Stable Isotope Applications for Studying Food Web Dynamics and Nutrient Sources II Posters
Presiding: C Kendall, U.S. Geological Survey
H51E-01 0830h
Importance of Chemolithoautotrophic Production to Mobile Benthic Predators in the Gulf of Mexico
The continental slope of the Gulf of Mexico is characterized by substantial hydrocarbon seepage which provides reduced energy sources, both CH4 and H2S, for chemolithoautotrophs existing as endosymbionts within mussels and tubeworms found in dense colonies that provide habitat for an array of endemic and colonial fauna. The extent of trophic export of chemosynthetic biomass to the seep communities and the surrounding benthic communities in the Gulf, however, remains an open question. To elucidate the nutritional associations between seep residents and the surrounding benthos the carbon, nitrogen and sulfur stable isotope values of the hagfish Eptatretus sp., the giant isopod Bathynomus giganteus and the predatory snail Phymorhyncus sp. were interpreted through a three source, dual isotope mixing model. The model was able to assess the contributions of different isotopic signals to a mixture and thus could distinguish between photosynthetic/phytodetritus based sources, methanotrophic sources and thiotrophic sources. Incorporation of chemosynthetic based food sources was minimal on the whole and species specific; however some of the organisms considered in this study did incorporate nutrition from chemolithoautotrophic sources.
H51E-02 0830h
Assessing the Influences of Mercury Bioaccumulation and Bioavailability in Everglades Food Webs
Eastern mosquitofish are an important sentinel species used to monitor mercury contamination of the aquatic ecosystem in the Everglades. Like other freshwater fish, the primary route of mercury uptake into mosquitofish tissues is through diet as bioavailable methylmercury. Yet, it is unclear whether variations in mosquitofish mercury observed across the Everglades are due primarily to differences in bioaccumulation (i.e., increase of mercury at higher trophic levels) or abundance of methylmercury available to the food web base. We use isotopic methods to investigate the importance of these two controls on mosquitofish mercury at the landscape scale. As part of the USEPA REMAP project, mosquitofish, periphyton, and sediments were collected during September 1996 from over one hundred sites throughout the Everglades and analyzed for mercury concentration and many other parameters. The USGS analyzed splits of the samples for d15N, d13C, and d34S. Mosquitofish were analyzed as composites of 5-10 fish and periphyton and 0-5 cm sediment samples were analyzed in bulk. Tissue d15N is widely used to estimate the relative trophic positions of organisms in a food web, and should correlate positively with tissue mercury if bioaccumulation is an important control on mosquitofish mercury concentration. Tissue d34S values potentially indicate the extent of dissimilatory sulfate reduction in sediments, a process used by sulfate-reducing bacteria (SRB) during conversion of inorganic Hg(II) to bioavailable methylmercury. Because this process increases the d34S value of remaining sulfate (assimilated by the food web base), mosquitofish d34S should show positive correlations with SRB activity, methylmercury production, and mosquitofish mercury concentrations. Mosquitofish and periphyton isotopes are significantly correlated (d15N-Mosq vs. d15N-Peri, d34S-Mosq vs. d34S-Peri), indicating that a component of the bulk periphyton analyzed in this study is part of the mosquitofish food web. Mosquitofish mercury does not correlate significantly with tissue d15N or the d15N difference between mosquitofish and periphyton (D15N). Thus, differences in trophic level (and bioaccumulation) among the fish do not contribute a detectable influence on mercury variations in the samples studied. In contrast with the d15N results, mosquitofish mercury levels show strong positive correlations with mosquitofish d34S and D34S. This suggests that during the period studied, mosquitofish mercury concentrations in the Everglades ecosystem were primarily influenced by the bioavailability of mercury, rather than by varied trophic position. Our findings also suggest that the d34S of biota may provide a useful indicator of potential hotspots of mercury methylation.
H51E-03 0830h
Differential Incorporation of stable carbon and nitrogen isotopes into Liver, Muscle, and Scales of California Sardine (Sardinops sagax) Fed Different Diets
Stable isotopes offer a unique approach to examine trophic structure of many organisms. Here we quantify the isotopic response of liver, red and white muscle tissue, and scales of the California Sardine, Sardinops sagax caerulea, raised on three different food types. The O13C and O15N of tissues were examined at approximately one month intervals to examine the turnover response time in each tissue type. The growth rates of fish fed squid, copepods, and pellets differed. Nonetheless, isotopic signatures tended towards the composition of their food but with differential rates for each tissue type. The isotopic composition of liver varies most rapidly while that of scales is the least variant. The varying turnover times in different tissues offers the opportunity to determine feeding histories integrated over varying timescales.