Impact of sea-ice melt on dimethyl sulfide (sulfoniopropionate) inventories in surface waters of Marguerite Bay, West Antarctic Peninsula

Abstract

The Southern Ocean is a hotspot of the climate-relevant organic sulfur compound dimethyl sulfide (DMS). Spatial and temporal variability in DMS concentration is higher than in any other oceanic region, especially in the marginal ice zone. During a one-week expedition across the continental shelf of the West Antarctic Peninsula (WAP), from the shelf break into Marguerite Bay, in January 2015, spatial heterogeneity of DMS and its precursor dimethyl sulfoniopropionate (DMSP) was studied and linked with environmental conditions, including sea-ice melt events. Concentrations of sulfur compounds, particulate organic carbon (POC) and chlorophyll a in the surface waters varied by a factor of 5-6 over the entire transect. DMS and DMSP concentrations were an order of magnitude higher than currently inferred in climatologies for the WAP region. Particulate DMSP concentrations were correlated most strongly with POC and the abundance of haptophyte algae within the phytoplankton community, which, in turn, was linked with sea-ice melt. The strong sea-ice signal in the distribution of DMS(P) implies that DMS(P) production is likely to decrease with ongoing reductions in sea-ice cover along the WAP. This has implications for feedback processes on the region's climate system.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Keywords: West Antarctic Peninsula; dimethyl sulfide; dimethyl sulfoniopropionate; haptophytes; phytoplankton community structure; sea ice.

Figure 1.

Sea-ice concentration at the West Antarctic Peninsula: (a) average concentration in October–November over the period 2013–2017; (b) average concentration in December–January over the period 2013–2018; (c) map of the study area showing the average sea-ice coverage just preceding the JR307 cruise (16–31 December 2014), and the 11 sampling stations.

Figure 2.

Hydrographic conditions of the surface waters along the JR307 cruise track: (a) potential temperature, (b) salinity, (c) fractional contribution of sea-ice melt to the surface water composition.

Figure 3.

Spatial distribution of biological parameters along the JR307 cruise track: (a) Chl a (in µg l⁻¹); (b) POC (in mg l⁻¹); (c) ¹³C-isotopic signature of POC. Note different depth scales in (c): data below 40 m depth were capped as they may be unreliable due to low POC loading of the samples.

Figure 4.

Patterns of surface-water chemistry along the JR307 cruise track: (a) pH; (b) CO₂ concentration calculated from DIC and pH measurements; (c) oxygen concentration as measured with the CTD sensor; (d) nitrate concentration (adapted from [24]).

Figure 5.

Phytoplankton-pigment distributions along the JR307 cruise track: (a) Chl a as in figure 3, for comparison; (b) Fuco; (c) Hex-Fuco; (d) Chl b. (e) Species contribution as percentage of total Chl a as calculated with CHEMTAX. (f) Absolute species contribution to the total Chl a burden of the upper 25 m, as calculated with CHEMTAX. All data in panels (a) through (d) in microgram pigment l⁻¹.

Figure 6.

Spatial distribution of (a) DMS(P)t and (b) DMS(P)d along the JR307 cruise track.

Figure 7.

Ordination plot of a CCA analysis of species composition and abiotic parameters. The first two axes explain 92% of the variance. Sea-ice melt, nitrate and DIC concentration and O₂ saturation are the strongest drivers (blue arrows) of algal species composition (in red), with a less differentiating role for glacial melt. The green circle clusters 5 m surface samples. Deeper waters (15 and 25 m) are distinguished in two more clusters: the black circle (first quadrant) clusters coastal stations (T05–T10) and the blue circle (second quadrant) shelf stations (T02–T04). Numbers denote station ID and depth.

Figure 8.

Correlations of the full dataset between (a) DMSPp and POC, (b) DMSPp and Chl a, (c) DMSPp and the haptophyte pigment Hex-kFuco, and (d) total DMS(P)t pool and salinity. The colour coding shows the fractional contribution of sea-ice melt to the water composition.