Direct effects of elevated temperature, reduced pH, and the presence of macroalgae (Dictyota spp.) on larvae of the Caribbean coral Porites astreoides

Through an aquarium-based study, we provide evidence that exposure to macroalgae (Dictyota spp.), seawater with reduced pH (7.6 vs 8.1), and elevated temperature (31.8 vs 28.8 °C) causes distinct and additive repercussions for larval settlement and condition (photochemical efficiency and oxidative stress). Larvae from the common Caribbean coral Porites astreoides (Lamarck, 1816) were provided settlement substrate and exposed to each factor for 72 hrs in isolation and combination under an orthogonal design. Largely, biotic and abiotic factors did not interact in their impact on coral planulae; instead, stressors independently affected the survival, condition, and settlement of P. astreoides. Dictyota spp. and low pH each individually reduced the survival of coral larvae. Furthermore, the presence of Dictyota spp. and elevated temperature distinctly inhibited larval settlement and photochemical efficiency, respectively. When combined, stressors additively increased cellular oxidative damage (lipid peroxidation) approximately four fold compared to larvae maintained under control conditions. The results indicate that each stressor independently impacts distinct and overlapping facets of coral settlement, and suggest that their combined effects could have severe collective consequences for coral demography