δ15N evidence for nitrogen fixation associated with macroalgae from a seagrass-mangrove coral reef system

Early studies using acetylene reduction have suggested that macroalgae may contribute to nitrogen enrichment of waters near coral reefs via nitrogen fixation by their epiphytic cyanophytes. Our objectives were to investigate the potential of stable nitrogen isotope analysis for detecting nitrogen fixation in near-reef macroalgae, and to compare these findings with those for different fixing and non-fixing autotrophs from other systems. We made collections of near-reef algae in Puerto Rico, seagrasses and macroalgae from the northeast coast of the U.S., and freshwater algae from streams in Quebec, Canada. The mean +/- SD delta(15)N for our near-reef algae was 0.3 +/- 1.0 parts per thousand, Values which were significantly lower than those from our other sampling areas and also well below published Values for other marine benthic nitrogen fixers (ca 2 parts per thousand) and non-fixers (ca 6 parts per thousand). Our results provide a useful test of the value of stable nitrogen analysis in detecting nitrogen fixation of near-reef algae, thereby supporting previous non-isotope work in suggesting that this macroalgal-cyanophyte complex may provide an important source of fixed nitrogen to reef systems.