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Stable Isotope Studies of Red Mangroves and Filter Feeders from the Shark River Estuary, Florida

Brian Fry1 and Thomas J. Smith III2

1Institute of Pacific Islands Forestry, USDA Forest Service, 1151 Punchbowl St. Suite 323, Honolulu HI 96813, email: bfry@lsu.edu
2United States Geological Survey/Biological Resources Division, Center for Coastal and Regional Marine Studies, 600 Fourth Street South, St. Petersburg, FL 33701

Brian Fry and Thomas J. Smith III. 2002. Stable Isotope Studies of Red Mangroves and Filter Feeders from the Shark River Estuary, Florida. Bulletin of Marine Science 70(3): 871-890. Posted, in full, with permission from the Bulletin of Marine Science.

ABSTRACT

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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, micro algal 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.


map showing Shark River estuary of southwestern Florida and nine sampling locations indicated by dark circles
Figure 1. Shark River estuary of southwestern Florida and nine sampling locations indicated by dark circles. Approximate kilometers upestuary is given within each circle. For geoposition reference, coordinates for the 6 km station at the eastern edge of Ponce de Leon Bay are approximately 25°21'11"N, 81°06'55"W (see also Chen and Twilley, 1999). [larger image]
In tropical and subtropical estuaries, mangroves form the basis of important, productive coastal ecosystems. Mangroves provide many environmental benefits to coastal systems, one of which is thought to be food web support of coastal fisheries. In a series of now-classic studies, William Odum and Eric Heald articulated the idea that detrital mangroves colonized by bacteria and fungi are important, dominant food resources in coastal mangrove systems (Heald, 1970; Odum, 1970; Odum and Heald, 1972, 1975). Fisheries species such as pink shrimp and mangrove snappers are common in the south Florida estuaries studied by Odum and Heald, and these species were thought to benefit from mangrove food subsidies. A refinement of these views was that mangrove inputs would be strongest and most dominant in small local drainage basins, with influence declining downestuary as tidal flushing dispersed inputs from the terrestrial mangroves (Odum, 1984). While these ideas seem intuitive in the large mangrove forests of south Florida, mangrove inputs to food webs have been difficult to document in many subsequent studies conducted elsewhere (e.g., Rodelli et al., 1984; Newell et al. 1995; Primavera, 1996; Loneragan et al., 1997), leading to questions about the accuracy of concepts arising from the original south Florida investigations (Stoner and Zimmerman, 1988; Rodriguez and Stoner, 1990; France 1998).

To test the food web dominance of mangroves in south Florida where the original work on this subject was performed, we assayed elemental and isotopic compositions of mangroves and filter feeders across the Shark River estuary. Both the Shark River estuary and the nearby North River estuary, the original site of Odum and Heald's work, are in the mainstem outflow of the Everglades watershed and in Everglades National Park (Fig. 1). We followed suggestions that isotope values of source materials such as mangroves or mangrove detritus need to be carefully checked on a site-specific basis (Gearing, 1988), and that using a combination of tracers is frequently more powerful than use of a single tracer (Peterson and Howarth, 1987). Results showed that sulfur isotopic determinations, which were not used in previous investigations of mangrove food webs in south Florida (Fry, 1984; Zieman et al., 1984; Harrigan et al., 1989; Fleming et al., 1990; Fry et al., 1999), may be particularly important for assaying food web use of mangrove materials.

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