Bottom-up control of coral reef sponges: for better and for worse

Controversy over whether coral reef sponges are controlled bottom-up or top-down has distracted attention from more nuanced questions about what circumstances promote either bottom-up or top-down control, and how coral reef ecosystems are affected by what controls their sponges. Experiments now suggest that predators do not control coral reef sponges in their home habitat, shifting attention to effects of variations in sponge food supply. Comparing growth and survival of eleven of the most common and ubiquitous Caribbean coral reef sponge species on reefs in Belize and Panama differing in water column richness revealed faster sponge growth (by factors of 1.79-3.22 depending on the species) in the more picoplankton/nutrient-rich water at the Panama site. This could: a) benefit reefs by increasing positive effects of sponges, or b) harm reefs by sponges overgrowing corals. 'Sponges' is not a homogeneous category, so the balance between benefit and harm to reefs by increases in 'sponges' depends on both the species involved and environmental context. Predictions are complicated because sponge responses to increased nutrients/picoplankton are not monotonic: moderate increases boost sponge growth rates, but very dense blooms can devastate reef sponge populations. The same sponge species which grew faster in a somewhat richer water column (2 times as much picoplankton) lost nearly 2/3 of their biomass during a dense (8 times denser than normal) cyanobacteria bloom on the Belize Barrier Reef, reducing their unique positive functional roles of promoting water column clarity, symbiont habitat, food for charismatic megafauna, coral survival, and reef regeneration.