Primary succession of coral-reef algae: Differing patterns on fished versus unfished reefs

Abstract

Experimental coral plates were placed on four coral reefs to determine the effect that sea urchin and herbivorous fish grazing, and river sediments have on successional changes in algae. Algal functional-group composition, standing crop and chlorophyll concentrations were made eight times over a 450 day period. Despite large differences in the species composition of the grazing community, algal development and composition were distinct depending on whether the dominant grazers were sea urchins or herbivorous fishes. In the two reefs dominated by sea urchin grazers, the plates were colonized by filamentous algal turfs and the full succession was complete in less than 50 days. Algal wet weight and functional group composition were stable over the entire period despite strong seasonality in environmental conditions. In contrast, coral plates established on reefs dominated by herbivorous fishes did not complete their successional development for about 120 days, passed through more successional stages, and when fully developed had a greater diversity of functional groups, standing crop and chlorophyll concentrations. Seasonal changes on the plates were also more pronounced in the fish-dominated reefs and the highest standing crops were found during the southeast monsoon when solar radiation is low, and waters are cool and low in nitrogen. River discharge onto one of the fish-dominated sites reversed the algal succession from dominance by crustose coralline algae to filamentous turfs rather than accelerating the succession towards brown macroalgae—the expected climax. Metabolic effects of fish grazers can modify the effects of environmental conditions on algal community structure and succession. Beyond simply cropping algae, grazers can influence sedimentation and nutrient concentrations and ratios, which makes the specific mechanisms responsible for these observations difficult to determine. Grazing habits that increase nutrient concentrations, sedimentation and disturbance appear to arrest succession in the turf stage. A general conceptual model of successional development under different grazer functional groups is presented.