Seagrass beds are important coastal ecosystems worldwide that are shaped by facilitative interactions. Recent theoretical work has emphasized the potential for facilitative interactions involving foundation species to be destabilized in the face of anthropogenic change. Consequently, it is important to identify which taxa facilitate seagrasses. In other ecosystems, sponges contribute to the maintenance of diverse and productive systems through their facilitation of foundation species (e.g., mangroves) and the retention and recycling of energy and nutrients. Sponges are common in tropical and subtropical seagrass beds, yet we know little about how their presence impacts these communities. Here, we examine the impact of the sponge Ircinia felix on primary producers in a Thalassia testudinum dominated seagrass bed using a long-term field experiment in The Bahamas. We transplanted live sponges into the center of 5 m x 5 m plots and monitored the response of seagrasses and macroalgae. Sponge presence increased seagrass nutrient content and growth, as well as the abundance of macroalgae and non-dominant seagrass species (Syringodium filiforme and Halodule wrightii). These changes were not seen in the control (unmanipulated) or structure (where we placed a polypropylene sponge replica) plots. We conclude that I. felix facilitates seagrass bed primary producers in oligotrophic systems, likely due to nutrients supplied by the sponge. Our study shows that sponges can have a positive influence on seagrass bed foundation species. Further work is needed to understand how this facilitation impacts the stability of seagrass beds in areas where human activities have increased ambient nutrient levels.

See https://github.com/ecophilina/ircinia.