Discrete data on hydrography (from CTD casts) and chemical analyses of dissolved nutrients and organic carbon on the Louisiana-Texas shelf in the Gulf of Mexico from R/V Pelican cruise 28 September – 11 October 2017 (NCEI Accession 0278012)

Acquisition Description:
Conductivity, Temperature and Depth (CTD) data and water samples were collected at various depths across 102 stations (total n = 471) in the Gulf of Mexico on the R/V Pelican between September 28, 2017 and September 19, 2017. At all stations, a vertical profile of salinity, dissolved oxygen concentration and temperature was collected using a Seabird SBE 911 CTD instrument package mounted onto a rosette containing 12, 5L Niskin bottles for collecting aqueous samples at specific depths. Water samples were collected using a single Niskin bottle per depth and were analyzed across various analytical instruments for nutrient concentrations.

Water samples were collected using a single Niskin bottle per depth on the CTD rosette. Samples for dissolved inorganic nutrients and dissolved organic carbon / total nitrogen were analyzed following the methods detailed in Roberts and Doty (2015) and Mason et al. (2016). Specifically, one niskin bottle, water samples for dissolved inorganic nutrients (NO₃⁻ + NO₂⁻, NO₂⁻, PO₄³−, SiO₂, and NH₄⁺) were analyzed in duplicate using a Lachat Instruments QuikChem FIA+ 8000 Series Automated Ion Analyzer with an ASX-400 Series XYZ autosampler after being filtered through acid-cleaned (10% HCl), 47 mm diameter, 0.2 µm pore size, membrane filters (Pall SuporR-200) under low vacuum pressure. Samples were analyzed for dissolved NO₃⁻ + NO₂⁻ (by Cu-Cd reduction followed by azo dye colorimetry), PO₄³− (by the automated ascorbic acid reduction method), SiO₂⁻, and dissolved NH₄⁺ (by phenate colorimetry) (APHA, 1992). Dissolved NO₂⁻ was determined separately by azo dye colorimetry (without Cu−Cd reduction). Standard curves were prepared using certified standard stock solutions (Hach, Loveland CO) and yielded r² values of ≥ 0.999. NO₃⁻ concentration was determined by difference between NO₃⁻ + NO₂⁻ and NO₂⁻.

Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations were measured at least in triplicate on a Shimadzu TOC-VCSH Analyzer with TNM-1 Module (MDL 1.5 μmol C L−1, 0.3 μmol N L−1; Shimadzu Scientific Instruments, Columbia, MD) after being filtered through combusted (450°C), 47mm diameter Whatman GF/F filters under low vacuum pressure. Concentrations of dissolved organic nitrogen (DON) were determined by difference [DON=TDN−DIN].

All water chemistry data went through quality control checks. CTD instrument probes were calibrated prior to cruise departure.