Abstract. The northeast subarctic Pacific (NESAP) is a globally important source of the climate-active gas dimethylsulfide (DMS), yet the processes driving DMS variability across this region are poorly understood. Here we examine the spatial distribution of DMS at various spatial scales in contrasting oceanographic regimes of the NESAP. We present new high-spatial-resolution measurements of DMS across hydrographic frontal zones along the British Columbia continental shelf, together with key environmental variables and biological rate measurements. We combine these new data with existing observations to produce a revised summertime DMS climatology for the NESAP, yielding a broader context for our sub-mesoscale process studies. Our results demonstrate sharp DMS concentration gradients across hydrographic frontal zones and suggest the presence of two distinct DMS cycling regimes in the NESAP, corresponding to microphytoplankton-dominated waters along the continental shelf and nanoplankton-dominated waters in the cross-shelf transitional zone. DMS concentrations across the continental shelf transition (range < 1–10 nM, mean 3.9 nM) exhibited positive correlations to salinity (r=0.80), sea surface height anomaly (SSHA; r=0.51), and the relative abundance of prymnesiophyte and dinoflagellates (r=0.89). In contrast, DMS concentrations in nearshore coastal transects (range < 1–24 nM, mean 6.1 nM) showed a negative correlation with salinity (r=-0.69; r=-0.78) and SSHA (r=-0.81; r=-0.75) and a positive correlation to relative diatom abundance (r=0.88; r=0.86). These results highlight the importance of bloom-driven DMS production in continental shelf waters of this region and the role of prymnesiophytes and dinoflagellates in DMS cycling further offshore. In all areas, the rate of DMS consumption appeared to be an important control on observed concentration gradients, with higher DMS consumption rate constants associated with lower DMS concentrations. We compiled a data set of all available summertime DMS observations for the NESAP (including previously unpublished results) to examine the performance of several existing algorithms for predicting regional DMS concentrations. None of these existing algorithms was able to accurately reproduce observed DMS distributions across the NESAP, although performance was improved by the use of regionally tuned coefficients. Based on our compiled observations, we derived an average summertime distribution map for DMS concentrations and sea–air fluxes across the NESAP, estimating a mean regional flux of 0.30 Tg of DMS-derived sulfur to the atmosphere during the summer season.

We describe a method for measuring trace concentrations of dimethyl sulfide (DMS) in seawater using a commercial tandem mass spectrometer configured for atmospheric pressure chemical ionization (PT-APCI-MS/MS), coupled with a custom-built purge and trap gas extraction system. DMS was ionized through proton transfer, generating abundant [M + H] + ions. The semiautomated method analyzes samples in under 6 min, and is capable of processing up to 10 samples in a single batch. A detection limit of 0.9 pmol L −1 was determined for the analysis of 5 mL sample volumes, with a precision of 3.9% between replicates. Practical performance was evaluated during two oceanographic research cruises within the coastal waters around Vancouver Island, British Columbia. To demonstrate method utility, a series of DMS depth profiles were obtained along two transects extending from the west coast of Vancouver Island into deep water off the continental shelf. Additional depth profile sampling was conducted in Saanich Inlet, a coastal anoxic fjord with active chemotrophic sulfur cycling. This method enabled us to capture the deep-water accumulation of subnanomolar DMS in the anoxic water of Saanich Inlet, providing evidence of cryptic sulfur cycling. The method was also leveraged to facilitate stable isotope rate measurement experiments, in which the consumption of isotopically labeled DMS, dimethylsulfoxide, and dimethylsulfoniopropionate tracers was monitored in the low picomolar range. These measurements enable metabolic rate determinations using low-level tracer additions that do not perturb in situ microbial activity. Our sensitive, high throughput method helps to improve understanding of the natural marine cycling of volatile sulfur compounds.

