Nitrogen inputs to coastal environments can considerably alter the abundance of primary producers. However, how herbivores modify their trophic signatures and adjust to changes in food resource conditions remains controversial. Here, we assess the effect of nutrient availability on the diet shifts of the two main Mediterranean herbivores, the Sparid fish Sarpa salpa L. and the sea urchin Paracentrotus lividus (Lmk.) that feed mostly on the seagrass Posidonia oceanica L. (Delile), epiphytes and benthic macroalgae. To do this, we (1) investigate the patterns of isotopic composition (d 13 C and d 15 N signatures) of the two herbivores and their potential food sources in three areas of contrasting nutrient conditions and, (2) we assess the diet shift along this nutrient gradient by estimating the isotopic nutrient enrichment (i.e., the contribution of d 13 C and d 15 N signatures in consumers’ tissues relative to potential food sources). Food web signatures of d 13 C were similar among the three study sites, and no patterns of d 13 C shift were observed in their diets. In contrast, there was a consistent increase in N contents and d 15 N along the nutrient gradient for all primary producers and their consumers. The rate of d 15 N enrichment was also clearly distinctive between the two herbivores: in P. lividus it increased by 61% along the nutrient gradient, while in S. salpa it remained constant. Our results suggest that sea urchins behave as facultative omnivores and feed on vegetable or mixed diets depending on the trophic status of the system. It is unclear, however, if this modification is behavioral or the consequence of mere changes in the availability of food items, as animal epiphytes (e.g., hydrozoans, bryozoans and ascidians) can also became more abundant on seagrass leaves under increased nutrient conditions. In contrast, adult fish appear to feed on vegetal material independent of nutrient availability in the ecosystem.

This work was supported by a FI scholarship from the Departament d’Universitats, Recerca i Societat de la Informacio´ (DURSI, Generalitat de Catalunya) and CGL2007-66771- C02-01/CGL2009-12562 grants from the Spanish Ministry of Science and Technology.

10 páginas,5 figuras, 3 tablas.

Peer reviewed