Lateral, Vertical, and Temporal Variability of Seawater Carbonate Chemistry at Hog Reef, Bermuda

Spatial and temporal carbonate chemistry variability on coral reefs is driven by a combination of seawater hydrodynamics, geomorphology, and biological processes, though the relative importance of each process varies from site to site. In addition, it is often assumed that the water column above most reefs is well-mixed with small to no gradients outside of the benthic boundary layer. However, few studies to date have explored the processes controlling these multi-dimensional gradients. Here, we investigated the lateral, vertical and temporal variability of seawater carbonate chemistry on a Bermudan rim reef using a combination of spatial seawater chemistry surveys and autonomous in situ sensors. A current profiler was deployed at Hog Reef along with instruments measuring seawater temperature, salinity, pH, pCO₂, dissolved oxygen, and total alkalinity (TA) on the benthos, and temperature, salinity, dissolved oxygen, and pCO₂ at the surface. Water samples from spatial surveys were collected from surface and bottom depths at 13 stations covering ~3km², across four days, to characterize changes in carbonate chemistry across the reef, with depth, and over time. High frequency temporal variability in carbonate chemistry was driven mainly by tidal and diel light cycles on the reef. Estimates of relative Net Community Calcification (NCC) and relative Net Community Production (NCP) on the reef indicated a larger contribution of organic carbon cycling over inorganic carbon cycling. Spatial variability in carbonate chemistry parameters was larger laterally than vertically and also varied between sampling dates and times. Lateral gradients in carbonate chemistry largely reflected the depth profile and benthic community composition of the survey area as well as the predominant direction of water flow. These results highlight the interacting effects of hydrodynamic, biological, and physical processes on the variability of reef carbonate chemistry across space, depth, and time.