Influence of Mangrove Detritus in an Estuarine Ecosystem

This study has determined the relationship and extent of mangrove reduced carbon flow and its contribution to the diet of higher consumers in a nearby seagrass bed using stable isotope techniques. In a seagrass bed approximately 80 m from a riverine mangrove community, detritus from seagrass or other marine sources of carbon such as epiphytes (Stilophora rhizodes) or phytoplankton the major contributor of reduced carbon. Mangroves make only a localized contribution to the food chain, thus contributing a relatively low proportion of reduced carbon to this seagrass community. The major primary producers in southern Florida fringe coastal communities are seagrasses and mangroves. Thalassia testudinum dominates the seagrass beds offshore, with Halodule wrightii as a secondary species. Rhizophora mangle dominates and Avicennia germinans and Laguncularia racemosa contribute significantly to the riverine and fringe forests. The detrital material formed by the partial decomposition of the leaves of these vascular plants may be the basis of trophic food chains in coastal ecosystems of southern Florida. Mangrove forests are net exporters of particulate organic carbon (Golley et a1., 1962; Heald, 1969; Lugo et al., 1976). Further, mangrove carbon supports food chains within riverine and basin communities along the southwest coast of Florida (Odum and Heald, 1975; Zieman et a1., 1984). In areas where seagrass and mangrove communities are in close association, however, the relative contribution of the seagrass and mangrove detritus to the food chain is difficult to determine. Mangrove estuarine environments are important nursery areas for fish populations, yet large numbers of organisms are supported by seagrass beds, the magnitude being related to the seagrass cover density (Zieman et a1., 1984). This paper addresses the question of whether the influence of mangrove detrital flow is far reaching or localized. To answer this question we used stable carbon isotope abundance analysis of organisms from a mangrove riverine community and two seagrass communities, one of which was geographically close to a mangrove stand and the other was without any apparent mangrove detritus input. Stable isotope analysis provides a powerful tool to determine carbon flow in this particular ecosystem because there is a significant difference in the carbon-I 3 content of the primary producers in question (mangroves and seagrasses, Zieman et al., 1984). Further, consumers of mangroves and seagrass can be identified by isotopic analysis, because diet switching and turnover experiments for carbon clearly show that diet is the primary determinant of the animal isotopic composition (see Fry and Arnold, 1982). Unfortunately, nitrogen isotope analysis cannot be used in this situation since in some cases there are no differences in natural 15N abundances between seagrass and mangrove detritus (Zieman et a1., 1984).