Samples from the submarine groundwater wells and overlying water column were collected quarterly from November 2022 through March 2024 aboard U.S. Geological Survey (USGS) small boats. This sampling effort was led by collaborator Chris Smith at USGS. Water column samples were collected by lowering the sampling line, acid-cleaned ½” OD Bev-A-Line tubing attached to ¼” synthetic line that was weighed down by a 30 lb kettlebell to target depths. Surface samples were collected at ~1 m depth and bottom samples were collected from ~1 m above the sea floor. A separate sampling line of acid-cleaned 3/8” OD Bev-A-line connected to a custom-made PVC adapter was used for sampling the submarine groundwater wells, which had been installed by USGS. Different Teflon diaphragm pumps were used for the water column and wells samples to avoid cross contamination. Samples for macronutrient analysis were filtered through 0.2 µm Pall Acropak Super membrane filter capsules, collected in acid-cleaned and triple-rinsed 15-mL Falcon tubes and stored in ziplock bags and frozen (-20 ºC) until analyzed following recommended practices (Becker et al., 2020).
Samples for riverine and estuarine end members were collected by collaborators from the University of South Florida as described in Conway and Hunt (submitted). These macronutrient samples were filtered using an acid-cleaned and triple-rinsed polypropylene syringe coupled with a PVDF 0.2 µm filter, collected in acid-cleaned and triple-rinsed 15-mL Falcon tubes and stored in ziplock bags and frozen (-20 ºC) until analyzed following recommended practices (Becker et al., 2020).
Macronutrients samples were analyzed for phosphate, nitrate+nitrite, silicate, and nitrite on a QuAAtro39 AutoAnalyzer (SEAL Analytical) according to standard colorimetric methods (Strickland and Parsons, 1972). All reagents were prepared in dedicated labware with high purity Milli-Q (>18 MΩ cm) water. Working standards were prepared fresh daily in an artificial seawater (ASW; 35 g/L sodium chloride, 0.5 g/L sodium bicarbonate) matrix using calibrated volumetric pipettes.
Nine-point standard curves were made in artificial seawater analyzed at the beginning of each run following a series of reagent blanks. Quality control checks were conducted every twelfth sample using a combination of ASW blanks and standards. The highest standard from the calibration curve was analyzed approximately every twenty samples to check/correct for drift during the runs. Subsamples of reference material for nutrients in seawater (Kanso) were measured in each run (see accompanying Table 1 in supplemental file).
Detection limits for each parameter were determined from three times the standard deviation of replicate lowest standards. Limits of detection were 0.01 µM for phosphate, 0.04 µM for nitrate+nitrite, 0.04 µM for silicate, and 0.01 µM for nitrite. Values below detection are reported as 0 µM with accompanying QC Flag 6. Limits of quantification (LOQ) for each parameter were determined as ten times the standard deviation of replicate analyses of the lowest standards. These are provided in the supplemental file, Table 2, as an alternative estimate of the analytical limitations for these parameters.
Sample analyses for macronutrients were performed by senior researcher Dr. Salvatore Caprara in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University.
