Dimethyl Sulfide and (Dimethylsulfonio)propionate in European

Chapter 12

Dimethyl Sulfide and (Dimethylsulfonio)propionate in European Coastal and Shelf Waters

Downloaded by TUFTS UNIV on June 23, 2017 | http://pubs.acs.org Publication Date: April 27, 1989 | doi: 10.1021/bk-1989-0393.ch012

S. M . Turner, G. Malin, and P. S. Liss School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom The seasonal and spatial variations in surface water dimethyl sulphide (DMS) concentrations have been determined, enabling an estimation of the flux of marine biogenic sulphur to the atmosphere for the European Shelf system. This natural emission may account for a significant amount of the excess atmospheric sulphate over Scandinavia in Spring and Summer. Several ecologically important, bloom-forming species of phytoplankton have been identified as DMS producers, and where the algal population is nearly monospecific, there are good correlations between DMS and biomass. Concentrations of dimethylsulphoniopropionate (DMSP), the precursor of DMS, are on average 14 times higher than concentrations of DMS and there is good correlation between the two compounds. Intracellular levels of DMSP vary according to algal species. Over the past few years we have been studying the waters around the United Kingdom, including the North Sea, Irish Sea and N.E. Atlantic, in order to characterise dimethyl sulphide (DMS) emissions and assess the significance of this natural contribution to acidity of rainfall and the sulphur cycle. Biogenic DMS concentrations in seawater vary considerably both temporally and spatially and coastal and shelf water systems often contain higher concentrations of volatile sulphur than the open oceans (1.2). In order to establish the sources of DMS we have made measurements of dimethylsulphoniopropionate (DMSP; the precursor of DMS) and attempted to relate the two compounds to phytoplankton species and abundance. We have also investigated variations in DMS and DMSP with depth through the water column, with respect to diel cycling and monitored the effect of high and low nitrate concentrations on DMSP levels in laboratory cultures and in tne N. Sea. Experimental Surface seawater samples were taken using ships' continuous non-toxic pump supplies, the average depth of intake being 3m. A 500ml bottle was carefully filled with the fast-flowing, bubble-free water and allowed to overflow for about a minute and then stoppered (ground glass). Hence, exposure to air and 0097-6156/89/0393-0183$06.00A) • 1989 American Chemical Society

Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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BIOGENIC SULFUR IN THE ENVIRONMENT

potential problems of degassing or contamination were minimised. Aliquots for analysis were immediately sub-sampled from the bottom of the bottle using glass syringes, to avoid possible storage effects and degassing by contact with air. One aliquot was purged with scrubbed N 2 for 20 minutes, to extract the dissolved gases which were cryo-focussed at -150°C. The sample was thawed and injected via a six-port valve into a gas chromatograph fitted with two Chromosil 330 columns ( S U P E L C O ) and two flame photometric detectors (Varian 3700 with Aerograph dual flame detector, 365nm optical filter ). Two further aliquots were prepared for D M S P analysis; one unfiltered and one filtered using Whatman G F / C filters, with a nominal, initial retention size of 1.2 μτη. The samples were reacted with 10M sodium hydroxide (final p H = 12) in ground glass stoppered bottles at room temperature, for at least 6 hours. The time for complete alkaline conversion of D M S P to D M S was determined using natural samples. The D M S P concentration was then determined as D M S by the analysis described above. Dissolved and particulate D M S P were calculated by subtraction : DMSP = DMSP d

f f l t e r e d

-DMS

and DMSP = DMSP p

t o t a l

- (DMS + DMSP ). d

(The "dissolved" fraction consists of D M S P that passes through the filter, and the "particulate" fraction is that retained. Hence, the particulate D M S P is associated with algal cells, gut contents of zooplankton and detrital material.) Standard hydrographie data (temperature, salinity, fluorescence and inorganic nutrients) were generally recorded continuously. Chlorophyll a concentrations of discrete samples were determined fluorometrically. Water samples were preserved with LugoPs iodine solution and 0.5% neutralised formaldehyde for onshore identification and enumeration of the phytoplankton using an inverted microscope.

