The base of nutritional support for the gray snapper (Lutjanus griseus): an evaluation based on a combined stomach content and stable isotope analysis

A combined stomach content and stable isotope analysis was used to determine ifseagrass provides a base of nutritional support to the gray snapper, Lutjanus griseus. The results provided a quantitative evaluation of the relative contribution of carbon and nitrogen from various primary organic sources to gray snapper taken from each a mangrove and a seagrass dominated location in south Florida. Stomach content analysis revealed that gray snapper from the two areas had similar diets which were primarily composed of penaeid shrimp (>60%). Isotopic results provided a distinction between food webs on the basis of carbon values. The 6'3C of components from the seagrass location were greater than -17%0 in contrast to values of less than 190/00 for those from the mangrove area. Quantitative estimates indicated that gray snapper from the seagrass area derived more than 90% of their carbon and nitrogen from sediment or water column particulate organic matter. Gray snapper from the mangrove area were supplied by carbon and nitrogen from these sources in addition to detritus. The main contributors appeared to be particulate organic matter from the water column and the brackish water grass, Ruppia maritima. Together, these sources accounted for 35 to 100% of the ultimate source of prey item dietary carbon and nitrogen. These results suggest that within both food webs carbon and nitrogen are transferred from a detrital base by similar mechanisms and emphasize the use of multiple isotopes as a tool for quantitatively evaluating food webs. Seagrasses are major primary producers in the shallow coastal waters of south Florida and the Caribbean. As a consequence of their ability to modify the physical environment and to provide a food source, seagrass beds are inhabited by a variety of marine organisms. A need to identify and quantify aspects of seagrasses that are important to marine organisms has emerged in response to the declining abundance of several marine species. This study focussed on the role of seagrass in the food web of the gray snapper, Lutjanus griseus, a commercially important fish species of south Florida. The nutritional role of seagrass in supporting populations of gray snapper has only been suggested qualitatively by analysis of stomach contents (Randall, 1960; Tabb and Manning, 1961; Starck and Schroeder, 1964; Odum and Heald, 1972). These studies indicated that gray snapper, particularly juveniles, obtain prey from seagrass beds. Despite an apparent association between gray snapper and seagrass, dependence of these animals on seagrass beds has not been firmly established. This study employed a combined stomach content and stable isotope analysis to trace the food web of the gray snapper and quantitatively determine if gray snapper are dependent on seagrass as a base of nutritional support. Stable carbon isotopes have been frequently used to trace the diets of animals (DeNiro and Epstein, 1978; Fry and Parker, 1979; Macko et aI., 1982). Such studies are based on the close resemblance of the l3c/12e ratio of the organism to that of its food source. The use of isotopes of a single element in a tracer study may be inadequate to distinguish between isotopically similar food sources or a mixture of more than two food sources. This inadequacy can be reduced by the