We are investigating 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.

Figure 1. Correlation of the log10 of total mercury concentration (THg as µg/g ww) with the square root of total fish length (TL in mm) for largemouth bass at Loxahatchee. Symbols represent individual fish. [larger image]

Of the twelve marsh and canal sites we are investigating, 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 (approximately 38 percent), which is consistent with locally differing influences (Kendall et al., 2001). Fish length explains approximately an additional 15 percent of THg variance, with relatively minor contributions by

N (approximately 5 percent) and interannual differences (approximately 3 percent).

Figure 2. Correlation of the log10 of total mercury concentration (THg as µg/g ww) with the log10 of 15N (‰) for largemouth bass at Loxahatchee. Symbols represent individual fish. [larger image]

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 at four sites (approximately 16-27 percent 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 at three sites. At one of these sites, ¹³C is the best predictor of THg variation (approximately 23 percent), whereas at the other two it explains only about 2-6 percent 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 microenvironmental variations). Interannual variations are a significant influence at ten of the twelve sites, where they explain approximately 5-31 percent 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.

Contact: Bryan, Bemis, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA, 94025, Phone: 650-329-5603, Fax: 650-329-5590, bebemis@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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