Seasonal and annual calcification rates of the Hawaiian reef coral, Montipora capitata, under present and future climate change scenarios

Abstract

The response of corals to future conditions of global warming and ocean acidification (OA) is a topic of considerable interest. However, little information is available on the seasonal interaction between temperature, pCO2, and irradiance under ecologically relevant experimental conditions. Controlled experiments were performed in continuous-flow mesocosms under full solar radiation to describe the direct and interactive effects of temperature, irradiance, and pCO2 on growth of a Hawaiian reef building coral (Montipora capitata) over an annual cycle. Corals were subjected to 12 experimental treatments consisting of two pCO2 levels (present-day levels, 2× present), two temperature regimes (ambient, heated +2°C), and three conditions of irradiance (ambient, 50 and 90% reduction). A multiple polynomial regression model with full factorial fixed factors (temperature, pCO2, irradiance) was developed. Temperature and irradiance were the primary factors driving net calcification (Gnet) rates of M. capitata, with pCO2 playing a lesser role. Gnet showed a curvilinear response to irradiance and temperature, which defines thresholds at the end members. Also, high irradiance regimes under elevated temperatures showed a negative synergistic effect on Gnet. Therefore, decreasing irradiance penetration resulting from greater depth and/or higher turbidity will lower the impact of ocean warming on M. capitata. Results suggest that under future climate conditions, the interaction of environmental parameters may shift seasonal patterns in Gnet and timing of growth optima for M. capitata. Ocean warming in shallow water environments with high irradiance poses a more immediate threat to coral growth than acidification for this dominant coral species. In the future, increased temperature and the interaction between high irradiance and high temperature will be the main factors controlling Gnet with OA playing a less important role. This observation is congruent with other reports that high temperature combined with high irradiance is the main cause of high coral mortality during mass bleaching events.