Seawater Dimethylsulfide Concentrations Research Articles

Characterizing spatio-temporal variations of dimethyl sulfide in the Yellow and East China Sea based on BP neural network

Dimethyl sulfide (DMS), an organic volatile sulfide produced from Dimethylsulfoniopropionate (DMSP), exerts a significant impact on the global climate change. Utilizing published literature data spanning from 2005 to 2020, a BP neural network (BPNN) model of the surface seawater DMS in the Yellow and East China Sea (YECS) was developed to elucidate the influence of various marine factors on the DMS cycle. Results indicated that the six parameters inputted BPNN model, that include the time (month), latitude and longitude, sea-surface chlorophyll a (Chl-a), sea-surface temperature (SST), and sea-surface salinity (SSS), yielded the optimized simulation results (R2 = 0.71). The optimized estimation of surface seawater DMS in the YECS were proved to be closely aligned with the observed data across all seasons, which demonstrated the model’s robust applicability. DMS concentration in surface seawater were found to be affected by multiple factors such as Chl-a and SST. Comparative analysis of the three environmental parameters revealed that Chl-a exhibited the most significant correlation with surface seawater DMS concentration in the YECS (R2 = 0.20). This underscores the pivotal role of chlorophyll in phytoplankton photosynthesis and DMS production, emphasizing its importance as a non-negligible factor in the study of DMS and its sulfur derivatives. Furthermore, surface seawater DMS concentration in the YECS exhibited positive correlations with Chl-a and SST, while displaying a negative correlation with SSS. The DMS concentration in the YECS show substantial seasonal variations, with the maximum value (5.69 nmol/L) in summer followed in decreasing order by spring (3.96 nmol/L), autumn (3.18 nmol/L), and winter (1.60 nmol/L). In the YECS, there was a gradual decrease of DMS concentration from the nearshore to the offshore, especially with the highest DMS concentration concentrated in the Yangtze River Estuary Basin and the south-central coastal part off the Zhejiang Province. Apart from being largely composed by the release of large amounts of nutrients from anthropogenic activities and changes in ocean temperature, the spatial and temporal variability of DMS may be driven by additional physicochemical parameters.

High-resolution distribution and emission of dimethyl sulfide and its relationship with pCO2 in the Northwest Pacific Ocean

Ocean-derived dimethyl sulfide (DMS) is widely concerning because of its hypothesized influence on global climate change. This study aims to explore the distribution characteristics and influencing factors of DMS and partial pressure of carbon dioxide (pCO2) in the Northwest Pacific Ocean, as well as the potential relationship between DMS and pCO2. A high-resolution, underway, shipboard measurement device was used to determine the DMS and pCO2 of the surface seawater and atmosphere in the Northwest Pacific and its marginal seas during November 2019. The result show that atmospheric and surface seawater DMS concentrations ranged from 3 to 125 pptv and 0.63 to 2.28 nmol L-1, respectively, with mean values of 46 ± 19 pptv and 1.08 ± 0.34 nmol L-1. The average sea surface pCO2 was 371 ± 16 μatm (range from 332 to 401 μatm). The trends in the surface seawater DMS in different current systems were primarily associated with phytoplankton abundance and composition. Biological activity and physical processes such as cooling jointly influenced the sea surface pCO2. A cold eddy along the transect in the Northwest Pacific Ocean increased DMS at the sea surface by 10% and CO2 uptake by 3%. We found a significant negative correlation between DMS and pCO2 in the Northwest Pacific Ocean at the 0.1° resolution [DMS]seawater = -0.0161[pCO2]seawater + 7.046 (R2 = 0.569, P < 0.01). The DMS and pCO2 sea-air fluxes were estimated to range from 0.04 to 25.3 μmol m-2·d-1 and from -27.0 to 4.22 mmol m-2·d-1 throughout the survey area. The Northwest Pacific Ocean, especially the Oyashio Current, is an important sink of CO2 and a source of DMS.

