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

Bottom-up and top-down controls regulate the structure and function of ecosystems through trophic resources and consumption pressure, respectively. The relative contributions of both controls over tropical sponges have been documented; however, it remains unknown how these controls regulate sponge populations in temperate environments. We focused on the globally distributed sponge Hymeniacidon perlevis inhabiting two tidal channels in San Antonio Bay (Argentine Patagonia) with different anthropogenic nutrient loads and experimentally tested the relative contribution of spongivores (i.e., sponge consumers) and trophic resources (i.e., dissolved inorganic nutrients and POC proxies) in sponge growth. The presence of spongivores was evaluated, as well as the relevance of trophic resource concentrations in the sponge abundance pattern. Hymeniacidon perlevis was more abundant (5.42% vs. 1.29% in cover), grew more (39.6% vs. −10.9% in volume, 89.5% vs 13.9% in surface area), and experienced less biomass reduction (−19.9% vs. −46.2% in dry weight) in the channel with the highest concentration of trophic resources compared to the non-enriched channel, while spongivores had a negligible effect. Among trophic resources, nitrate concentration was the one that best explained the abundance pattern of H. perlevis, with sponge cover changing by 1.02% for each μmol L− change in nitrate concentration. Overall, our results show that the population of H. perlevis is mostly bottom-up controlled. The role of a microbial symbiotic pathway in the fulfillment of the nutritional requirements of H. perlevis is also discussed.