Role of supratidal invertebrates in the decomposition of beach-cast green algae Ulva sp.

We examined the effects of supratidal invertebrates on the decomposition of the green alga Ulva sp. in Paracas Bay, southern Peru. The species identification of Ulva sp. was problematic; thus, we decided to refer to the broadbladed Ulva occuring in Paracas Bay by the generic name. We used stable isotopes of carbon and nitrogen to evaluate the flow of energy and nutrients from the decomposing algae to the community of supratidal consumers. We found that supratidal invertebrates had little effect on the decomposition rate of beach-cast macroalgae. Algae lost 46.5 % of their initial weight within 18 d from initial stranding ashore. The most abundant arthropods in samples of 100 g wet Ulva sp. packs were the springtail Paraxenylla peruensis (1165.9 ind. pack(-1)), a sphaerocerid fly (23.6 adults pack(-1)) and at least 3 species of mites (82.9 ind. pack(-1)). In terms of biomass, the main consumers were the talitrid amphipod Transorchestia chiliensis (42.8 mg pack(-1)), P. peruensis (2.8 mg pack(-1)) and mites (1.3 mg pack(-1)). Carbon isotopic values suggested that Ulva sp. was the most important source of energy to arthropod consumers, whereas nitrogen isotopic values indicated that mites and centipedes occupy high trophic positions in the supratidal food web. Paracas Bay abuts a hyper-arid desert wherein vegetation is extremely scarce and patchy. Supratidal invertebrates feeding upon decomposing Ulva sp., despite having little direct effect on the rate of Ulva sp. decomposition, establish a critical link between the marine and terrestrial ecosystems because they make marine-derived energy and nutrients available to desert consumers.