The Grazing Impact of Microherbivores on Algal Growth in Exposed and Sheltered Coral Reef Habitats

Abstract

Algal turfs are rapid colonizers of space on a reef, providing competition for space with coral recruits and often overgrowing healthy corals. Algal turf abundance is largely determined 1) nutrient uptake, which can be increased with flow, and 2) herbivory. Previous studies have highlighted the importance and impact of herbivorous fish on limiting algal turf biomass, but have minimized the impact of microherbivores. The results of these studies have caused many to ignore the grazing impact of microherbivores. Others have emphasized the necessity of more research, acknowledging the dearth of information in this area. This study sought to remedy this oversight by examining the impact of microherbivores in areas of high and low wave exposure, while also plotting out the algal growth curve – something that had yet to be done – in both site types. Experimental tiles were placed in exclusion cages at two exposed sites and two sheltered sites on Uelchelbeluu Reef, Palau, allowing the partitioning of the grazing impacts of microherbivores while examining the role of flow in algal growth. Microherbivores consumed algal turf growth more in exposed sites than sheltered sites but their impact was likely dampened by the cages in which the algae tiles were placed, as cages were found to reduce the abundance of microherbivores in all sites. Of the microherbivores, only diogenid hermit crabs were more abundant at exposed sites. Surprisingly, flow did not affect turf growth rate or the maximum growth of the turf. These results suggest microherbivores, especially hermit crabs, are more important than they have previously been given credit for, and that their role should not be ignored moving forward.