Variability of DMS in the East China Sea and its response to different ENSO categories

Dimethylsulfide (DMS) produced by marine phytoplankton is the most important volatile sulfur compound in ocean surface seawater and has a distinct contribution to global climate change. To assess DMS-climate feedbacks, the interannual variations in seawater DMS concentrations in the East China Sea (ECS) were examined, and their periodic characteristic patterns related to different El Niño-Southern Oscillation (ENSO) categories were discussed. Based on satellite surface seawater temperature (SST) and chlorophyll-a (Chl-a) datasets, a 23-year (1998–2020) dataset of DMS concentrations in the ECS was hindcasted by using a generalized additive mixed model (GAMM). The ENSO events from 1998 to 2020 were selected and divided into persistent, recession and transformed types according to their development process. The variability of surface seawater DMS concentration in the Yangtze River Estuary and its adjacent seas, the coast of Zhejiang and Fujian, and the coast of northern Jiangsu are more strongly influenced by ENSO events than the remainder of the ECS. Furthermore, ENSO alters the seasonal variability in DMS concentrations in the Yangtze River Estuary. The DMS concentrations of Apr-Jul were reduced (enhanced) by approximately 3.05 (0.61) nmol/L during Persistent El Niño (PEN) (Persistent La Niña (PLN)) events, which were reduced by approximately 0.51 and 1.10 nmol/L during recessionary El Niño (REN) event and recessionary La Niña (RLN) event, respectively. However, the DMS concentrations were enhanced by approximately 0.53 (1.29) nmol/L during El Niño-La Niña transition (ELT) (La Niña-El Niño transition (LET)) events. ENSO events affected the DMS concentrations in the surface seawater of the study area by influencing Chl-a and SST, where the short-period interannual variability (2–3 years) of DMS was mainly affected by SST, while the interannual variability of the long period (6 years) was mainly affected by Chl-a. The findings can be applied to the development of DMS concentration prediction methods.

Abundant microzooplankton possibly cause ultrahigh seawater dimethylsulfide during Southern Ocean algal blooms

Oceanic dimethylsulfide (DMS) is hypothesized to impact cloud formation and consequently the solar radiation budget of Earth’s surface. Ultrahigh seawater DMS concentrations, up to hundreds of nM, have been observed in the Southern Ocean, attributing to concurrent high phytoplankton biomass. However, phytoplankton biomass cannot fully explain the mechanism leading to those extreme values. Herein, measurements, including seawater DMS concentrations and other biological and environmental parameters, were collected in the water column during the austral summer of 2015–2016 at the tip of the Antarctic Peninsula. Notably, large-scale ultrahigh seawater DMS concentrations (up to 85.2 nM and generally above 40 nM in the upper layer) was observed only in areas with co-existing phytoplankton blooms and abundant microzooplankton (indicated by ciliates, whose abundance and biomass were above 1000 ind L-1 and 2 μg L-1, respectively), implying that the grazing of abundant microzooplankton possibly causes the ultrahigh seawater DMS concentrations during the bloom seasons in the Southern Ocean.

Unravelling Surface Seawater DMS Concentration and Sea‐To‐Air Flux Changes After Sea Ice Retreat in the Western Arctic Ocean

AbstractThe receding of the seasonal ice cover in the Arctic due to climate change has been predicted by models to increase climate‐active biogenic trace gas emissions, specifically those of dimethylsulfide (DMS). However, insufficient DMS measurements are currently available to either support or refute this hypothesis and to fully understand the various responses of oceanic DMS in a rapidly changing Arctic Ocean environment. Here, we present high‐resolution surface water DMS data collected in the summer of 2014 in combination with a suite of ancillary variables including sea ice cover, salinity, and nutrients. We show that surface seawater DMS concentrations, generally below 0.5 nmol L−1, remained unchanged in the Canada Basin after sea ice retreat probably due to insufficient nutrients supply to the upper mixed layer and resulting low primary production. Moreover, in the Chukchi shelf region, DMS concentrations decreased following a phytoplankton bloom due to the rapid depletion and slow resupply of nutrients. Although the DMS sea‐to‐air fluxes were not high from a global perspective, they increased by a factor of 4‐fold after sea ice retreat in the Arctic Ocean high latitudes. This increase in DMS flux was mainly driven by increased wind speed. This work provides unique observations and insights on how surface seawater DMS and flux to the atmosphere may change in the future Arctic Ocean.

