LAUSR

Some of the fundamental drivers of reef functioning are being rewritten through progress in understanding nutrient cycling. The sponge loop, described in 2013, revealed that some cryptic sponges can convert dissolved organic carbon (DOC) into detritus. In subsequent studies, researchers proposed that sponges and seaweeds could mutually reinforce their abundance by utilizing each other's waste products, thereby helping to suppress coral recovery. We build on these ideas, including the provision of a new but related hypothesis to explain the reinforcement of coral-algal phase shifts: Detritus and DOC resulting from fleshy algae reduce light penetration and increase the stress on corals, particularly in terms of calcification. We then consider the wider ecosystem implications of nutrient cycling and arrive at some novel conclusions. In particular, we propose that the combination of higher sponge biomass, which generates detritus, with the relative paucity of detritivory should generate a surplus of detritus on Caribbean reefs compared with that of the Indo-Pacific. New studies are needed to quantify the generality of the sponge loop and to track the fate of detritus to determine whether there is a surplus in the Caribbean. Although Charles Darwin identified enigmatic aspects of the nutritional status of coral reefs, this research field continues to surprise and redefine how we view the dynamics of this complex ecosystem.