The effects of light intensity and flow speed on biogeochemical variability within a fringing coral reef in Onna-son, Okinawa, Japan

Global warming and ocean acidification are driving gradual declines in seawater dissolved oxygen concentrations and pH. Predicting how these changes will affect shallow, near-shore environments including coral reefs is challenging due to high natural variability on both spatial (10 m to km) and temporal (diel to seasonal) scales. To make predictions, it is first necessary to identify and quantify the drivers of this natural variability. While significant efforts have been devoted to characterising the influence from metabolic processes on coral reef seawater chemistry, less attention has been devoted to physical processes such as flow speed and light intensity. Here, we measured seawater flow, photosynthetically active radiation (PAR), pH, and dissolved oxygen (DO) at three reef habitats (reef flat, lagoon, and outflow channel) in a fringing coral reef system in Okinawa, Japan for a duration of 3 weeks in October of 2019. During the study, circulation was primarily wave-driven with mean flow speeds ranging from 14-26 cm/s. Flow direction became increasingly consistent at higher flow speeds, which traced visual patterns in the benthos observed from satellite imagery. Multiple linear regression models of daytime changes in pH and DO versus daily mean flow speed and PAR described 25-74% of the observed variability across all sites while at night, flow speed alone accounted for 35-69% of the observed variability. The results demonstrate that flow speed, trajectory, and PAR play important and variable roles in controlling biogeochemical variability within coral reefs and need to be considered in assessing their vulnerability to global climate change.