Parameterizing the Impact of Seawater Temperature and Irradiance on Dimethylsulfide (DMS) in the Great Barrier Reef and the Contribution of Coral Reefs to the Global Sulfur Cycle

AbstractBiogenic emissions of dimethylsulfide (DMS) are an important source of sulfur to the atmosphere, with implications for aerosol formation and cloud albedo over the ocean. Natural aerosol sources constitute the largest uncertainty in estimates of aerosol radiative forcing and climate and thus, an improved understanding of DMS sources is needed. Coral reefs are strong point sources of DMS; however, this coral source of biogenic sulfur is not explicitly included in climatologies or in model simulations. Consequently, the role of coral reefs in local and regional climate remains uncertain. We aim to improve the representation of tropical coral reefs in DMS databases by calculating a climatology of seawater DMS concentration (DMSw) and sea‐air flux in the Great Barrier Reef (GBR), Australia. DMSw is calculated from remotely sensed observations of sea surface temperature and photosynthetically active radiation using a multiple linear regression model derived from field observations of DMSw in the GBR. We estimate that coral reefs and lagoon waters in the GBR (∼347,000 km2) release 0.03–0.05 Tg yr−1 of DMS (0.02 Tg yr−1 of sulfur). Based on this estimate, global tropical coral reefs (∼600,000 km2) could emit 0.08 Tg yr−1 of DMS (0.04 Tg yr−1 of sulfur), with the potential to influence the local radiative balance.

Natural dimethyl sulfide gradients would lead marine predators to higher prey biomass

Finding prey is essential to survival, with marine predators hypothesised to track chemicals such as dimethyl sulfide (DMS) while foraging. Many predators are attracted to artificially released DMS, and laboratory experiments have shown that zooplankton grazing on phytoplankton accelerates DMS release. However, whether natural DMS concentrations are useful for predators and correlated to areas of high prey biomass remains a fundamental knowledge gap. Here, we used concurrent hydroacoustic surveys and in situ DMS measurements to present evidence that zooplankton biomass is spatially correlated to natural DMS concentration in air and seawater. Using agent simulations, we also show that following gradients of DMS would lead zooplankton predators to areas of higher prey biomass than swimming randomly. Further understanding of the conditions and scales over which these gradients occur, and how they are used by predators, is essential to predicting the impact of future changes in the ocean on predator foraging success.

Predictability of Seawater DMS During the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES)

This work presents an overview of a unique set of surface ocean dimethylsulfide (DMS) measurements from four shipboard field campaigns conducted during the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) project. Variations in surface seawater DMS are discussed in relation to biological and physical observations. Results are considered at a range of timescales (seasons to days) and spatial scales (regional to sub-mesoscale). Elevated DMS concentrations are generally associated with greater biological productivity, although chlorophyll a (Chl) only explains a small fraction of the DMS variability (15%). Physical factors that determine the location of oceanic temperature fronts and depth of vertical mixing have an important influence on seawater DMS concentrations during all seasons. The interplay of biomass and physics influences DMS concentrations at regional/seasonal scales and at smaller spatial and shorter temporal scales. Seawater DMS measurements are compared with the global seawater DMS climatology and predictions made using a recently published algorithm and by a neural network model. The climatology is successful at capturing the seasonal progression in average seawater DMS, but does not reproduce the shorter spatial/temporal scale variability. The input terms common to the algorithm and neural network approaches are biological (Chl) and physical (mixed layer depth, photosynthetically active radiation, seawater temperature). Both models predict the seasonal North Atlantic average seawater DMS trends better than the climatology. However, DMS concentrations tend to be under-predicted and the episodic occurrence of higher DMS concentrations is poorly predicted. The choice of climatological seawater DMS product makes a substantial impact on the estimated DMS flux into the North Atlantic atmosphere. These results suggest that additional input terms are needed to improve the predictive capability of current state-of-the-art approaches to estimating seawater DMS.

