δ15N studies of nitrogen use by the red mangrove, Rhizophora mangle L. in South Florida

To help define nitrogen (N) sources and patterns of N processing in mangrove ecosystems, mangrove leaf nitrogen contents and delta(15)N values were assayed in three marshes along the south Florida coast. In each marsh, leaf samples were collected from dwarf mangroves at interior locations and taller mangroves at the ocean fringe. Leaf % N and delta(15)N values did not differ consistently between dwarf and tall mangroves, even though there were large variations in delta(15)N (18 parts per thousand range, -5 to +13 parts per thousand) and % N (1.2% range, 0.9-2.1%). Highest % N and delta(15)N values occurred along the western margin of Biscayne Bay where canals draining agricultural lands deliver high-nitrate waters to fringing mangrove marshes. High mangrove delta(15)N values may be good biomonitors of anthropogenic N loading to south Florida estuaries. Lower values likely reflect less anthropogenic N entering the mangrove marshes, as well as differences in plant physiology that occur along the fringe-dwarf gradient. (C) 2000 Academic Press.