The role and contribution of the seagrass Posidonia oceanica (L.) Delile organic matter for secondary consumers as revealed by carbon and nitrogen stable isotope analysis

The delta(13)C and delta(15)N values of primary producers and consumers were studied to obtain information on the trophic role of Posidonia oceanica L. Delile, the dominant primary producer, in a Mediterranean shallow environment (the Stagnone di Marsala, western Sicily). delta(13)C strongly discriminated between pelagic and benthic pathways, with the former based on phytoplankton and the latter on a mixed pool of seagrass detritus, epiphytes and benthic algae as carbon sources. A particularly important trophic role appears to be performed by the vegetal epiphytic community on seagrass leaves (delta(13)C = -14.9 +/- 0.1parts per thousand), which supports most of the faunal seagrass community (i.e. Amphipoda, Isopoda, Tanaidacca; delta(13)C = -14.9 +/- 0.1parts per thousand, -12.5 +/- 0.1parts per thousand and -14.8 +/- 1.0parts per thousand, respectively). Although P. oceanica (delta(13)C = -11.3 +/- 0.3parts per thousand) does not seem to be utilised by consumers via grazing (apart from a few Palaemonidae species with delta(13)C value of -10.8 +/- 1.8parts per thousand), its trophic role may be via detritus. P. oceanica detritus may be exploited as a carbon source by small detritivore invertebrates, and above all seems to be exploited as a nitrogen reservoir by both bottom and water column consumers determining benthic-pelagic coupling. At least three trophic levels were detected in both the pelagic (mixture of phytoplankton and cyanobacteria, zooplankton, juvenile transient fish) and benthic (sedimentary organic matter and epiphytes, small seagrass-associated invertebrates, larger invertebrates and adult resident fish) pathways. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.