Ocean Acidification Impacts on the Physiology and Calcification of Reef-Building Corals in the Eastern Gulf of Guinea: A Multi-Stressor Experimental and Modelling Study

Ocean acidification represents a fundamental perturbation to the carbonate chemistry environment of reef-building corals, with impacts compounded by simultaneous thermal stress and turbidity in the eastern Gulf of Guinea. This research examines the interactive effects of ocean acidification, elevated sea surface temperature, and turbidity on the physiology, calcification rates, and cellular stress responses of two dominant coral species (Porites asteroides and Siderastrea radians) through controlled multi-stressor mesocosm experimentation and a mechanistic physiological model. Coral nubbins were exposed to a full factorial combination of current and projected CO2 concentrations (400 and 900 ppm), temperatures (current ambient and plus 2 degrees Celsius), and turbidity levels (low and high). Calcification rates were measured using buoyant weight technique. Photosynthetic efficiency of zooxanthellae was assessed by pulse amplitude modulated fluorometry. Cellular indicators including heat shock protein expression, reactive oxygen species production, and lipid peroxidation were quantified. Symbiodiniaceae density and clade composition were characterised using cell counting and ITS2 metabarcoding. Acidification reduced calcification rates by 18 to 34 percent under current temperatures. Thermal stress at elevated CO2 produced synergistic calcification reductions of 52 to 71 percent exceeding the sum of individual effects. Turbidity partially ameliorated thermal bleaching by reducing photoinhibition under high CO2 and temperature, but simultaneously suppressed zooxanthellae photosynthesis in non-thermally stressed treatments. A bioenergetics physiological model parameterised with the experimental data accurately reproduced the non-linear interactive responses and was extended to project coral growth under RCP2.6 and RCP8.5 scenarios. The study makes original contributions to multi-stressor interaction theory in tropical coral physiology. Keywords: ocean acidification, coral calcification, multi-stressor experiment, Gulf of Guinea, bleaching.