Dataset: Chlorophyll-a concentrations in seawater near the BATS station, cruises AE2113 and AE2303

Seawater samples were collected using Niskin bottles from CTD casts taken at the Bermuda Atlantic Time-series Study (BATS) station aboard the R/V Atlantic Explorer during a summer cruise in 2021 (AE2113) and a winter cruise in 2023 (AE2303). Seawater samples were collected in pre-cleaned opaque brown polyethylene bottles and then analyzed for chlorophyll a (Chl-a) following extraction into 90% acetone:10% high purity water according to the method outlined in Welschmeyer, 1994. Briefly, after a seawater sample was collected from the CTD rosette, a known volume of that seawater was filtered through a 25 mm diameter GF/F glass fiber filter under subdued lighting and at ~100 mm Hg pressure differential. Samples were filtered within 30 min of sample collection. The filter-funnel walls were then rinsed with previously filtered seawater. The filter was folded with a flat-tipped forcep and placed in a borosilicate test tube that was tightly capped. The test tube was wrapped in Al foil and frozen. Once several samples were processed and stored this way, they were removed from the freezer and 4 mL of 90% (v/v) acetone (Burdick and Jackson, spectrophotometric grade) diluted with high-purity laboratory water (Milli-Q water from a Barnstead water system) was added. The samples were vigorously shaken, wrapped in Al foil, and returned to the freezer without letting the samples return to room temperature. The Chl-a extracted overnight in the freezer (ca. 12 hr) in the dark. Chl-a in the acetone extracts was quantified by fluorescence under subdued lighting using a Turner Designs model AU-10 fluorometer. Samples were removed from the freezer and warmed to room temperature in the dark prior to analysis.