Temporal variability in seawater Sr/Ca ratios within a coral atoll as an indicator of marine calcifier community diversity

Coral reefs are marine environments where calcium carbonate production and dissolution along with photosynthesis and respiration can have a large impact on the daily biogeochemical cycles of seawater dissolved inorganic carbon (DIC), total alkalinity (TA), oxygen (O2), and major nutrients. Using seawater samples from Tetiaroa Atoll, French Polynesia, we apply isotope dilution thermal ionization mass spectrometry (ID-TIMS) to show that these daily cycles of variability extend to the dissolved seawater Sr/Ca ratio (Sr/Casw). Hourly measurements over a 27+ hour period indicate that Sr/Casw covaries with DIC and TA and exhibits up to a 2% relative difference compared to the regional Pacific Ocean source water Sr/Casw value of 8.53 mmol/mol. We hypothesize that the primary mechanism of hourly variability in Sr/Casw is carbonate precipitation/dissolution by corals and other marine calcifiers on the Tetiaroa reef, while sedimentary redox processes may significantly contribute to an apparent decoupling between net calcification rates estimated from TA compared to dissolved calcium [Ca]sw concentrations. Using a simple Rayleigh distillation model and a two-member mixing equation, we estimate the net partition coefficient of Sr (KD-Sr) for each hourly sample as a mixture of coral (aragonite; KD-Sr -1.06) and crustose coralline algae (high Mg-calcite; KD-Sr -0.39) contributions to the measured variability of [Ca]sw and Sr/Casw. As the high Mg-calcites of crustose coralline algae are (1) integral components to the structure and stability of the reef framework and (2) more soluble than aragonite under more acidic conditions, we propose that continued analyses of Sr/Casw within coral reef environments could be a useful tool for monitoring the ecological distribution and contributions of marine calcifiers on reefs as ocean warming and acidification intensify in response to anthropogenic climate change. Additionally, seasonal, interannual, and/or centennial-scale offsets between the Sr/Casw of open ocean source seawater and ambient reef seawater may have significant implications for interpreting past changes in sea surface temperatures derived from the coral Sr/Ca paleothermometer.