δ15N Studies of Nitrogen Use by the Red Mangrove, Rhizophora mangle L. in South Florida

Abstract

To help define nitrogen (N) sources and patterns of N processing in mangrove ecosystems, mangrove leaf nitrogen contents and δ15N 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 δ15N values did not differ consistently between dwarf and tall mangroves, even though there were large variations in δ15N (18‰ range, −5 to +13‰) and %N (1·2% range, 0·9–2·1%). Highest %N and δ15N values occurred along the western margin of Biscayne Bay where canals draining agricultural lands deliver high-nitrate waters to fringing mangrove marshes. High mangrove δ15N 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.