KINANE, S.*; THOMAS, F.I.M.; University of South Florida; University of South Florida: Bleaching in the coral Porites divaricata: effects of temperature, light, and water motion

Coral bleaching, the loss of endosymbiotic dinoflagellates (zooxanthellae) or their photosynthetic pigments, is affecting coral reefs throughout the world. High water temperature and strong solar irradiance contribute to coral bleaching and there is evidence that water motion may lessen these effects. We examined the effects of temperature, light, and water motion on coral bleaching in the shallow-water coral Porites divaricata. A fully-factorial randomized block design was used to test the effects of two levels of each of these three variables on bleaching. Bleaching was measured as a change in zooxanthellae density and chlorophyll a concentration over the course of each four-day experiment. Stress on zooxanthellae Photosystem II was monitored using a pulse-amplitude modulated (PAM) fluorometer. We measured minimum dark-acclimated fluorescence (F_o), maximum dark-acclimated fluorescence (F_m), and variable fluorescence (F_v = F_m – F_o). Decline in the ratio F_v/F_m during the course of each experiment was used to quantify photoinhibition and bleaching stress. High temperature and high light caused a reduction in zooxanthellae density, chlorophyll a concentration, and F_v/F_m. Temperature- and light-induced bleaching was reduced in high-velocity water flow (15-20 cm s^-1) compared with low velocity flow (2-3 cm s^-1).