Context-dependent interaction between an intertidal sponge and a green macroalga in a variable temperate Patagonian bay

The stress gradient hypothesis states that positive interactions become more important as physical stress increases. Organisms living in Patagonian intertidal ecosystems are subjected to high desiccation stress due to elevated temperatures and strong winds. In coastal environments, sponges and macroalgae are common benthic organisms, and they occur from the subtidal to intertidal zones. As human pressure has increased in these systems, both sponges and macroalgae have become more abundant, which has led to more frequent interactions between them. To evaluate the interaction between Hymeniacidon perlevis and Ulva lactuca in 2 contrasting environments, we sampled their abundance and conducted field factorial experiments consisting of sponge transplants and manipulation of algal presence. Our results show that in harsher environments, U.lactuca facilitates H.perlevis likely through the reduction of direct solar radiation or high substrate temperature beneath its canopy, whereas in milder environments, U.lactuca negatively affects H. perlevis throughout other physical or chemical mechanisms (e.g. abrasion, feeding interference through changing current dynamics, depletion of oxygen, allelochemical compound production).