Macroalgal community response to herbivores and sediment deposition: an indicator of coral reef degradation

Abstract

The effects of herbivores and sediment deposition on the structure and composition of the macroalgal communities on coral reefs were tested together experimentally and, in the field, in one of the most disturbed reefs of the Caribbean, the leeward reefs of the Bocas del Toro region, Panama. In this area, the potential natural herbivores have been replaced by small-bodied herbivores, mainly the urchin Echinometra viridis, and sediment deposition has increased as a result of land use in the zones close to the reefs. Experimental studies consisted of herbivorous exclusion and artificial increase in sediment deposition; while, in different sites of the studied area, relationships between macroalgal communities, herbivore abundance and sediment deposition were examined in situ. Both approaches suggest that changes on macroalgal species and the structure of the communities are mediated by herbivores and sediment deposition. The experimental approach showed an interaction between the studied factors. Sediment deposition, at the levels used in the experiment, did not limit the grazing of E. viridis. This urchin controlled the development of communities, mainly the erect macroalgae, independently of the level of sediment deposition tested. However, with the treatment of herbivores present and increase of sediment deposition, Ceramiales turf replaced cyanophytes. This contrasted with our findings in the field, where extreme values of sediment deposition coincided with minimal urchin populations. These last conditions favoured communities dominated by erect macroalgae, but their composition and abundance differ from those of communities that develop under low herbivory and low sedimentation because the deposited sediments also influence the community structure.