Area of Study Figure 1 is a map of the study area and shows the regions covered by the cruises and the position of a sampling station just off the coast of East Angha. We have tried to maximise geographical and seasonal coverage, but as most of the work has been done on ships of "opportunity", certain areas, such as the Wadden Sea, German Bight and eastern North Sea, have not yet been properly sampled. Seasonal Variation in Surface Water Concentrations D M S i n seawater is produced by marine phytoplankton (2*1) and i n the latitudes delineated by the European Shelf system would be expected to show some seasonality. Figure 2a shows the variation of D M S concentration (log scale) at the fixed station just off the coast of East Anglia. For the two years of monitoring, there was a marked increase i n D M S concentration i n spring, reaching a maximum in summer and then decreasing. The maxima coincided with blooms of the alga, Phaeocystis pouchetii, which occur annually in this area, but vary in timing from year to year. Figure 2b is a compilation of all the data obtained on cruises showing mean values and ranges of D M S concentration plotted on a logarithmic scale. The mean values for winter and summer are 0.1 and 9.4 nmol D M S (S) H respectively, a seasonal difference of two orders of magnitude. A s yet, the data set for D M S P is not as comprehensive as for D M S , but as the former is the precursor of the latter, a similar seasonal pattern might be expected. For


DMS and DMSP in European Waters

185


12. TURNER ET AL.

Figure 1. Map to show the areas covered by cruises, 1984 to 1987, and the position of a sampling station, · , off the East Arglian coast.


186

BIOGENIC SULFUR IN THE ENVIRONMENT

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Figure 2. Seasonal variation of D M S in surface water (approx. 3m depth); (a) log D M S concentration at the East Anglian sampling station for 1983 — O — and — · — 1984. (b) compilation of data from all cruises showing the log values for mean D M S concentrations, ranges of D M S concentration and duration of each cruise.


12. TURNER ET AL.

187

DMS and DMSP in European Waters

example, for the 3 cruises falling i n the period 22nd A p r i l to 3rd July, the average concentration of total D M S P was 134.4 nmol D M S F W S ) l " , with a mean value of 30.0 nmol D M S P (S) H for an October cruise. The mean D M S concentrations for the two periods were about 9.4 and 2.2 nmol D M S (S) H. Thus, from Spring to Autumn, D M S P , and D M S concentrations decreased proportionally (by a factor of 4.3 to 4.5) and appeared to maintain a ratio of about 14:1. 1

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Spatial Variation In addition to temporal variation i n D M S concentration Figure 2b also illustrates the large spatial variations that can be encountered in relatively small sampling areas. A s a case study, Figure 3a shows the location of a cruise in ApntyMay 1987. Figure 3b describes the distribution of D M S in surface water on this cruise, when the mean concentration was 9.2 nmol D M S (S) H, (n = 162). There were large transverse concentration gradients ranging from 0.8 to nearly 90 nmol D M S (S) Κ In an area between the Shetland Islands and the Norwegian coast D M S concentrations ranged from 15.6 to 89.8 nmol D M S (S) 1\ with two discrete areas having concentrations greater than 32.8 nmol D M S (S) l " . The highest D M S levels were of a similar magnitude to those reported tor the approaches to the R i o de la Plata estuary (5) and in a coastal salt pond (£). The distribution of D M S P in European Shelf surface waters is similar to that of D M S , in that large ranges of concentrations are found over relatively small distances. Total D M S P concentrations are on average 15.6 times higher than those of D M S (means from 5 cruises, S D = 3.3), but the ratio is very variable for individual samples. The ratio of D M S P to DMSProduction also varies with the stage of algal growth (2) and this may also be a actor i n natural populations. Figure 4a shows the relationship between D M S and chlorophyll a for a suite of samples taken on a circumnavigation of Britain in 1985. The samples were taken from areas with very different hydrographie characteristics. Again, there appears to be little correlation between D M S and chlorophyll. However, if the phytoplankton are identified and enumerated, samples containing a dominant species or group can be abstracted from the total data set. When D M S is plotted against chlorophyll for these sub-sets, some improved correlations appear. F o r coccolithophores, the dinoflagellate, Gyrodinium aureolum and dinoflagellates excluding G. aureolum (Figure 4; c, e and f) there is reasonable correlation and the difference in slopes indicates how D M S production per unit chlorophyll varies for different algal species. The plots for flagellates and diatoms (fa and f), however, do not snow clear correlations.

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COCCOLITHOPHORES

Figure 4. Relationship between D M S and chlorophyll a, July/August 1985. A l t e r identification and enumeration of phytoplankton, cell numbers were converted to carbon and each particular group or species was expressed as a percentage of total phytoplankton biomass. D M S and chlorophyll data values or samples containing an identifiable dominant group or species were plotted. Where groups were represented by a small number of samples, data sets were expanded by including samples which contained rocesses. However, as so little is known about the mechanisms and rates of oss and production of these compounds, any conclusions are highly speculative. p

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Dimethyl Sulfide and (Dimethylsulfonio)propionate in European

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