Increased Algal Dominance Despite Presence of <I>Diadema Antillarum</I> Populations on a Caribbean Coral Reef

Abstract

Populations of Diadema antillarum (Philippi, 1845) have been slow to recover after their 1983 Caribbean-wide massive mortality. The continued absence of this important coral reef herbivore as well as further impacts on local and regional-scales have severely threatened the health of coral reefs. From 2008 to 2011, the population of D. antillarum and structure of the benthic community were monitored within Akumal Bay, Mexico, a heavily impacted ecosystem. Across the 4 yrs of the study, densities of adult D. antillarum did not significantly change and were highest on backreef sites (mean 1.30 and 1.44 m−2). Coral cover was low at all sites (maximum 14%) and decreased during the 4 yrs of the study. Changes in crustose coralline algae (CCA) cover varied by year but was significantly higher on forereef sites, while cover by turf-algal-sediment (TAS) increased at all four sites. Diadema antillarum densities correlated positively with coral and CCA cover in the backreef sites where sea urchin densities were higher but no correlation occurred with TAS and turf. Where Echinometra lucunter (Linnaeus, 1758) sea urchins were more prevalent, their densities correlated negatively with coral and positively with CCA. Although densities of adult D. antillarum were comparable to other regions of the Caribbean, increases in TAS over the 4 yrs of this study may be evidence that even an important herbivore cannot prevent continued decline of a reef in an impacted ecosystem. Coral reefs in the Caribbean have experienced major disturbances over the past few decades and continue to be under a combination of stressors including bleaching, overexploitation of herbivores, coral diseases, and declining water quality, which has resulted in an increase in macroalgae cover (Knowlton 1992, Aronson and Precht 2001, Hughes et al. 2003, Bellwood et al. 2004). The importance of long-spined sea urchin, Diadema antillarum (Philippi, 1845), in removing macroalgae cover became evident after their massive mortality in 1983, when algal biomass abruptly increased (de Ruyter van Steveninck and Bak 1986, Carpenter 1988, Hughes 1994). As the populations of D. antillarum recover after their 1983 Caribbean-wide massive mortality (Lessios et al. 1984b, Lessios 2005), it remains unclear if grazing by this herbivore provides a suitable driver from which to expect a decrease in macroalgal cover on impacted coral reefs. Modeled scenarios indicate that sea urchins impart significant resilience to Caribbean reefs such that continued decline is inevitable when sea urchins are scarce (Mumby et al. 2006). However, abundant literature indicates it is a combination of factors which impacts the benthic dynamics on coral reefs and the propensity for shifts between dominant states [e.g., competition for space (Sandin and McNamara 2012), herbivore diversity (Cheal et al. 2010), biotic histories (Aronson et al. 2004), global climate change and marine diseases (Aronson and Precht 2006)]. CORAL REEF PAPER FastTrack➲ publication