France R.

Estimating the assimilation of mangrove detritus by fiddler crabs in Laguna Joyuda, Puerto Rico, using dual stable isotopes

France, R. 1998. Estimating the assimilation of mangrove detritus by fiddler crabs in laguna joyuda, puerto rico, using dual stable isotopes. Journal of Tropical Ecology 14 (JUL): 413-25.

Dual stable isotope analyses (delta 13C and delta 15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assim- ilation of material from ingested sediments. Fiddler crabs preferentially selected foi niitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the prin- cipal source of energy to estuariiie consumers. Previous research fiom this lagoon as well as from Amazonia suggests that the magnitude of lan-d-wvater ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.

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

France, R., J. Holmquist, M. Chandler, and A. Cattaneo. 1998. Delta N-15 evidence for nitrogen fixation associated with macroalgae from a seagrass-mangrove coral reef system RID H-3188-2011. Marine Ecology-Progress Series 167 : 297-9.

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 cynaphytes. Our objectives were to investigate the potential of stable nitrogen isotope analysis for detecting nitrogen fixation in near-reef macroalgae, and to compare these finding 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 streams in Quebec, Canada. The mean + SD 6 1 5 ~fo r near-reef algea was 0.3 r i.G%, vaiues whlch were significantly lower than those from our other sampling areas and also well below published values for other marine benthic nitrogen fixers (ca 2%) and non-fixers (ca 6%0). 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 macroalgalcyanophyte complex may provide an important source of fixed nitrogen to reef systems.
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