Relative contribution of top-down and bottom-up controls on the regulation of the sponge Hymeniacidon perlevis (Montagu, 1814) in Patagonia: an experimental and observational approach.

Abstract

Abstract Two main perspectives explain the structure of food webs: bottom-up and top-down controls, respectively driven by trophic resources and consumption pressure. There is evidence of the relative contribution of both controls over sponges in tropical environments, but it is still unknown how these controls mediate the regulation of sponge populations in temperate environments. To assess this, we performed in situ manipulative experiments and samplings on the worldwide-distributed sponge Hymeniacidon perlevis in two tidal channels from San Antonio Bay (Argentine Patagonia), with different anthropic nutrients loads. We first experimentally tested the relative contribution of the top-down and bottom-up controls on the individual sponge growth. Then, we identified the consumers of H. perlevis, assessing their consumption pressure. Finally, we assessed the abundance of H. perlevis and the concentration of particulate and dissolved trophic resources for the sponge at both tidal channels. We found that H. perlevis was more abundant and grew more in the channel with the highest concentration of trophic resources, despite the consumption pressure of several consumers (ophiuroids, polychaetes, crabs and fishes). Among the available trophic resources, nitrate, phosphate and total suspended solids best explained the abundance pattern of H. perlevis throughout a year. Our results show that the abundance and individual growth of H. perlevis is mostly bottom-up controlled, through particulate but mainly dissolved trophic resources. The role of a symbiotic pathway in the fulfillment of the nutritional requirements of H. perlevis is also discussed.