Modeling the Impact of Marine DMS Emissions on Summertime Air Quality Over the Coastal East China Seas

[1] Biogenic emission of dimethyl sulfide (DMS) from seawater is the major natural source of sulfur into the atmosphere. In this study, we use an advanced air quality model (CMAQv5.2) with DMS chemistry to examine the impact of DMS emissions from seawater on summertime air quality over China. A national scale database of DMS concentration in seawater is established based on a five-year observational record in the East China seas including the Bohai Sea, the Yellow Sea and the East China Sea. We employ a commonly used global database and also the newly developed local database of oceanic DMS concentration, calculate DMS emissions using three different parameterization schemes, and perform five different model simulations for July, 2018. Results indicate that in large coastal areas of China, the average DMS emissions flux obtained with the local database is three times higher than that resulting from the global database, with a mean value of 9.1 μmol m-2 d-1 in the Bohai Sea, 8.4 μmol m-2 d-1 in the Yellow Sea and 13.4 μmol m-2 d-1 in the East China Sea. The total DMS emissions flux calculated with the Nightingale scheme is 42% higher than that obtained with the Liss and Merlivat scheme, but is 15% lower than that obtained with the Wanninkhof scheme. Among the three parameterizations, results of the Liss and Merlivat scheme agree better with the ship-based observations over China's coastal waters. DMS emissions with the Liss and Merlivat parametrization increase atmospheric sulfur dioxide (SO2) and sulfate (SO4 2-) concentration over the East China seas by 6.4% and 3.3%, respectively. Our results indicate that although the anthropogenic source is still the dominant contributor of atmospheric sulfur burden in China, biogenic DMS emissions source is nonnegligible.

Regional and Urban-Scale Environmental Influences of Oceanic DMS Emissions over Coastal China Seas.

Marine biogenic dimethyl sulfide (DMS) is an important natural source of sulfur in the atmosphere, which may play an important role in air quality. In this study, the WRF-CMAQ model is employed to assess the impact of DMS on the atmospheric environment at the regional scale of eastern coastal China and urban scale of Shanghai in 2017. A national scale database of DMS concentration in seawater is established based on the historical DMS measurements in the Yellow Sea, the Bohai Sea and the East China Sea in different seasons during 2009~2017. Results indicate that the sea-to-air emission flux of DMS varies greatly in different seasons, with the highest in summer, followed by spring and autumn, and the lowest in winter. The annual DMS emissions from the Yellow Sea, the Bohai Sea and the East China Sea are 0.008, 0.059, and 0.15 Tg S a−1, respectively. At the regional scale, DMS emissions increase atmospheric sulfur dioxide (SO2) and sulfate concentrations over the East China seas by a maximum of 8% in summer and a minimum of 2% in winter, respectively. At the urban scale, the addition of DMS emissions increase the SO2 and levels by 2% and 5%, respectively, and reduce ozone (O3) in the air of Shanghai by 1.5%~2.5%. DMS emissions increase fine-mode ammonium particle concentration distribution by 4% and 5%, and fine-mode nss- concentration distributions by 4% and 9% in the urban and marine air, respectively. Our results indicate that although anthropogenic sources are still the dominant contributor of atmospheric sulfur burden in China, biogenic DMS emissions source cannot be ignored.

Atmospheric dimethyl sulfide and its significant influence on the sea-to-air flux calculation over the Southern Ocean

Our understanding about the atmospheric dimethyl sulfide (DMS) and its influence to sea-to-air flux calculation in the Southern Ocean is still limited due to insufficient investigations. Herein, high-resolution shipboard underway simultaneous surface seawater and atmospheric DMS measurements were conducted in the Southern Ocean from February 23 to March 31, 2018. A larger variation of DMS levels was found in atmosphere compared with that in seawater. Remarkably, a large-scale area with high seawater and atmospheric DMS concentrations up to 27.9 nM and 3.92 ppbv, respectively, was investigated outside of Ross Sea sector. Atmospheric DMS levels were strongly impacted by wind speed and air mass convection. The relationship between atmospheric DMS and air mass exposure to oceanic chlorophyll varied greatly depending on the area of investigation. Some other regions with high DMS production capacity were examined as well beside those along the cruise tracks based on the results of positive correlations with high slopes and back trajectories. Moreover, significant uncertainty of sea-to-air DMS flux over the Southern Ocean could be caused by follows: (1) the selecting of different gas transfer coefficients; (2) the negative flux values calculated under high atmospheric DMS levels together with low seawater DMS concentrations; and (3) the greatly overestimated flux, approximately 47.1–76.9%, without considering the atmospheric DMS. This study highlights the urgent demand of high-resolution observations of atmospheric DMS over the Southern Ocean to estimate DMS emission with high accuracy.

