The effects of ocean acidification, elevated temperature and herbivory on tropical crustose coralline algae

The coral reef ecosystem supports the greatest taxonomic diversity in the ocean and is shaped by a suite of biotic and abiotic factors. One ecological interaction that is important in structuring coral reefs is herbivory. A series of field surveys found that sea urchin grazing on crustose coralline algae was a frequent occurrence on the fringing reefs of Nanwan Bay, southern Taiwan, where more than 50% of grazing scars had been inflicted by sea urchins. Grazing assays conducted in southern Taiwan in August 2009 showed that the closely related sea urchin species Echinothrix diadema and Diadema savignyi had similar grazing effects on the tropical coralline alga Hydrolithon onkodes, where each species grazed similar amounts of coralline algal surface area. Hydrolithon onkodes is an abundant species of tropical coralline algae throughout the Indo-Pacific, and on the northshore of Moorea, French Polynesia H. onkodes accounted for up to 11% of the benthic cover on the shallow backreef. A series of grazing assays conducted in Moorea in January 2010 showed that the sea urchin D. savignyi had a greater effect on the cryptic species of coralline alga Goniolithon improcerum by grazing more surface area than the heavily calcified species H. onkodes. These results suggest that sea urchins are important grazers of tropical coralline algae, and that the extent of thallus calcification may impact susceptibility to grazing. Two environmental factors that are becoming an increasingly important influence on coral reefs are due to the increasing concentration of carbon dioxide in the atmosphere. The effects of elevated carbon dioxide (pCO2) on rates of calcification in coral reef habitats have been explored primarily for scleractinian corals with few studies addressing the impacts on crustose coralline algae. Laboratory experiments conducted in May-June 2010 found that elevated pCO2 significantly decreased calcification rates of the coralline alga H. onkodes by up to 51% and decreased rates of photosynthetic performance by 33%. Experiments conducted in Hawaii in August 2010 showed that elevated pCO2 and warmer seawater temperature combined had a significant interactive effect on the calcification rate of H. onkodes. Calcification was significantly reduced by elevated pCO2, but the effect was non-linear depending on temperature. In addition, elevated pCO2 and warmer temperature also increased the susceptibility of H. onkodes to grazing by sea urchins, where E. diadema grazed 60% more of H. onkodes that had been exposed to elevated pCO2. These results suggest that elevated pCO2 and warming seawater temperature have negative implications for the physiological calcification response in tropical crustose coralline algae, and that this physiological effect may have community-wide effects such as increased susceptibility to grazing by excavating herbivores. Coralline algae serve many important ecological roles in coral reefs, and the abundance of coralline algae, as well as their ecological functions, may be significantly reduced in the future with the progression of ocean acidification and sea surface temperature warming.