The trophic role of the macrophyte Cymodocea nodosa (Ucria) Asch. in a Mediterranean saltworks: Evidence from carbon and nitrogen stable isotope ratios

A multiple stable isotope analysis (δ13C and δ15N) was used to determine the trophic role of the seagrass Cymodocea nodosa (Ucria) Asch. for consumers in a Mediterranean saltworks. Stable carbon and nitrogen isotope ratios performed on primary producers and consumers in February and July 1999 showed that C. nodosa is not a direct dietary source via grazing. No animal presented carbon values as enriched as the seagrass (-7.8‰). The trophic role of Cymodocea occurs through the detritus route via SOM. Sedimentary organic matter (SOM) (δ13C = -16.5‰) seems to be a mixture of particulate organic matter (δ13C = -19.0‰), algae (δ13C = -18.6‰) and seagrass detritus (δ13C = -7.7‰). The results of a mixing model suggest that Cymodocea detritus contribution to SOM varied temporally, a greater role in February than in July. In July the enriched values of both stable carbon and nitrogen isotopes of several primary producers and consumers in comparison with February (about +2‰ for primary producers, +3.5‰ for invertebrates and +1‰ for fish) suggest that less depleted carbon and nitrogen (probably from seagrass detritus) influence food web structure. Cymodocea has an important structuring role as seagrass blades are extensively colonised by a complex vegetal community which provides food and habitats for invertebrates. The δ13C of the animals suggests that epiphytes may represent an important ultimate organic matter source together with SOM.