studies (Lange et al., 1999); larger fish are typically older and have had more time to accumulate mercury in their tissues.

Figure 1. Correlation of the log10 of total mercury concentration (THg as Figure 2. Correlation of the log10 of g/g ww) with the square root of total total mercury concentration (THg as 15 fish length (TL in mm) for largemouth g/g ww) with the log10 of  N (‰) bass at Loxahatchee. Symbols for largemouth bass at Loxahatchee. represent individual fish. Symbols represent individual fish. In contrast with the findings for the site-grouped analysis, the site-specific analyses indicate that 15N, 13C, and temporal variations can be locally important predictors of THg in largemouth bass (fig. 2). 15N is a significant predictor of THg variation in four sites (~16-27% variance explained), and explains the most variance in two of these (likely an indicator of relative trophic level differences among largemouth bass within a site). 13C is a significant predictor of THg in three sites. In one of these sites, 13C is the best predictor of THg variation (~23%), whereas in the other two it explains only ~2-6% of THg variation. The small expected consumer-diet enrichment of 13C (compared to 15N) and large 13C spatial variations (Kendall et al., 2001) suggest that 13C is tracking something other than trophic position (perhaps micro-environmental variations). Interannual variations are a significant influence in ten of the twelve sites, where they explain ~5-31% of variation in THg. The importance of these predictor variables in explaining largemouth bass mercury does not appear to correlate with marsh versus canal sites.

In summary, 15N and 13C can provide locally important information about mercury variations in largemouth bass tissue in the Everglades. However, fish size and temporal changes in the ecosystem are typically much more important influences that may mask information that could contribute to better understanding the transfer of mercury within food webs.

References

Gilmour, C.C. et al., 1998. Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades. Biogeochemistry, 40: 327-345. Kendall, C. et al., 2001. Use of Stable Isotopes for Determining Foodwebs and Explaining Spatial Variability in Hg in the Everglades, Proceedings of the Workshop on the Fate, Transport, and Transformation of Mercury in Aquatic and Terrestrial Environments, West Palm Beach, Florida. Lange, T.R., Richard, D.A. and Royals, H.E., 1999. Trophic Relationships of Mercury Bioaccumulation in Fish from the Florida Everglades, Florida Department of Environmental Protection, Florida Game and Fresh Water Fish Commission, Fisheries Research Laboratory.

Bryan, Bemis, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA, 94025, Phone: 650-329-5603, Fax: 650-329-5590, [email protected]