STABLE ISOTOPE ANALYSIS OF SEDIMENTARY ORGANIC MATTER FROM BIOLUMINESCENT BAYS IN VIEQUES, PUERTO RICO, AS AN INDICATOR OF POSSIBLE LINK BETWEEN MANGROVES AND DINOFLAGELLATE ABUNDANCE

The three Puerto Rican bays considered as part of this research project - Puerto Mosquito (PM), Bahia Tapon (BT), and Puerto Ferro (PF) - have similar ecology, geomorphology and geography. The vegetation present in each of these bays has two main components: marine seagrass and terrestrial mangrove (Keck 2006). Only PM contains high concentrations of the dinoflagellate Pyrodinium bahamense that causes brilliant luminescence. Mangrove organic matter has been suggested to be a source of vitamin B12 and other nutrients critical to dinoflagellate growth. This study investigated the importance of mangrove organic matter in causing Pyrodinium bahamense growth-and thus luminescence- by using carbon isotope ratios to trace source vegetation in bay sediment. Higher carbon isotope ratios (-3 to -15‰) indicate that marine vegetation, most likely Halimeda, a calcareous algae, or Thalassia, a seagrass, is a major source of organic matter (Bouillon et al., 2002; Coffin et al., 1989; Fry et al., 1984), while lower carbon isotope ratios (-26 to -31‰) indicate that terrestrial vegetation, most likely mangroves, is a major source of organic matter in the sediment. Carbon isotope ratios of marine plankton are intermediate, from -17 to -22‰ (Ku 2007). Carbon isotope ratios of sediment organic matter from each bay vary, suggesting some degree of marine and terrestrial vegetation mixing. PF has the highest organic carbon isotope ratios (-14.6 ± 1.7 ‰) while PM has the lowest (-20.2 ± 2.0 ‰). These differences in carbon isotope ratios correlate with differences in bioluminescence. In addition to other corroborative chemical data (e.g. total inorganic carbon, C/N) these data suggest a causal relation between bay waters dominated by input from decaying mangroves, and bioluminescence. In turn, this relative abundance of mangrove plant material most likely reflects the longer residence time of water in PM versus BT and PF.