Bioerosive impact of Diadema mexicanum on southern Mexican Pacific coral reefs

Abstract

Reef ecosystem growth and maintenance is closely related to sea urchin feeding activities. Hence, quantifying the bioerosive capacity of the sea urchin Diadema mexicanum is necessary to evaluate its impact on coral communities and reefs that develop off the Pacific coast of southern Mexico. Sea urchin density, size, bioerosion rate, and type of carbonate removed were estimated, and the carbonate budget of 15 coral communities and reefs found off Guerrero and Oaxaca was calculated. Sea urchin density (H = 13.4, P = 0.001) and size (H = 139.5, P = 0.000) varied over space and were significantly (density, r² = 0.79, P = 0.00; size, r² = 0.28, P = 0.03) related to substrate characteristics. In general, size (test diameter) and density were directly related to rock coverage but inversely related to live coral cover. Carbonate removal was directly related to sea urchin size (<3.4 cm = 0.052 g/ind/d, 3.4–6.1 cm = 0.202 g/ind/d, >6.1 cm = 0.325 g/ind/d). Of the total carbonate removed, 22% corresponded to aragonite and 78% to magnesium calcite, indicating that sea urchin impact on coral reef skeletons is small. When reef carbonate budget was calculated, only 3.1% of the total carbonate deposited by reef corals was removed. The above data indicate that sea urchin feeding activity alone does not compromise reef ecosystem growth and maintenance in southwestern Mexico. Nonetheless, in the near future, the current policy of use and development in and around reef systems may boost the bioerosive impact of D. mexicanum in the area.