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.