Sensitivity of Arctic sulfate aerosol and clouds to changes in future surface seawater dimethylsulfide concentrations

Abstract. Dimethylsulfide (DMS), outgassed from ocean waters, plays an important role in the climate system, as it oxidizes to methane sulfonic acid (MSA) and sulfur dioxide (SO2), which can lead to the formation of sulfate aerosol. Newly formed sulfate aerosol resulting from DMS oxidation may grow by condensation of gases, in-cloud oxidation, and coagulation to sizes where they may act as cloud condensation nuclei (CCN) and influence cloud properties. Under future global warming conditions, sea ice in the Arctic region is expected to decline significantly, which may lead to increased emissions of DMS from the open ocean and changes in cloud regimes. In this study we evaluate impacts of DMS on Arctic sulfate aerosol budget, changes in cloud droplet number concentration (CDNC), and cloud radiative forcing in the Arctic region under current and future sea ice conditions using an atmospheric global climate model. Given that future DMS concentrations are highly uncertain, several simulations with different surface seawater DMS concentrations and spatial distributions in the Arctic were performed in order to determine the sensitivity of sulfate aerosol budgets, CDNC, and cloud radiative forcing to Arctic surface seawater DMS concentrations. For any given amount and distribution of Arctic surface seawater DMS, similar amounts of sulfate are produced by oxidation of DMS in 2000 and 2050 despite large increases in DMS emission in the latter period due to sea ice retreat in the simulations. This relatively low sensitivity of sulfate burden is related to enhanced sulfate wet removal by precipitation in 2050. However simulated aerosol nucleation rates are higher in 2050, which results in an overall increase in CDNC and substantially more negative cloud radiative forcing. Thus potential future reductions in sea ice extent may cause cloud albedos to increase, resulting in a negative climate feedback on radiative forcing in the Arctic associated with ocean DMS emissions.

Global trend analysis in primary and secondary production of marine aerosol and aerosol optical depth during 2000–2015

The global trends in sea spray aerosol (SSA) emissions, dimethyl sulfide (DMS) flux from sea to air, and aerosol optical depth (AOD) during 2000–2015 were analyzed using the satellite-based, ECMWF reanalysis, and model-predicted data. The SSA emissions were estimated using a widely used whitecap method with a sea surface temperature (SST) dependence. The sea-to-air DMS fluxes were estimated by a thin film model based on the DMS concentration in seawater and its gas transfer velocity. The yearly global mean anomaly in DMS fluxes showed a significant downward trend during the study period, whereas the SSA emissions showed an upward trend. In terms of regional trends, the increases in SSA emissions during 2000–2015 occurred over the tropical southeastern Pacific Ocean and Southern Ocean, whereas any downward trends in SSA emissions were localized. The DMS fluxes during the study period showed a clear downward trend over most oceans, except for the strong upward trend at low latitudes. In general, the AOD from the MODerate resolution Imaging SpectroRadiometer (MODIS) showed upward trends across the entire Southern Hemisphere (SH), whereas there were downward trends in most areas of the Northern Hemisphere (NH). Therefore, the upward trend of AOD in the SH were derived mainly from the SSA emission trend, due to smaller contribution of anthropogenic sources in the SH.

