Sampling Plan:
This study was conducted during the California Current Ecosystem Long Term Ecological Research (CCE LTER) Process Cruise P2105 from July 10 to August 8, 2021, on board the R/V Roger Revelle (RR2105) where we sampled two water parcels and a seven station transect. Water parcels were sampled with a Lagrangian-style process study using drifter floats and modeled after previous CCE LTER cruises (Krause et al., 2015; Landry et al., 2009). During these process studies, an upwelled water parcel was identified by satellite temperature and chlorophyll-a concentrations. We followed an upwelled water parcel for 11 days and a water parcel that had already moved to the eastern edge of the oligotrophic gyre for three days. Profiles for dissolved Hg concentrations, including total mercury (THg), dimethylmercury (DMHg), monomethylmercury (MMHg), and elemental mercury (Hg0), were taken every other day during the water parcel studies. A seven station transect from stations S1-S7 was conducted across the California Current water mass to the coast, and profiles for THg, DMHg, and Hg0 were taken for all stations in the transect. A MMHg profile was only taken for Station S4 in the transect. Stations are colored based on their corresponding water mass in Figure 1 of Adams et al. (2024). Profiles for Hg speciation typically covered the upper water column to 200 meters (m), and those data are presented here. One station within the California Undercurrent was sampled to 600 m, and the concentrations of Hg species in the waters deeper than 200 m are used as a reference for upwelled waters in our mass budget model. Benthic Boundary Layer (BBL) stations were selected based on previous work in the region and are locations where the continental shelf drops off and can have high levels of suspended sediments within the water column (Biller and Bruland, 2013). Samples at BBL stations were taken at one depth around 40 to 70 m identified as 5 m above the sea floor (Biller and Bruland, 2013).

Sampling Methods:
Vertical dissolved Hg profiles were sampled using 5-liter (L) X-Niskin bottles (Ocean Test Equipment) mounted on a trace metal rosette (Seabird) deployed on a non-metallic hydroline (Brzezinski et al., 2015; Cutter and Bruland, 2012) and triggered automatically by pressure on upcasts using a Seabird Auto Fire Module. BBL samples were collected using a 30 L Teflon™-coated GO-Flo™ bottle (General Oceanics) deployed on a non-metallic hydroline and triggered with an acid-cleaned Teflon™ messenger (Bruland et al., 1979). The Niskin or GO-Flo™ bottles were transported into a dedicated Class 100 laboratory van under trace metal-clean conditions (Cutter and Bruland, 2012). Samples were pressure-filtered (N2 gas, 99.99%) directly from the Niskin or GO-Flo™ bottles through 0.2 um capsule filters (Acropak 200, Pall Laboratory) into 2 L acid-cleaned Teflon™ bottles.

An aliquot of sample was transferred from the 2 L Teflon™ bottle into 0.25 L pre-cleaned borosilicate glass bottles (I-Chem) for THg analysis and oxidized with 0.04% bromine monochloride at least 12 hours prior to analysis (U.S. Environmental Protection Agency Method 1631, Revision E).

The remainder of the sample in the 2 L Teflon™ bottle was analyzed for gaseous Hg0 and DMHg with a purge-and-trap method (Bowman et al., 2011; Tseng et al., 2004). Samples were purged with Hg-free N2 gas for 60 minutes ;at a rate of 0.5 L per minute. Effluent gas was passed through a soda lime trap to remove water vapor and aerosols, then DMHg was concentrated onto a Carbotrap® (graphitized carbon black, Sigma-Aldrich) matrix downstream of the soda lime trap, and Hg0 was concentrated onto a gold trap downstream of the Carbotrap® (Tseng et al., 2004; Lamborg et al., 2012). DMHg was thermally desorbed from the Carbotrap® and quantified on board via Gas Chromatographic Cold Vapor Atomic Fluorescence Spectrometry (GC-CVAFS) on a Tekran 2500 (Bowman et al., 2011; Baya et al., 2013). The detection limit for DMHg was 2 fM. Hg0 was quantified by dual gold amalgamation Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS) on a Tekran 2600 following thermal desorption from the gold trap (Tseng et al., 2004; Bloom and Fitzgerald, 1988). The method detection limits for Hg0 were 40 fM (n=14). Sample concentrations for DMHg and Hg0 were determined by a calibration curve based on a gaseous Hg0 standard (Tekran 2505 Mercury Vapor Primary Calibration Unit).

Seawater purged of DMHg and Hg0 was transferred into a 0.25 L precleaned amber borosilicate glass bottles (I-Chem) for MMHg analysis, acidified with 1% sulfuric acid (Trace Metal Grade, Fisher Scientific), stored at 4°C, and analyzed at Scripps Institution of Oceanography within 2 months of collection.

THg samples were analyzed following U.S. EPA Method 1631 on board the ship (U.S. Environmental Protection Agency Method 1631, Revision E; Lamborg et al., 2012). Samples were reduced to Hg0 with 20% wt:vol tin (II) chloride solution (ACS grade, Fisher Chemical) in 10% hydrochloric acid (ACS grade, Fisher Chemical). Hg0 was purged onto a gold trap with Hg-free argon gas and thermally desorbed via CVAFS for detection using a Tekran 2600 Automated Mercury Analyzer. Sample concentrations were determined by a calibration curve based on a gaseous Hg0 standard (Tekran 2505 Mercury Vapor Primary Calibration Unit). The method detection limit was 0.22 pM (n=8 blanks) and replicates had an average precision of 6.4% (n = 73).

MMHg samples were analyzed by ascorbic acid-assisted direct ethylation following Munson et al. (2014) and U.S. EPA Method 1630 (U.S. Environmental Protection Agency Method 1630; Munson et al., 2014). Samples were adjusted to a pH of 4.8 using a 2 M acetate/glacial acetic acid buffer (J.T. Baker) in ultrapure water (Milli-Q, 18.2 MW per centimeter) and 8 M potassium hydroxide (J.T. Baker) in ultrapure water (Milli-Q, 18.2 MW per centimeter). 2.5% wt:vol ascorbic acid (J.T. Baker) in ultrapure water (Milli-Q, 18.2 MW per centimeter was added to the samples, then samples were ethylated with sodium tetraethylborate (NaTEB) solution (1% NaTEB in 2% potassium hydroxide, Strem Chemicals) to convert MMHg to volatile methylethylmercury. Ethylation was allowed to proceed for 10 minutes before sample analysis. Samples were analyzed by GC-CVAFS on a Tekran 2700 Automated Methylmercury Analyzer. Concentrations were determined by a calibration curve based on standards prepared from a certified 1000 ppm MMHg (II) chloride standard (Alfa Aesar). The method detection limit was 11.3 fM (n=9 blanks), and ongoing precision and recovery was 98.4 ± 7.9% (n=25). Replicates had an average precision of 9.2% (n=18) and matrix spike recovery was 112 ± 13% (n=15).