Stable isotope studies of red mangroves and filter feeders from the Shark River estuary, Florida

Abstract

This study focused on the Shark River estuary in the central drainage of the Everglades watershed, testing whether mangrove subsidies to estuarine food webs were especially strong in south Florida sites, as suggested in pioneering studies by Odum and Heald (1975). Samples of red mangroves (Rhizophora mangle L.) and filter feeding barnacles and mussels were collected in a transect from the mouth of the estuary to the freshwater end of the estuary during August 1997. Along this transect, elemental and stable isotopic compositions of green, yellow and submerged detrital mangrove leaves showed relatively constant carbon (C) compositions, while nitrogen (N) and sulfur (S) compositions varied strongly. For example, mangrove N contents increased at the upper freshwater end of the Shark River estuary, and N isotopic compositions declined, probably indicating relatively N-rich inputs from the upstream Everglades watershed. In contrast, mangrove sulfur contents varied most in mid-estuary where green leaf S contents were highest and S isotopic compositions lowest. Mixing models based on S isotopes showed that mangrove swamps supported up to 60% of filter feeder nutrition at mid-estuarine stations. However, microalgal support of the filter feeders was also generally strong (40-75%) throughout the estuary. Although this study did not confirm a dominant nutritional role for mangrove detritus at most sites, results did support a second hypothesis from earlier work in this area (Odum, 1984), namely that landscape position may generally control development of mangrove-based food webs.