The Effects of Varying Thermal Stress and Light Intensity on the Growth of Orbicella faveolata

Llenos AL, Hills JN, Coddington NC, Coelho VR. 2017. The effects of varying thermal stress and light intensity on the growth of Orbicella faveolata. Abstract: Coral reefs are one of the most diverse ecosystems in the world, yet due to bleaching caused by the synergy between thermal stress and high light intensity, these ecosystems are being threatened. This study aimed to test the effects of different shade intensities paired with high temperature on the growth of the endangered Caribbean species, Orbicella faveolata. It was hypothesized that by decreasing light intensity, the corals would be better able to withstand thermal stress. Two hundred and fifty-nine coral fragments, from four coral colonies were used in the experiment. Ten to twelve fragments from each colony were placed per aquaria, for a total of 43 per tank. Six aquaria were placed under metal halide light fixtures, with two tanks per treatment: 26.5 degrees Celsius, no shade (control); 31.5 degrees Celsius, no shade (HTNS); and 31.5 degrees Celsius, 80 percent shade (HT80S). We determined cumulative thermal stress using degree heating week (DHW) measurements, following NOAA’s Coral Reef Watch methodology. Our results showed that over an approximate one-month period, up to DHW 8, the greatest growth was observed in the HTNS treatment (0.1 g buoyant weight), with no statistically significant difference between controls and HT80S. This suggests, at least in the short-term, that high temperature and high light intensity promote faster growth in this species. Further studies would be necessary in order to understand the long-term consequences of this rapid growth, particularly considering this is a type of boulder coral, which naturally exhibits slower growth. KEYWORDS: corals, Orbicella faveolata, coral bleaching, thermal stress, light intensity, climate change, degree heating week (DHW), buoyant weigh