Effects of ocean acidification and short-term light/temperature stress on biogenic dimethylated sulfur compounds cycling in the Changjiang River Estuary

Environmental contextContinuous anthropogenic CO2 emissions have led to an increase in seawater acidity, potentially affecting the growth of phytoplankton and their production of the climate-moderating biogenic gas, dimethyl sulfide. Our simulation experiments showed that ocean acidification, coupled with light and temperature changes, had a significant influence on dimethyl sulfide concentrations. This research provides fundamental data for predicting the biogeochemical cycle of dimethyl sulfide under various global change scenarios. AbstractOcean acidification (OA) affects marine primary productivity and community structure. Therefore, OA may influence the biogeochemical cycles of volatile biogenic dimethyl sulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP) and photochemical oxidation product dimethyl sulfoxide (DMSO). A 23-day shipboard incubation experiment investigated the short-term response of the production and cycling of biogenic sulfur compounds to OA in the Changjiang River Estuary to understand the effects of OA on biogenic sulfur compounds. Phytoplankton abundance and community composition showed a marked difference at three different pH levels at the late stage of the experiment. Significant reductions in chlorophyll a (Chl-a), DMS, particulate DMSP (DMSPp) and dissolved DMSO (DMSOd) concentrations were identified under high CO2 levels. Moreover, minimal changes were observed in the productions of dissolved DMSP (DMSPd) and particulate DMSO (DMSOp) among the treatments. The ratios of DMS, total DMSP (DMSPt) and total DMSO (DMSOt) to Chl-a were not affected by a change in pH. Furthermore, the concentrations of DMS and DMSOd were closely related to the mean bacterial abundance at the three pH levels. Additional short-term (8h) incubation experiments on the light and temperature effects showed that the influence of pH on the production of dimethylated sulfur compounds also depended on solar radiation and temperature. Under natural and UVB light, DMS photodegradation rates increased by 1.6 to 4.2 times at low pH levels. Thus, OA may lead to decreasing DMS concentrations in surface seawater. Light and temperature conditions also play important roles in the production and cycling of biogenic sulfur compounds.

Temporal and spatial distributions of carbonyl sulfide, dimethyl sulfide, and carbon disulfide in seawater and marine atmosphere of the Changjiang Estuary and its adjacent East China Sea

AbstractCarbonyl sulfide (COS), dimethyl sulfide (DMS), and carbon disulfide (CS2) concentrations were determined in seawater and the overlying atmosphere of the Changjiang Estuary and the adjacent area during two cruises from 22 February 2016 to 15 March 2016 and from 03 July 2016 to 25 July 2016. The concentration ranges of COS, DMS, and CS2 in seawater during winter were 121–388 pmol L−1, 0.321–1.59 nmol L−1, and 10.3–46.8 pmol L−1, respectively, and those during summer were 98–346 pmol L−1, 0.397–9.13 nmol L−1, and 13.3–65.4 pmol L−1. The DMS concentration in surface seawater varied seasonally, whereas the COS and CS2 concentrations varied little. The atmospheric mixing ratios of COS, DMS, and CS2 were 459–777 pptv, 5.2–96.8 pptv, and 96.9–164 pptv during winter and 417–644 pptv, 133–365 pptv, and 15.8–89.8 pptv during summer, respectively. A correlation analysis revealed that COS concentration was related to the concentrations of DMS and CS2. The estimated fluxes of COS, DMS, and CS2 were in the range of 1.19–2.06 μg m−2 d−1, 15.11–164.81 μg m−2 d−1, and 0.27‐1.32 μg m−2 d−1 during winter and 0.61–0.97 μg m−2 d−1, 71.76–2511 μg m−2 d−1, and 0.38–1.47 μg m−2 d−1 during summer, respectively. These results indicate that the study area is a sink for COS, but served as a source of atmospheric DMS and CS2 during the study period.

