Dimethylsulfide (DMS) in the Bering Sea and Adjacent Waters:
In-situ and Satellite Observations

This project is funded by the NASA Office of Earth Science Oceanography Program and the NOAA Office of Oceanic and Atmospheric Research.

PMEL Project Description

Oceanic dimethylsulfide (DMS) is the major natural source of sulfur to the atmosphere (Bates et al., 1992). As a volatile odiferous sulfur compound, DMS serves as an olfactory attractant for sea birds (Nevitt et al., 1995). In the atmosphere DMS is oxidized to produce aerosol particles, which affect the acid-base chemistry of the atmosphere (Charlson and Rodhe, 1982) and the radiative properties of marine stratus clouds (Shaw 1983, 1987; Charlson et al., 1987; Andreae and Crutzen, 1997). An increase in the DMS flux has the potential to increase the number of atmospheric aerosol particles and the cloud drop number concentration. An increase in the number of small cloud drops suppresses precipitation leading to longer-lived clouds (Albrecht, 1989; Lohmann and Feichter, 1997). This effectively increases the cloud cover, which leads to more radiation reflected back to space.

The source of atmospheric DMS is the surface ocean. The production of DMS and its precursor, dimethylsulphoniopropionate (DMSP) is confined largely to a few classes of marine phytoplankton, specifically the Dinophyceae and the Prymnesiophycease, which include coccolithopheres (Keller et al., 1989). Blooms of the cocolithophore, Emiliania huxleyi, in the Gulf of Maine and Northeast Atlantic produced concentrations of DMS and DMSP that were as much as an order of magnitude higher than in the surrounding waters (Matrai and Keller, 1993; Malin et al., 1993). Although actively growing cells release only small quantities of DMSP and DMS, DMS is produced during cell senescence (Nguyen et al., 1988; Turner et al., 1988), zooplankton grazing (Dacey and Wakeham, 1986; Wolfe and Steinke, 1996), and the interaction of bacterioplankton (Kiene and Bates, 1990; Bates et al., 1994; Ledyard and Dacey, 1994) and viral pathogens (Malin et al., 1998; Hill et al., 1998).

Since the production and seawater consumption of DMS are highly species specific and dependent upon the bacteria, phytoplankton, and zooplankton communities, the concentration of DMS will be strongly influenced by the physical processes controlling water mass interactions and nutrient availability (Turner et al., 1996). This includes the input of atmospheric nutrients. Coccolithophore blooms have been shown to occur in seasonally stratified waters after the spring diatom bloom has depleted inorganic nutrient levels (Holligan, 1987). It has also been suggested that iron limitation might lead to coccolithophore dominated populations in regions such as the Gulf of Alaska (Martin et al., 1989).

The first true color images over the Bering Sea after the launch of SeaWiFS in August 1997 showed the presence of an extensive coccolithophore bloom. This was the first reported occurrence of a coccolithophore bloom in the Bering Sea, and it has since been associated with anomalous oceanographic and atmospheric conditions. The bloom has reoccurred annually since 1997.

Have these coccolithophore blooms in the Bering Sea affected the flux of DMS to the atmosphere? The complex interaction of physical (ocean and atmospheric circulation, temperature, salinity), chemical (availability of macro and micro nutrients), and biological (species composition and food web dynamics) factors that control the production and consumption of DMS makes it difficult to predict oceanic DMS concentrations and thus the flux to the atmosphere. It is evident that the Bering Sea ecosystem is changing and that coccolithophores have been a dominant plankton species during the past few years. There is also strong evidence from studies in the North Atlantic Ocean that these blooms lead to high concentrations of DMS.

Previous DMS measurements in the Bering Sea

DMS was measured in the Bering Sea in the spring of 1981 as part of the PROBES (Processes and Resources of the Bering Sea Shelf) program (Barnard et al., 1984). DMS concentrations in seawater were strongly correlated with the cell density of the haptophyte, Phaeocystis poucheti and ranged from 1 to 17 nM with a mean of 3 nM. DMS measurements were also made in the Bering Sea on a ship-of-opportunity in September 1985 by Bates and co-workers (1987). Concentrations ranged from 0.5 to 20 nM with a mean of 6 nM. Since DMS is biologically produced, the concentrations in the subarctic regions are very seasonally dependent (Bates et al., 1987).

Why is DMS important to the Bering Sea ecosystem?

An increase in seawater DMS concentrations in the Bering Sea could affect the ecosystem in several ways.

  1. DMS is the precursor of the background sulfate aerosol over the ocean (Bates et al., 1998 and other references in the ACE-1 Special Issue of the Journal of Geophysical Research, July 1998). An increase in atmospheric sulfur aerosol could affect the pH of precipitation falling on coastal regions around the Bering Sea.
  2. An increase in DMS emissions could increase the cloud condensation nuclei concentration and change cloud properties over the Bering Sea. This could affect precipitation frequency, cloud amount, and the regional radiation budget (Albrecht, 1989; Lohmann and Feichter, 1997).
  3. An increase in odiferous seawater sulfur compounds could be carried through the food web to higher order species. While there is evidence in studies of Baltic herring that this flavor compound may be transferred to fish from phytoplankton (Granroth and Hattula, 1976), this has never been proven.
  4. An increase in atmospheric DMS concentrations could affect the foraging behavior of Bering Sea birds. To date there has only been one such study that was carried out in the Antarctic (Nevitt et al., 1995).
Scientific Question Addressed in this Study: Has the increased abundance of coccolithophores in the Bering Sea caused an increase in the production of DMS in sea water and an increase in the flux of DMS to the atmosphere? We are addressing this question by deploying an automated underway DMS sampling/analysis system in the Bering Sea and adjacent waters. Our goal is to quantify the regional and seasonal concentrations of DMS in this region. Have the coccolithophore blooms significantly increased the concentration of DMS in the Bering Sea compared to the concentrations measured in the 1980s (Barnard et al., 1984; Bates et al., 1987)? The data are being made available to the scientific community on this web site. Our co-investigators on this project at Northwest Research Associates are monitoring the coccolithophore blooms, cloud cover and cloud reflectivity in area via satellite. With will combined our data sets in the final year of this project to address this important scientific question.

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