Carbon and nitrogen stable isotopic variation in Laguncularia racemosa (L.) (white mangrove) from Florida and Belize: implications for trophic level studies

Carbon and nitrogen stable isotopic data from the primary producers in mangrove ecosystems are needed to investigate trophic links and biogeochemical cycling. Compared with other mangrove species (e.g. Rhizophora mangle) very few measurements have been conducted on the white mangrove, Laguncularia racemosa. The carbon and nitrogen stable isotopic and elemental compositions of L. racemosa were analyzed and compared from Florida and Belize. δ13C values of L. racemosa from Florida (mean = –26.4‰) were slightly higher than those from Twin Cays, Belize (mean = -27.4‰), which may be due to higher salinity in some parts of the Florida site. There was no difference between the δ15N values from L. racemosa from these two sites (Florida mean = 0.6‰; Belize mean = 0.3‰), which are indicative of nitrogen derived from nitrogen fixation in a planktonic marine system. However, higher δ15N values from L. racemosa at Man of War Cay in Belize (11.4‰ and 12.3‰), which is fertilized by roosting marine birds (∼14.0‰), illustrate that L. racemosa can sensitively reflect alternative nitrogen sources. Although the isotopic data could not distinguish between Avicennia germinans, R. mangle and L. racemosa in Belize the L. racemosa had considerably higher C/N ratios (46.5 – 116.1) compared with the Florida samples (42.2 – 76.0) or the other mangrove species. Unlike some previous findings from R. mangle, substrate characteristics (e.g. salinity, NH4 +, and H2S) were not related to the isotopic or elemental composition of L. racemosa. δ13C, δ15N and C/N were analyzed for ecosystem components from L. racemosa habitats at Twin Cays, including other plants (e.g. R. mangle, A. germinans and seagrass), detritus, microbial mats and sediments. Results from mass-balance calculations show that mangrove detritus composes very little of the sediment, which is principally composed of microbial biomass (80 – 90%). Detritus at some sites is also influenced by sources other than that from L. racemosa, including seagrass leaves.