Dataset: Bottle sample data from CTD casts from R/V Hugh R. Sharp HRS1610 in the Mid-Atlantic coastal waters from August 2016 (CyanateInTheSea project)

Nutrient samples at each depth were collected directly from Niskin bottles through a 0.2 μm capsule filter (Pall Supor®) using a peristaltic pump at pressures ≤5 mm Hg. Filtrate was collected directly into two 50 mL sterile polypropylene centrifuge tubes (Falcon®) to measure nitrate (NO3–), nitrite (NO2–), orthophosphate ions (simplified as PO4, sum of HPO42– and PO43–), and urea concentrations. Filtrate was also pumped into three 2 mL sterile polypropylene tubes for cyanate determinations, two 15 mL OPA-treated polypropylene tubes (Falcon®) for ammonium (NH4+) analyses, and two 40 mL combusted amber glass vials for analysis of total dissolved free primary amine. To minimize contamination from filtration and between samples, filters and tubing were rinsed thoroughly with site water before sample collection. NO3–+ NO2–, NO2–, PO4, and urea samples (duplicates) were stored at 4 °C until analysis within 48 hours of their collection using a nutrient autoanalyzer (Astoria-Pacific, Inc., USA) according to the manufacturer’s specifications. The method detection limits for NO3–+ NO2–, NO2–, PO4, and urea were 0.14 μmol L–1, 0.07 μmol L–1, 0.03 μmol L–1, and 0.08 μmol L–1, respectively. NH4+ samples (duplicates) were kept at 4 °C until analysis within 24 hours of collection. The concentrations of NH4+ were measured onboard using the OPA-fluorescence method of Holmes et al. (1999) with a spectrofluorometer. The method detection limit was 10.0 nmol L–1. Cyanate samples (triplicates) were stored in liquid nitrogen aboard the ship and at -80 °C once samples were returned to the land-based laboratory. Cyanate concentrations were measured by high-performance liquid chromatography (HPLC) using a precolumn fluorescence derivatization method (Widner et al., 2013; Widner and Mulholland, 2017). The method detection limit was 0.4 nmol L–1.