Characteristics of the surface water DMS and pCO2 distributions and their relationships in the Southern Ocean, southeast Indian Ocean, and northwest Pacific Ocean

AbstractOceanic dimethyl sulfide (DMS) is of interest due to its critical influence on atmospheric sulfur compounds in the marine atmosphere and its hypothesized significant role in global climate. High‐resolution shipboard underway measurements of surface seawater DMS and the partial pressure of carbon dioxide (pCO2) were conducted in the Atlantic Ocean and Indian Ocean sectors of the Southern Ocean (SO), the southeast Indian Ocean, and the northwest Pacific Ocean from February to April 2014 during the 30th Chinese Antarctic Research Expedition. The SO, particularly in the region south of 58°S, had the highest mean surface seawater DMS concentration of 4.1 ± 8.3 nM (ranged from 0.1 to 73.2 nM) and lowest mean seawater pCO2 level of 337 ± 50 μatm (ranged from 221 to 411 μatm) over the entire cruise. Significant variations of surface seawater DMS and pCO2 in the seasonal ice zone (SIZ) of SO were observed, which are mainly controlled by biological process and sea ice activity. We found a significant negative relationship between DMS and pCO2 in the SO SIZ using 0.1° resolution, [DMS] seawater = −0.160 [pCO2] seawater + 61.3 (r2 = 0.594, n = 924, p < 0.001). We anticipate that the relationship may possibly be utilized to reconstruct the surface seawater DMS climatology in the SO SIZ. Further studies are necessary to improve the universality of this approach.

Assessing the potential for dimethylsulfide enrichment at the sea surface and its influence on air–sea flux

Abstract. The flux of dimethylsulfide (DMS) to the atmosphere is generally inferred using water sampled at or below 2 m depth, thereby excluding any concentration anomalies at the air–sea interface. Two independent techniques were used to assess the potential for near-surface DMS enrichment to influence DMS emissions and also identify the factors influencing enrichment. DMS measurements in productive frontal waters over the Chatham Rise, east of New Zealand, did not identify any significant gradients between 0.01 and 6 m in sub-surface seawater, whereas DMS enrichment in the sea-surface microlayer was variable, with a mean enrichment factor (EF; the concentration ratio between DMS in the sea-surface microlayer and in sub-surface water) of 1.7. Physical and biological factors influenced sea-surface microlayer DMS concentration, with high enrichment (EF > 1.3) only recorded in a dinoflagellate-dominated bloom, and associated with low to medium wind speeds and near-surface temperature gradients. On occasion, high DMS enrichment preceded periods when the air–sea DMS flux, measured by eddy covariance, exceeded the flux calculated using National Oceanic and Atmospheric Administration (NOAA) Coupled-Ocean Atmospheric Response Experiment (COARE) parameterized gas transfer velocities and measured sub-surface seawater DMS concentrations. The results of these two independent approaches suggest that air–sea emissions may be influenced by near-surface DMS production under certain conditions, and highlight the need for further study to constrain the magnitude and mechanisms of DMS production in the sea-surface microlayer.

Abundance and distribution of dimethylsulfoniopropionate degradation genes and the corresponding bacterial community structure at dimethyl sulfide hot spots in the tropical and subtropical pacific ocean.

Dimethylsulfoniopropionate (DMSP) is mainly produced by marine phytoplankton but is released into the microbial food web and degraded by marine bacteria to dimethyl sulfide (DMS) and other products. To reveal the abundance and distribution of bacterial DMSP degradation genes and the corresponding bacterial communities in relation to DMS and DMSP concentrations in seawater, we collected surface seawater samples from DMS hot spot sites during a cruise across the Pacific Ocean. We analyzed the genes encoding DMSP lyase (dddP) and DMSP demethylase (dmdA), which are responsible for the transformation of DMSP to DMS and DMSP assimilation, respectively. The averaged abundance (±standard deviation) of these DMSP degradation genes relative to that of the 16S rRNA genes was 33% ± 12%. The abundances of these genes showed large spatial variations. dddP genes showed more variation in abundances than dmdA genes. Multidimensional analysis based on the abundances of DMSP degradation genes and environmental factors revealed that the distribution pattern of these genes was influenced by chlorophyll a concentrations and temperatures. dddP genes, dmdA subclade C/2 genes, and dmdA subclade D genes exhibited significant correlations with the marine Roseobacter clade, SAR11 subgroup Ib, and SAR11 subgroup Ia, respectively. SAR11 subgroups Ia and Ib, which possessed dmdA genes, were suggested to be the main potential DMSP consumers. The Roseobacter clade members possessing dddP genes in oligotrophic subtropical regions were possible DMS producers. These results suggest that DMSP degradation genes are abundant and widely distributed in the surface seawater and that the marine bacteria possessing these genes influence the degradation of DMSP and regulate the emissions of DMS in subtropical gyres of the Pacific Ocean.

