Urchins Are Important Herbivores and Mediate Positive Effects of Sediment on Algal Turfs

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Abstract Summary

While the importance of herbivorous fishes in supporting the health of coral reefs has been established globally, the role of herbivorous invertebrates has only been evaluated in the Caribbean, where urchins were found to have compensated for a long history of overfishing when a catastrophic decline in sea urchins resulted in transitions from healthy short algal turfs to long sediment-laden turfs or macroalgae that prevent coral recovery. It is unknown whether urchins can compensate for overfishing on reefs of the tropical Pacific subjected to multiple human stressors. We conducted a two-factor field experiment varying access to urchins and sedimentation on the algal turf community of an overfished fringing reef of Moorea, French Polynesia. In the absence of urchins, turf height increased, especially with the addition of marine sediments, possibly due to increased nutrients supplied by sediments. In contrast, mixed terrestrial sediments did not enhance turf growth. Urchins maintained short algal turf, mitigatig the positive effect of marine sediments. However, urchins avoided turf covered by terrestrial sediments. Across all sediment treatments, urchin grazing removed sediments. Overall, urchins contributed to the maintenance of healthy turf on Pacfic coral reefs by heavily grazing turfs and removing sediment, which may promote coral reef resilience.

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Environmental Science