Dataset: Dissolved organic carbon, and chromophoric and fluorescent DOM measurements of sea surface microlayer and subsurface samples from the R/V Hugh R. Sharp cruises HRS2213, HRS2215 and HRS2407 in the Northwestern Atlantic Ocean in 2022 and 2024

Sea surface microlayer (SML) samples were collected using the rosette-based glass plate sampler aboard the R/V Hugh R. Sharp. Subsurface seawater (SSW) was collected from the starboard side of the ship using a modified pump profiler system (Hudson et al. 2019) from ~0.5 meters (m) depth, immediately after each SML sampling. The samples were filtered using 0.7 micrometer (µm) pre-combusted glass fiber filters. An aliquot of filtered were frozen at -20 degrees Celsius (C) for DOC measurements, while another portion was stored in the fridge at 4 degrees C for colored dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) analysis.

To measure dissolved organic carbon (DOC) concentrations, ~20 milliliters (mL) filtered water samples were acidified to a pH of ~2 using 4 drops of 2 molar (M) HCl. DOC was then quantified using the non-purgeable organic carbon method on a high temperature combustion instrument (Shimadzu TOC-V) that has a low blank (<5 micromolar (µM) C) and high precision (~1% RSD; (Qian and Mopper 1996)), following published studies (e.g., (Wozniak et al. 2012)). Sample responses were calibrated and validated against consensus surface seawater reference material from the Hansell laboratory at the University of Miami. DOC enrichment factors were calculated as a ratio of DOC concentration in the SML to DOC concentration in the paired SSW sample.

To characterize the optical properties of the SML and SSW DOM, both absorbance and excitation emission matrix (EEMs) fluorescence spectra were collected using a Horiba Aqualog spectrofluorometer. Absorbance spectra were acquired from 230 to 700 nanometers (nm) at 2 nm intervals. DOM aromaticity was estimated using SUVA254, which was calculated by dividing the absorption coefficient at 254 nm (a254) by DOC concentration (milligrams carbon per liter) (mg C L⁻¹). EEMs were obtained by scanning excitation wavelengths at 2 nm intervals between 230 and 700 nm, with emission spectra recorded at 4.65 nm intervals from 254 to 822 nm. The humification and biological indices (HIX and BIX) were calculated from the EEMs data using the staRdom and eemR R packages following standard procedures (Ohno 2002; Ouyang et al. 2024).

Additional measurements of the physical properties of seawater and meteorological conditions were made with the ship's instrumentation. Sea surface temperature and salinity were measured and recorded continuously with the shipboard instrumentation (SBE 45, Sea-Bird, at 3.7 m depth). Chlorophyll a concentrations were continuously measured with the shipboard fluorometer (AU10, Turner). Discrete water samples were also collected and used for chlorophyll a concentration measurements in the lab, using the spectrophotometric method described by Aminot and Rey (2000), to calibrate the shipboard fluorometer measurements. Wind velocity and air temperature were measured and recorded continuously with the ship's meteorological station (RM Young 26700).