Suzanne M. Turner

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Volcanic carbon dioxide vents show ecosystem effects of ocean acidification

The atmospheric partial pressure of carbon dioxide (pCO2) will almost certainly be double that of pre-industrial levels by 2100 and will be considerably higher than at any time during the past few million years. The oceans are a principal sink for anthropogenic CO2 where it is estimated to have caused a 30% increase in the concentration of H+ in ocean surface waters since the early 1900s and may lead to a drop in seawater pH of up to 0.5 units by 2100 (refs 2, 3). Our understanding of how increased ocean acidity may affect marine ecosystems is at present very limited as almost all studies have been in vitro, short-term, rapid perturbation experiments on isolated elements of the ecosystem. Here we show the effects of acidification on benthic ecosystems at shallow coastal sites where volcanic CO2 vents lower the pH of the water column. Along gradients of normal pH (8.1–8.2) to lowered pH (mean 7.8–7.9, minimum 7.4–7.5), typical rocky shore communities with abundant calcareous organisms shifted to communities lacking scleractinian corals with significant reductions in sea urchin and coralline algal abundance. To our knowledge, this is the first ecosystem-scale validation of predictions that these important groups of organisms are susceptible to elevated amounts of pCO2. Sea-grass production was highest in an area at mean pH 7.6 (1,827 μatm pCO2) where coralline algal biomass was significantly reduced and gastropod shells were dissolving due to periods of carbonate sub-saturation. The species populating the vent sites comprise a suite of organisms that are resilient to naturally high concentrations of pCO2 and indicate that ocean acidification may benefit highly invasive non-native algal species. Our results provide the first in situ insights into how shallow water marine communities might change when susceptible organisms are removed owing to ocean acidification.

A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month

A database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1°×1° latitude-longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. [1994]. In this approach, a first-guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise present.

A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North Atlantic

The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA-AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite-measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore-rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June-July 1987. Coccolith production (1 × 106 tonnes calcite-C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

Dimethylsulphide and dimethylsulphoniopropionate in the Northeast atlantic during the summer coccolithophore bloom

Abstract Concentrations of dimethylsulphide (DMS) and dissolved and particulate pools of its precursor, dimethylsulphoniopropionate (DMSP), were surveyed at the time of the summer bloom of coccolithophores in the Northeast Atlantic. The average DMS concentration was 12 nmol dm −3 ( n = 158, range 1.06−93.8 nmol dm −3 , σn − 1 = 12.4). Statistically significant positive correlations between particulate DMSP and chlorophyll were found for samples from areas where coccolithophores accounted for 50% or more of the total carbon biomass. In these areas correlations between DMS and chlorophyll were not as strong but still significant. An estimate of the flux of DMS from the Northeast Atlantic in June–July (721 nmol m −2 h −1 ) is of the same order as estimates for the southern North Sea at the same time of year (646 nmol m −2 h −1 ). The data provide strong evidence for the importance of coccolithophores in the emission of DMS to the atmosphere. Comparison of flux data with budgets for airborne sulphur in Europe, reported by the European Monitoring and Evaluation Programme (EMEP), suggests that in summer the Northeast Atlantic may be a source of the sulphur deposited on adjacent land areas not strongly affected by anthropogenic sulphur sources. The overall results are discussed in relation to present knowledge of the global distribution of coccolithophores.

Sulfur: The plankton/climate connection

Question (Aiken et al.): Given the uncertainties involved in the calculation of air-sea exchange of gases such as DMS, is it possible to assess by other methods the importance of natural versus anthropogenic sulfur emissions? Answer: In our paper we discuss various approaches that have been taken to estimate the rate of emission of DMS from the oceans including models (Erikson et al. 1990, Thompson et al. 1990) and the use of observed concentration fields of DMS combined with knowledge of air-sea transfer velocity (Andreae 1986, Bates et al. 1987b). In addition measurements of MSA in atmospheric aerosols can be used to infer the emission of DMS into the atmosphere (J. Prospero, University of Miami, pers. commun.). Many studies (not reviewed in this paper) have investigated the emissions of sulfur from manmade sources. In all these attempts, an assessment is often made of the relative importance of the two sources, although the methodology used to calculate biogenic and anthropogenic sources are fundamentally different. The only consistent approach we are aware of, which has the potential to directly ascribe sulfur in the atmosphere to its major sources, is through the use of sulfur isotope signature measurements. The method relies on the sulfur isotope signature of fossil fuels being significantly different from that of DMS and its oxidation products. This approach shows great promise and is currently being investigated in our laboratory.

