Complex ecological associations: competition and facilitation in a sponge-algal interaction

Over the past few decades, Caribbean coral reefs have undergone a phase shift from coral-dominated to algal-dominated communities due to several factors, including increased input of anthropogenic nutrients. With the decline in coral cover, sponges have also become more dominant members of Caribbean coral reef communities. Increased algal and sponge dominance on Caribbean reefs has led to an increase in the frequency of interaction between these 2 groups. This study used a factorial design to assess the independent and interactive effects of contact and elevated nutrient levels on 2 common members of these communities, the sponge Aplysina cauliformis, and the macroalga Microdictyon marinum. Algal contact had a significant negative physiological effect on A. cauliformis, affecting both the host sponge and its cyanobacterial symbionts. While elevated nutrient levels had some positive effects on the sponge photosymbionts, this only occurred in the absence of algal contact or a shading/abrasion control, and elevated nutrient levels had a negative effect on the sponge holobiont. In contrast, M. marinum responded positively to experimentally enhanced nutrient levels and to sponge contact under ambient nutrient regimes, but was not affected by sponge contact under elevated nutrient concentrations. Stable isotope enrichment experiments showed that the alga's positive response to sponge contact was associated with nitrogen transfer from the sponge over the course of the experiment. Thus, while A. cauliformis facilitates increased productivity in M marinum, algal contact competitively inhibits sponge condition.