Stable isotopes of carbon and nitrogen were used to assess the relevance of 4 primary carbon sources (the macroalga Cystoseira balearica, decaying blades of the seagrass Posidonia oceanica, seagrass epiphytes and pelagic particulate organic matter [POM]) for consumers inhabiting 3 adjoining subtidal habitats in the Western Mediterranean: seagrass meadows, unvegetated sandy patches and the overlying pelagic ecosystem. MANOVA and ANOVA revealed statistically significant differences between the δ 13 C signal of decaying blades of P. oceanica (-13.0 ± 0.6 ‰), epiphytes (-19.7 ± 0.4 ‰) and a third group including POM (-23.1 ± 0.3 ‰) and C. balearica (-22.7 ± 0.8 ‰). However, these primary sources did not differ in their δ 15 N signals (average: 1.0 ‰; range: 0.8 to 1.2 ‰). IsoSource mixing model software was used to calculate the contribution of each primary source to the carbon assimilated by each animal species. Nested ANOVA showed that the 3 trophic webs differed in the average contribution of the decaying blades of P. oceanica to the carbon assimilated by animals. The average contribution was 46.0 ± 14.0% for meadow-dwelling species, 18.3 ± 6.0% for sand-dwelling species and 12.8 ± 3.0% for pelagic species, once zooxanthellae-bearing jellyfish were removed from the analysis. However, IsoSource provided solutions for all the pelagic species and for half the sand-dwelling species that do not use decaying blades of P. oceanica as a carbon source. Conversely, IsoSource identified the decaying blades of P. oceanica as a relevant carbon source in all the solutions calculated for 3 different sand-dwelling species and in all of the meadowdwelling species. Thus, we conclude that organic detritus from P. oceanica is a relevant carbon source for species inhabiting seagrass meadows and for those sand-dwelling species living close to the meadows, but not for species exploiting deeper and more distant unvegetated patches.
KEY WORDS: Detritus · Export · Seagrass meadows · Stable-isotope analysisResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 335: 123-131, 2007 extremely high (Cebrián et al. 1997), but only a small fraction of it is consumed by local herbivores (Cebrián et al. 1996;Mateo & Romero 1997, but see Tomas et al. 2005; most of it is actually exported as fine detritus or coarser debris to adjoining ecosystems, where it is thought to fuel detritus-based food webs (Boudouresque et al. 1994), although reliable evidence is lacking. This export process is thought to be more important in the unvegetated sandy patches scattered within the meadows and adjoining the lower fringe of the meadows than elsewhere (Boudouresque et al. 1994). However, the relevance of seagrass detritus as a carbon source for unvegetated areas close to the seagrass beds has never been carefully investigated, despite recent efforts devoted to assessing the relevance of several carbon sources in the food webs of Mediterranean ecosystems (Dauby 1989, Jennings et al. 1997, Lepoint et al. 2000, P...