Measurements of various sulphur gases in a coastal marine environment

Measurements of several sulphur gases have been made in coastal seawaters (including microlayers) and marine air off Great Yarmouth, U.K., and in a freshwater lake. The results show dimethyl sulphide to be the dominant sulphur gas in all the waters examined, with lesser amounts of carbonyl sulphide and carbon disulphide. For the marine air and water samples carbonyl sulphide showed no significant seasonal variation in concentration. The seawater was always supersaturated with respect to the carbonyl sulphide concentration in the air; the mean saturation value being 4.6. Likewise the seawater was always supersaturated with dimethyl sulphide, but for this gas the concentrations in the water showed substantial seasonal variation (× 40), with a maximum value of about 500 ng(S) l-1 in late June, approximately contemporaneous with the second plankton bloom in the region.Sea surface microlayers harvested cryogenically showed a mean enrichment of 2.4 relative to subsurface water for carbonyl sulphide. Some part of the observed microlayer enrichment for this gas may be due to freezing-on of atmospheric carbonyl sulphide onto the frozen microlayer sample. In general, microlayer samples did not exhibit a significant enrichment for dimethyl sulphide. However, under conditions of high biological production, enrichments of several-fold were found, but may be attributable, at least in part, to biological production of dimethyl sulphide in the microlayer water in the period between collection and analysis.

Seasonal variation of dimethyl sulphide in the North Sea and an assessment of fluxes to the atmosphere

The distribution of DMS concentrations in surface waters of the southern North Sea is described for nine months (February–October) in 1989. Minimum concentrations in winter were 0.13 nM and the maximum, monthly mean concentration was 25 nM, in May, coincident with large blooms of Phaeocystis pouchetii, off the continental coast. Comparison with other North Sea data suggests that the interannual seasonal pattern of DMS concentrations is similar. Transfer velocities, for sea-to-air transfer of DMS are derived, comparing a number of methods, and some of the uncertainties in the flux calculation are discussed. Optimised flux data for the North Sea show a distinct annual cycle with monthly averages ranging from 0.2 to 16.4 μmol m−2 day−1 for February and June, respectively. Comparison with other data suggests that North Sea fluxes are very similar to other ocean areas in a similar latitude band and on an annual and seasonal basis. The potential impact of North Sea DMS fluxes on the European atmospheric sulphur budget is discussed.

Dimethyl sulphide and Phaeocystis: A review

Dimethyl sulphide (DMS) is the dominant sulphur gas found in surface marine waters and there is compelling evidence that it is formed biologically in these environments. In all areas so far investigated the oceans are found to be highly supersaturated (typically by two orders of magnitude) with respect to atmospheric levels of DMS, which indicates a net flux of the gas out of the oceans. In this paper, we first briefly review the environmental importance of the gas and particularly the role of its sea-to-air flux on atmospheric chemistry and physics. Then we discuss what is known of its mode of formation and cycling in seawater, before looking more specifically at the role and significance of Phaeocystis as a producer of DMS.

Interlaboratory calibration and sample analysis of dimethyl sulphide in water

An intercalibration exercise carried out by the Universities of Stockholm and East Anglia, for the determination of natural levels of dimethyl sulphide in aqueous samples, is described. Good agreement between the two groups was obtained for natural samples, but for cultures containing high numbers of marine phytoplankton some differences were observed. The problems associated with the analysis of samples with high densities of phytoplankton are discussed. Four calibration techniques were tested, and their relative merits are assessed.