Coral reefs are in dramatic global decline, with seaweeds commonly replacing corals. As a result, coral-seaweed contacts will increase for both remaining adult corals and new coral recruits-yet the consequences of increasing contacts are poorly understood. It remains unclear whether seaweeds cause coral mortality via direct competition or whether they colonize reefs opportunistically following coral death from other causes. Thus, we know little about how the outcome of seaweed-coral competition varies among species, the relative importance of different competitive mechanisms involved in these interactions, or how herbivory mediates seaweed-coral competition. To address these questions, we conducted pair-wise seaweed-coral contact manipulations (8 seaweeds x 4 corals, n=10-12 per interaction) on a reef flat in Fiji. To assess the most plausible mechanism for the patterns we observed in these experiments, we then tested the effect of lipid-soluble extracts from each seaweed on corals. Because our seaweed-coral manipulations were conducted in the absence of herbivory, seaweeds were then transplanted onto reefs to determine how herbivory may mediate seaweed-coral competitive interactions by limiting seaweed abundance.
Results/Conclusions
Here, we show that lipid-soluble metabolites from seaweed surfaces directly damage corals – causing suppressed photosynthesis, coral bleaching, and occasionally death in 75% of 32 seaweed-coral interactions in Fiji. Effects of seaweeds on corals were limited to areas of direct contact for 94% of the interactions assayed, and declining corals like Acropora were more susceptible than more stable corals like Porites. Herbivory appears critical for minimizing seaweed damage to corals, and for maintaining reef resilience. In replicate areas protected from fishing, seaweeds from our competition studies were rapidly consumed by fishes, corals were abundant, and seaweeds rare. In adjacent fished areas, grazing was low, seaweeds abundant, and corals rare. Thus, numerous seaweeds can damage corals directly via surface-associated allelotoxins when reefs are fished and seaweeds released from herbivore control. Management of processes such as herbivory can suppress seaweeds, minimize seaweed-coral contacts, prevent allelopathic damage to corals, and thus improve reef resilience.