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

Abstract

Spatial and temporal carbonate chemistry variability on coral reefs is influenced by a combination of seawater hydrodynamics, geomorphology, and biogeochemical processes, though their relative influence varies by site. 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 and properties 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. Instruments were deployed at Hog Reef measuring current flow, seawater temperature, salinity, pHT, pCO2, dissolved oxygen (DO), and total alkalinity (TA) on the benthos, and temperature, salinity, DO, and pCO2 at the surface. Water samples from spatial surveys were collected from surface and bottom depths at 13 stations covering ∼3 km2 across 4 days. High frequency temporal variability in carbonate chemistry was driven by a combination of diel light and mixed semi-diurnal tidal cycles on the reef. Daytime gradients in DO between the surface and the benthos suggested significant water column production contributing to distinct diel trends in pHT, pCO2, and DO, but not TA. We hypothesize these differences reflect the differential effect of biogeochemical processes important in both the water column and benthos (organic carbon production/respiration) vs. processes mainly occurring on the benthos (calcium carbonate production/dissolution). Locally at Hog Reef, the relative magnitude of the diel variability of organic carbon production/respiration was 1.4–4.6 times larger than that of calcium carbonate production/dissolution, though estimates of net organic carbon production and calcification based on inshore-offshore chemical gradients revealed net heterotrophy (−118 ± 51 mmol m–2 day–1) and net calcification (150 ± 37 mmol CaCO3 m–2 day–1). These results reflect the important roles of time and space in assessing reef biogeochemical processes. The spatial variability in carbonate chemistry parameters was larger laterally than vertically and was generally observed in conjunction with depth gradients, but varied between sampling events, depending on time of day and modifications due to current flow.

Characterizing Reef Net Metabolism Via the Diel Co‐Variation of pH and Dissolved Oxygen From High Resolution in Situ Sensors

“…The DO ranges recorded at our sites 30.7 μmol/L at Goff's Caye fore reef (30.0 μmol/kg), 32.0 μmol/L at Goff's Caye back reef (31.3 μmol/kg), 11.9 μmol/L at English Caye (11.6 μmol/kg), and 7.6 μmol/L in Dominica (7.4 μmol/kg) are generally lower than those reported in other studies (e.g., 22-54 μmol/kg in Bermuda (Pezner et al, 2021;Takeshita et al, 2018) and in Puerto Rico 60 μmol/L (fall) and 120 μmol/L (winter) (Gray et al, 2012)).…”

“…More recently, mean daily DO concentrations and ranges from 32 reefs around the globe have been reported as 173 ± 28 μmol/kg and 81 ± 52 μmol/kg respectively (Pezner et al, 2023). The greater DO amplitudes reported in the literature do not always correspond to greater pH ranges (0.067 (winter) and 0.093 (fall) in Puerto Rico (Gray et al, 2012) and 0.07 at Hog Reef (Pezner et al, 2021;Takeshita et al, 2018)) and could imply different biogeochemical and metabolic regimes, reflecting variations in benthic ecosystem compositions. Minimum DO values recorded at English Caye and Goff's Caye (both fore reef and back reef sites) were on occasion below 156 μmol/L, a threshold below which reef ecosystems are considered "weakly hypoxic" and therefore under stress (Vaquer-Sunyer & Duarte, 2008).…”

“…At a fixed TA, pH correlates negatively with dissolved CO 2 according to the speciation of carbonic acid in seawater (Zeebe & Wolf‐Gladrow, 2003). Therefore, a positive correlation between DO and pH can be expected in productive marine habitats such as kelp forests (Frieder et al., 2012), seagrass beds (Hendriks et al., 2014), and coral reef systems (Gray et al., 2012; Pezner et al., 2021). Divergence from the predicted P:R model could indicate that other physical, chemical, or biological processes may be at play (besides photosynthesis and respiration).…”

The Effects of Light Intensity and Flow Speed on Biogeochemical Variability Within a Fringing Coral Reef in Onna‐Son, Okinawa, Japan

“…Two-sample t-tests performed at each site indicated that mean flow speed did not differ significantly between day and night (p > 0.6, Table S4 in Supporting Information S1) suggesting that mean primary production rates during the day were greater than mean respiration rates at night and the Onna-son reef system was net autotrophic throughout the study period. Although calcification and CaCO 3 dissolution also affect seawater pH, they do not directly affect DO concentrations, and rates and variability of organic carbon cycling on reefs are typically greater than rates and variability of net community calcification over the diel timescales considered in this study (Cyronak et al, 2018;Pezner et al, 2021;Takeshita et al, 2018).…”

“…Calcification and CaCO 3 dissolution decrease and increase pH, respectively, with no direct influence on DO. These processes occur within both benthic and water column communities (Kinsey, 1985; Pezner et al., 2021; Smith et al., 2013; Tilstra et al., 2018). The degree to which the biogeochemistry of reef seawater is modified depends on the relative rates of these processes, which vary over diel (Gruber et al., 2017; Kekuewa et al., 2021) and seasonal timescales (Davis et al., 2020; Page et al., 2019), and the benthic communities modifying the overlying seawater (e.g., seagrass, seaweed, sand, rubble, and corals; Jokiel et al., 2014; Page et al., 2016).…”