Seasonal changes in seawater calcium and alkalinity in the Sargasso Sea and across the Bermuda carbonate platform

Alyssa Jean Griffin1, Zachary Anderson2, John Ballard1, Nicholas Robert Bates2, Rebecca Garley3, Rodney Johnson2, Todd R Martz1, Fernando Pacheco3, Yuichiro Takeshita4 and Andreas J Andersson1, (1)Scripps Institution of Oceanography, La Jolla, CA, United States, (2)Bermuda Institute of Ocean Sciences, St. George's, Bermuda, (3)Bermuda Institute of Ocean Sciences, St.George's, Bermuda, (4)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States
Ocean acidification may shift reefs from a state of net ecosystem calcification (+NEC) to net ecosystem dissolution (–NEC). Evidence of –NEC is typically inferred from either an increase in measured or calculated total alkalinity (TA) or the dissolved calcium (Ca) to salinity ratio relative to source water values. Due to the analytical uncertainty and challenges associated with Ca measurements, the alkalinity anomaly technique is the preferred method to measure changes in NEC. However, certain assumptions must be made in order to use TA as a proxy for NEC, because other processes can influence seawater TA (e.g., nutrient fluxes, organic acid-base systems, redox processes). Since Ca concentrations are mostly unaffected by these other processes, Ca and TA anomalies relative to open ocean source water can be used in conjunction to identify changes in NEC with greater confidence.

Here, we present a two-year time series of monthly seawater Ca and TA measurements taken from locations across the Bermuda carbonate platform and the nearby Bermuda Atlantic Time-series Study (BATS) location offshore. High precision (2-5 µmol/kg) Ca measurements were conducted using a novel spectrophotometric titration technique and closely followed changes in alkalinity over both spatial and temporal scales. Inshore measurements of Ca and TA showed seasonal fluctuations with depletion during summer and no difference/repletion during winter relative to offshore waters. These fluctuations were due to +NEC during summer and near zero or –NEC (dissolution) during winter across the Bermuda platform. The seasonal patterns were most pronounced at the farthest inshore location where water was older than at the other inshore locations. Measurements of Ca and TA concentrations across coral reefs and in adjacent source waters over multi-annual timescales are important, as they could improve our predictions for when and where a reef system, such as the Bermuda platform, may shift from +NEC to –NEC.