Dissolved Oxygen:

Samples were collected into volume-calibrated flasks and preserved for Winkler dissolved oxygen analysis following standard protocols (Langdon, 2010). After collection, samples were stored until laboratory analysis following the procedures of Zhang et al. (2002). All samples were titrated within one week of collection at the Boston College Marine Biogeochemistry Lab using a custom-built Winkler titrator with automated potentiometric end point detection (control software available here: Dnicholson et al., 2023).

Precision of the sample collection and analysis procedure is determined by agreement between replicate measurements from the same location. Due to the need to preserve samples for short term storage, the majority of samples were collected in triplicate. After removal of outlier replicates in cases with triplicate sampling, median agreement among replicates was 0.18%. Larger discrepancies among replicates may reflect errors in sample collection and/or preservation; these results are reported for completeness but are flagged as questionable and/or bad (details of flagging scheme provided under “Problems/Issues” section).

Accuracy of sample measurements depends on standardization of the sodium thiosulfate titrant based on a reference standard. The sodium thiosulfate titrant used was determined by standardization with a 0.01N potassium iodate reference solution from Ocean Scientific International Ltd (OSIL). Lab-prepared potassium iodate standards, measured routinely throughout the monitoring period to verify titration accuracy and stability, were verified and adjusted by measurements against the OSIL standard.

Dissolved Inorganic Carbon and Total Alkalinity:

Samples were collected for dissolved inorganic carbon (DIC) and total alkalinity (TA) analysis following standard protocols (Dickson et al., 2007). Samples were collected into either 250 mL or 500 mL borosilicate glass bottles and preserved with saturated mercuric chloride (100 μL in 250 mL bottles, 200 μL in 500 mL bottles) for later analysis.

Samples were analyzed at the Boston College Marine Biogeochemistry Laboratory. DIC was analyzed using an Apollo SciTech AS-C6L DIC Analyzer and TA was analyzed using an Apollo SciTech AS-ALK2 TA Analyzer. Both DIC and TA were measured from each sample bottle. All DIC measurements were made on the day the bottle was opened for analysis, and TA measurements were made within the same week. DIC and TA instruments were calibrated daily and monitored throughout each analysis session by measuring Certified Reference Materials (Andrew Dickson, UCSD).

Analytical replicates were measured for all samples such that after analytical outliers (1 sigma) were removed, all samples for both DIC and TA retained at least two replicate measurements (median number of replicates for DIC = 3; median number of replicates for TA = 4). Analytical precision was determined for each sample as the standard deviation of analytical replicates. Mean analytical precision for all DIC samples in this dataset is 1.06 µmol/kg. Mean analytical precision for all TA samples in this dataset is 1.84 µmol/kg.