These data were collected on R/V Endeavor cruise EN665 from three stations in the Wilkinson and Jordan Basins of the Gulf of Maine (approximately 42.6 North, 69.6 West, depth 0-260 meters). Water samples were collected using Niskin bottles on the CTD rosette.

Nutrients:
Nutrient samples were collected directly from Niskin bottles using clean 60-milliliter (mL) plastic syringes. Samples were then passed through a 25-millimeter (mm) 0.2-micron filter (either polycarbonate or PTFE) into clean, 60-mL HDPE bottles. Bottles were rinsed 3 times with filtered water before filling and capping. Bottles were then frozen at -20 degrees Celsius for approximately 30 days until analysis. Analyses were performed at the WHOI Nutrient Analytical Facility on a SEAL Analytical AA3 HR following established procedures.

Total Alkalinity, Dissolved Inorganic Carbon (DIC), and d13C-DIC:
Samples for total alkalinity (TA), dissolved inorganic carbon (DIC), and d13C-DIC were collected simultaneously. Water from the Niskins was passed through a 0.45-micron cartridge filter and all bubbles were removed from the line. Ground-glass stoppered 250-mL were rinsed 3 times with flowing, filtered seawater and then filled. Each sample bottle was left to overflow for approximately double the amount of time it took to fill the bottle. Excess water was gently dumped out to leave a ~2-3 mL headspace below the ground glass fitting. Following collection, samples were poisoned with 100 microliters of saturated mercuric chloride solution. Bottles were sealed with a greased stopper (Apiezon-L). A rubber band was placed over the stopper to ensure sample closure. Samples were stored cool and in the dark prior to analysis at the Woods Hole Oceanographic Institution.

DIC and d13C analyses were performed first immediately after opening the bottle. DIC and d13C-DIC were determined simultaneously using an Apollo AS-D1 analyzer connected to a Picarro G-2121i cavity ringdown system on a 5 mL sample of seawater. Samples were run in at least triplicate and calibrated against seawater Certified Reference Materials. Isotopic values were calibrated against an in-house seawater standard that was intercalibrated against known solid materials (NBS-19, IAEA-C2, and NBS-20). Intercalibration was performed on the same Picarro instrument using an Automate-Liaison front-end unit. Total alkalinity was determined using an open-system Gran titration on 5-mL samples in triplicate, using a Metrohm 805 Dosimat and a robotic Titrosampler, calibrated against seawater Certified Reference Materials.

pH:
pH samples were measured on the total scale with pure meta cresol purple indicator (mCP) at 25C +/- 0.1C on an Agilent 8453 spectrophotometer. A subset of the samples (18 out of 30 total) was used to derive the R-value perturbation by the indicator, for which all of the samples were then corrected.