Abstract :
[en] Polyspecific seagrass meadows are of critical ecological importance in tropical coastal zones. These ecosystems provide a wide range of socio-economical services to local populations. Meadows however undergo multiple threats linked to human activities (increased nutrient input, overfishing, invertebrate overharvesting, etc.). It is currently difficult to assess how seagrass meadows could respond to anthropogenic impacts due to poor knowledge of their functional ecology.
Here, stable isotope ratios of carbon, nitrogen and sulfur were used to unravel trophic interactions ruling the food webs associated to seagrass beds of the Toliara Great Reef (SW Madagascar). The contributions of various potential food items (sediment-associated and suspended particulate organic matter, plankton, leaves, roots and epiphytes of 7 seagrasses and thalli of 7 dominant macroalgae) to the diet of 20 invertebrate taxa (one sea urchin, 2 sea stars, 2 sea cucumbers, 5 gastropods including one sea hare, one bivalve, 2 amphipods, one leptostracan, one cumacean 2 hermit crabs and 3 shrimps) were assessed using a Bayesian stable isotope mixing model.
Model outputs revealed that important trophic diversity existed among the invertebrate assemblage. In some groups (e.g. hermit crabs and amphipods), resource use by morphologically and taxonomically close taxa was markedly different. Many of the dominant taxa heavily relied on macroalgae for their nutrition. On the other hand, few species apparently consumed seagrass tissues. Moreover, when they did, seagrass generally accounted for a minor portion of the diet only. Overall, our results suggest that seagrass grazing in meadows of the Toliara Great Reef could be lower than in other tropical areas. These discrepancies could be linked with seasonal variation in resource availability or with eutrophication. Higher nutrient load is indeed known to cause ecosystem phase shifts and it may induce diet shift to algivory in some invertebrate consumers.