MEPS 306:125-131 (2006)  -  doi:10.3354/meps306125

ABSTRACT: Despite increasing evidence that herbivory in seagrass systems is more important than previously considered, many factors regulating seagrass herbivory still need to be elucidated. In this study we evaluate the importance of epiphytes and seagrass blades in sea urchin nutrition using a multiple stable-isotope approach. Our aim is to contribute to the understanding of plant–herbivore interactions in seagrass beds. Stable-isotope ratios of carbon, nitrogen and sulphur of the sea urchin Paracentrotus lividus (Lamarck) were measured and compared to those of the potential food sources (i.e. the seagrass Posidonia oceanica [L.] Delile and its epiphytes) in a temperate seagrass meadow. Epiphytes and seagrass leaves had distinct δ¹³C (–18.7 vs. –12.8‰, respectively) and δ¹⁵N values (+6.4 vs. +3.6‰, respectively), while values for δ³⁴S were closer (+19.4 vs. +17.0‰, respectively). Data from δ¹³C and δ³⁴S indicated that both food sources contributed in approximately equal proportions (50%) to the bulk organic matter assimilated by Paracentrotus lividus, whereas δ¹⁵N measurements showed that approximately 90% of the nitrogen assimilated by the sea urchin is provided by epiphytes. Given the generally low nutritional quality of P. oceanica leaves, the data obtained suggest that epiphyte nitrogen is crucial in regulating the trophic relationships between the herbivore and the seagrass. Thus in P. oceanica meadows, epiphytes, particularly through nitrogen contribution, appear to be an essential component of the diet of important herbivores such as sea urchins.

KEY WORDS: Plant–herbivore interactions · Epiphytes · Stable isotopes · Nitrogen · Food quality · Mediterranean · Posidonia oceanica