Dataset: Compound-specific nitrogen stable isotope ratios of amino acids in size-fractionated particles from Monterey Bay, CA, 2017

Particles were collected off the coast of Monterey Bay, California using in situ filtration (WTS-LV, McLane Research Laboratories) on four consecutive days (July 31-August 3, 2017) at three discrete depths per day aboard Monterey Bay Aquarium Research Institute's R/V Paragon. Pumping duration was between 90 and 165 minutes, longer at deeper sampling depths and pumps filtered approximately 400-600 L of seawater. The vessel drifted freely during collection; start and end locations are recorded in the dataset. One pump failed to initiate pumping, and filters recovered from this pump were used as full process blanks. The three size classes were collected using three filters mounted on mini-MULVFS 3-tiered filter holders (Bishop, Lam, and Wood, 2012: 100 µm nylon (Nitex) mesh with 150 µm nylon (Nitex) mesh backing, 20 µm nylon (Nitex) mesh with 150 µm nylon (Nitex) mesh backing, and two, stacked 0.7 µm pre-combusted glass microfiber filters (GF/F). Nitex was pre-cleaned using 10% hydrochloric acid and methanol. After collection, filter holders were stored in a cooler with ice packs frozen at -80°C for two hours until processing in the lab, where filters were photographed, folded, and stored in combusted foil at -80°C until analysis.

Before analysis, particles collected on the 100 µm nylon (Nitex) and 20 µm nylon (Nitex) filters were resuspended in 0.2 μm filtered seawater in an acid-cleaned plastic bottle with gentle shaking and sonication, then re-filtered onto combusted 47-mm GF/Fs; this process was repeated three times. All filters were lyophilized and inspected under a dissecting microscope (10-40x magnification) to remove Nitex, plastic fibers, or swimmers. Filters were quantitatively split; approximately 1/8 to 1/4 of the filter was used for compound-specific isotope analysis.

Filter splits for Compound-Specific Stable Isotope Analysis of Amino Acids (CSIA-AA) were hydrolyzed to extract amino acids (20 hours at 110° C, 6N hydrochloric acid), purified on cation exchange resin columns (50W-X8, 100-200 mesh) and derivatized to trifluoroacetyl/isopropyl esters for gas chromatography following the methods of Hannides et al. (2013) and identical to Doherty et al. (2021). The derivatized amino acids were analyzed for nitrogen isotopic composition on a Thermo Trace 1310 gas chromatograph with a BPX5 column (50 m x 0.32 mm, 1.0 μm film thickness) through a combined combustion/reduction interface (Thermo Isolink II, 1000° C) and liquid nitrogen cold trap, interfaced to a Thermo Conflo IV and MAT 253 isotope ratio mass spectrometer. Three injections were made for each sample where possible, with norleucine and aminoadipic acid with known δ¹⁵N values as co-injection standards. Analytical uncertainty for each amino acid δ¹⁵N value was derived from the standard deviation of replicate injections. For some samples, only one injection was obtained due to low particle concentration. For these samples a conservative uncertainty estimate of 1‰ was assumed. A standard solution of 14 amino acids with known δ¹⁵N values was also analyzed with each set of three injections to track instrument performance, and to correct for instrument drift, including that due to oxidation state of the reactor (Hannides et al., 2013). Data from standards injected over the lifetime of the instrument were used to correct for relationships between measured δ¹⁵N values and peak area on the instrument.