Quantifying the impacts of an updated global dimethyl sulfide climatology on cloud microphysics and aerosol radiative forcing

AbstractOne of the critical parameters in assessing the global impacts of dimethyl sulfide (DMS) on cloud properties and the radiation budget is the estimation of phytoplankton‐induced ocean emissions, which are derived from prescribed, climatological surface seawater DMS concentrations. The most widely used global ocean DMS climatology was published 15 years ago and has recently been updated using a much larger database of observations. The updated climatology displays significant differences in terms of the global distribution and regional monthly averages of sea surface DMS. In this study, we use the ECHAM5‐HAMMOZ aerosol‐chemistry‐climate general circulation model to quantify the influence of the updated DMS climatology in computed atmospheric properties, namely, the spatial and temporal distributions of atmospheric DMS concentration, sulfuric acid concentration, sulfate aerosols, number of activated aerosols, cloud droplet number concentration, and the aerosol radiative forcing at the top of the atmosphere. Significant differences are observed for all the modeled variables. Comparison with observations of atmospheric DMS and total sulfate also shows that in places with large DMS emissions, the updated climatology shows a better match with the observations. This highlights the importance of using the updated climatology for projecting future impacts of oceanic DMS emissions, especially considering that the relative importance of the natural sulfur fluxes is likely to increase due to legislation to “clean up” anthropogenic emissions. The largest estimated differences are in the Southern Ocean, Indian Ocean, and parts of the Pacific Ocean, where the climatologies differ in seasonal concentrations over large geographical areas. The model results also indicate that the former DMS climatology underestimated the effect of DMS on the globally averaged annual aerosol radiative forcing at the top of the atmosphere by about 20%.

Effects of Harpacticus sp. (Harpacticoida, copepod) grazing on dimethylsulfoniopropionate and dimethylsulfide concentrations in seawater

We conducted 9d and 24h ingestion experiments to investigate the effects of copepod grazing on the concentrations of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in seawater. Data from the 9d trial showed that copepod Harpacticus sp. (Harpacticoida, copepod) grazing increased DMS (0–20%) and dissolved DMSP (DMSPd) (0–128%) apparently, accompanied by a significant reduction of particulate DMSP (DMSPp) in algal culture (0–30%). Ingestion rates (IRs) and pellet production rates (PPRs) of Harpacticus sp. varied with diet species (Platymonas subcordiformis (PS), Nitzschia closterium (NC), Skeletonema costatum (SC), Isochrysis galbana (IG), Prymnesium parvum (PP) or Heterosigma akashiwo (HA)), algal concentration, salinity and temperature. Harpacticus sp. fed on PP showed the lowest IRs (female/male, 0.72/0.53×104cells copepod−1h−1) and PPRs (female/male, 0.75/0.5 pellets copepod−1h−1), accompanied with the largest amounts of DMS and DMSPd,p (sum of DMSPd and DMSPp). IRs, PPRs, DMS and DMSPf (DMSP in fecal pellet) increased with the increase of food concentration and peaked at 25×104cellsmL−1I. galbana. High salinity decreased IRs, PPRs, DMS and DMSPf and increased DMSPz (DMSP in copepod body) and DMSPd,p. IRs, PPRs, DMS and DMSPf increased with the increase of temperature from 15 to 25°C, whereas DMSPz and DMSPd,p contents decreased. Pearson correlation analysis results showed that DMS concentrations presented positive relationships with IRs in algal concentration, salinity and temperature experiments (r=0.746; P<0.01). The contribution of DMSPz, DMSPf, DMS and DMSPd,p concentration to the total amounts (DMSPz+DMSPf+DMS+DMSPd,p) was 4–37%, 3–36%, 8–42% and 9–89%, respectively, indicating that DMSP was transferred to copepod tissue and fecal pellet via grazing. Our results are helpful for further understanding of the role of copepod grazing on DMS biogeochemical cycle.