We measured the concentration and turnover rates of dimethylsulfoxide (DMSO) across the northeast Subarctic Pacific (NESAP) to better understand the role of this compound in regional sulfur cycling and phytoplankton physiological ecology. Using ship-track observations and Lagrangian drifter surveys, we examined the spatial and temporal components of DMS and DMSO variability, comparing rates of DMSO reduction with simultaneous measurements of dimethylsulfide (DMS) oxidation and dimethylsulfoniopropionate (DMSP) cleavage. Our results show high concentrations and rapid turnover of DMSO across much of our NESAP survey region. DMS and DMSO concentrations ranged from < 1 nM to maxima of 26 nM and 183 nM, respectively, and exhibited a nonlinear positive relationship. Rate constants for DMSO reduction to DMS ranged from < 1 to 6.5 d −1 and were correlated to bacterial production and phytoplankton taxonomic composition. DMSO reduction rates exceeded dissolved DMSP cleavage at nearly all stations, and exceeded DMS oxidation rates at four stations where DMSO reduction and DMS oxidation were measured simultaneously. At these stations, net DMS production from DMSO ranged from 0.50 to 7.18 nM d −1. During our Lagrangian survey, DMSO concentrations decreased during periods of peak midday irradiance. A strong correlation between DMSO concentrations, nonphotochemical quenching and photochemical efficiency of photosystem II (F v /F m) suggest a potential role for DMSO in photo-protection, supporting previous suggestions of an anti-oxidant function for this molecule. Our findings highlight the potential contribution of DMSO to net DMS production in the NESAP, and suggest a role for this compound in phytoplankton physiological ecology.

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&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The northeast subarctic Pacific (NESAP) is a globally important source of the climate-active gas dimethylsulfide (DMS), yet the processes driving DMS variability across this region are poorly understood. Here we examine the spatial distribution of DMS at various spatial scales across contrasting oceanographic regimes of the NESAP. We present a new data set of high spatial resolution DMS measurements across hydrographic frontal zones along the British Columbia continental shelf, together with key environmental variables and biological rate measurements. We combine these new data with existing observations to produce a revised summertime DMS climatology for the NESAP, yielding a broader context for our sub-mesoscale process studies. Our results demonstrate sharp DMS concentration gradients across hydrographic frontal zones, and suggest the presence of two distinct DMS cycling regimes corresponding to microphytoplankton-dominated waters along the continental shelf, and nanoplankton-dominated cross-shelf transitional waters. DMS concentrations across the continental shelf transition (range &lt;&amp;#8201;1&amp;#8211;10&amp;#8201;nM, mean 3.9&amp;#8201;nM) exhibited positive correlations to salinity (r&amp;#8201;=&amp;#8201;0.80), sea surface height anomaly (SSHA; r&amp;#8201;=&amp;#8201;0.51) and relative prymnesiophyte abundance (r&amp;#8201;=&amp;#8201;0.88). In contrast, DMS concentrations in near shore coastal transects (range &lt;1&amp;#8211;24&amp;#8201;nM, mean 6.1&amp;#8201;nM) showed a negative correlation with salinity (r&amp;#8201;=&amp;#8201;&amp;#8722;0.69, r&amp;#8201;=&amp;#8201;&amp;#8722;0.78) and SSHA (r&amp;#8201;=&amp;#8201;&amp;#8722;0.81, r&amp;#8201;=&amp;#8201;&amp;#8722;0.75), and a positive correlation to relative diatom abundance (r&amp;#8201;=&amp;#8201;0.88, r&amp;#8201;=&amp;#8201;0.86). These results highlight the importance of bloom-driven DMS production in continental shelf waters of this region, and the role of prymnesiophytes in DMS cycling further offshore. In all areas, the rate of DMS consumption appeared to be an important control on observed concentration gradients, with higher DMS consumption rate constants associated with lower DMS concentrations. A compiled dataset of all available summertime DMS observations for the NESAP (including previously unpublished results) was used to examine the performance of several existing algorithms to predict regional DMS concentrations. We found that none of these existing algorithms was able to accurately reproduce observed DMS distributions across the NESAP, although performance was improved by the use of regionally tuned-coefficients. Based on our compiled observations, we derived an average summertime distribution map for DMS concentrations and sea&amp;#8211;air fluxes across the NESAP. We estimated that this region emits 0.30&amp;#8201;Tg of sulfur to the atmosphere during the summer season.&lt;/p&gt;