Albert Jerome Gabric

Australian dust storms in 2002–2003 and their impact on Southern Ocean biogeochemistry

During late 2002 and early 2003 southern Australia was in the grip of drought and experienced one of its most active dust storm seasons in the last 40 years with large dust plumes frequently advected over the adjacent Southern Ocean. We use meteorological records of dust activity, satellite ocean colour and aerosol optical depth data, and dust transport modeling to investigate the transport and deposition of mineral dust from Australia over adjacent ocean regions and to correlate it with biological response in phytoplankton standing stock as measured by chlorophyll-a concentration in five-degree latitude bands from 40-60°S. Seasonal maxima in mean surface chlorophyll-a of ~0.5 mg m-3 were not achieved until late Jan 2003 or during February in the more southerly bands, which when compared with a 9-year satellite mean climatology suggests the phenology of the bloom in 2002-03 was atypical. Contemporaneous field data on CO2 fugacity collected on transects between Tasmania and Antarctica show that significant atmospheric CO2 drawdown occurred as far south as 60°S during February 2003. Our results provide strong evidence for a large-scale natural dust fertilization event in the Australian sector of the Southern Ocean, and highlight the importance of dust-derived nutrients in the marine carbon cycle of the Southern Ocean.

Modelling the production of dimethylsulfide during a phytoplankton bloom

Dimethylsulfide (DMS) is an important sulfur-containing atmospheric trace gas of marine biogenic origin. DMS emitted from the oceans may be a precursor of tropospheric aerosols and cloud condensation nuclei (CCN), thereby affecting the Earths radiative balance and possibly constituting a negative feedback to global warming, although this hypothesis is still somewhat controversial. The revised conceptual model of the marine pelagic food web gives a central role to planktonic bacteria. Recent experiments have shown that consumption of dissolved DMS by microbial metabolism may be more important than atmospheric exchange in controlling its concentration in surface waters and hence its ventilation to the atmosphere. In this paper we investigate the effect of the marine food web on cycling of dissolved DMS in surface waters during a phytoplankton bloom episode. A nitrogen-based flow network simulation model has been used to analyze the relative importance of the various biological and chemical processes involved. The model predictions suggest that the concentration of DMS in marine surface waters is indeed governed by bacterial metabolism. Environmental factors that affect the bacterial compartment are thus likely to have a relatively large influence on dissolved DMS concentrations. The ecological succession is particularly sensitive to the ratio of phytoplankton to bacterial nutrient uptake rates as well the interaction between herbivore food chain and the microbial loop. Importantly for the design of field studies, the model predicts that peak DMS concentrations are achieved during the decline of the phytoplankton bloom with a typical time lag between peak DMS and peak phytoplankton biomass of 1 to 2 days. Significantly, the model predicts a relatively high DMS concentration persisting after the phytoplankton bloom due to excretion from large protozoa and zooplankton, which may be an additional explanation for the lack of correlation between DMS and chlorophyll a field measurements. Comparison of the model predictions has been made with tank algal bloom experiments.

The simulated response of dimethylsulfide production in the Arctic Ocean to global warming

Sulfate aerosols (of both biogenic and anthropogenic origin) play a key role in the Earth’s radiation balance both directly through scattering and absorption of solar and terrestrial radiation, and indirectly by modifying cloud microphysical properties. However, the uncertainties associated with radiative forcing of climate due to aerosols substantially exceed those associated with the greenhouse gases. The major source of sulfate aerosols in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and is synthesized by plankton. Climate models point to significant future changes in sea-ice cover in the Arctic Ocean due to warming. This will have consequences for primary production and the sea-to-air flux of a number of biogenic compounds, including DMS. In this paper we discuss the impact of warming on the future production of DMS in the Arctic Ocean. A DMS production model has been calibrated to current climate conditions with satellite ocean colour data (SeaWiFS) using a genetic algorithm, an efficient non-derivative based optimization technique. We use the CSIRO Mk 2 climate model to force the DMS model under enhanced greenhouse climate conditions. We discuss the simulated change in DMS flux and its consequences for future aerosol production and the radiative budget of the Arctic. Significant decreases in sea-ice cover (by 18.5% annually and 61% in summer–autumn), increases in mean annual sea surface temperature of 1◦C, and a decrease of mixed layer depth by 13% annually are predicted to result in annual DMS flux increases of over 80% by the time of equivalent CO2 tripling (2080). Estimates of the impact of this increase in DMS emissions suggest significant changes to summer aerosol concentrations and the radiative balance in the Arctic region.

Correlations between the satellite-derived seasonal cycles of phytoplankton biomass and aerosol optical depth in the Southern Ocean: Evidence for the influence of sea ice

The relationship between the production of dimethylsulfide (DMS) in the upper ocean and atmospheric sulfate aerosols has been confirmed through local shipboard measurements, and global modeling studies alike. In order to examine whether such a connection may be recoverable in the satellite record, we have analyzed the correlation between mean surface chlorophyll (CHL) and aerosol optical depth (AOD) in the Southern Ocean, where the marine atmosphere is relatively remote from anthropogenic and continental influences. We carried out the analysis in 5-degree zonal bands between 50°S and 70°S, for the period (1997-2004), and in smaller meridional sectors in the Eastern Antarctic, Ross and Weddell seas. Seasonality is moderate to strong in both CHL and AOD signatures throughout the study regions. Coherence in the CHL and AOD time series is strong in the band between 50°S and 60°S, however this synchrony is absent in the sea-ice zone (SIZ) south of 60°S. Marked interannual variability in CHL occurs south of 60°S, presumably related to variability in sea-ice production during the previous winter. We find a clear latitudinal difference in the cross correlation between CHL and AOD, with the AOD peak preceding the CHL bloom by up to 6 weeks in the SIZ. This suggests that substantial trace gas emissions (aerosol precursors) are being produced over the SIZ in spring (October-December) as sea ice melts. This hypothesis is supported by field data that record extremely high levels of sulfur species in sea ice, surface seawater, and the overlying atmosphere during ice melt.

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the micro-physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS-climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea-to-air DMS flux for the period 1960 to 2080, corresponding to equivalent CO2 tripling relative to pre-industrial levels. The results confirm a minor but non-negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.

Modeling the biogeochemical cycle of dimethylsulfide in the upper ocean: a review

An important focus of climate-change research is the understanding of the role of ecosystems in shaping climate. Central to this aim is the identification of any feedbacks by which ecosystems may moderate anthropogenic forcing of climate. One possible ecosystem feedback involves the marine food-web and the biogenic sulfur compound dimethylsulfide (DMS). DMS is produced by algae containing the precursor compound dimethylsulfoniopropionate (DMSP), and once ventilated to the atmosphere can be transformed to sulfate aerosols and global climate. It was hypothesized that an increase in biogenically produced sulfate aerosols leading to formation of more cloud condensation nuclei (CCN), and brighter clouds, could stabilize the climate against perturbations due to greenhouse warming. Although a large database of DMS seawater measurements exist, attempts to statistically correlate DMS concentrations with other biological parameters, such as chlorophyll a or nutrients, have failed. This underscores the complex and dynamic nature of the DMS cycle, and means that simple regression-type predictive models are unlikely to be useful, except at local scales. Regional-scale simulations of the DMS cycle have involved multi-parameter, deterministic formulations based on ecological food-web approaches but with the added challenge of properly simulating the behavior of coupled sulfur and nitrogen (or carbon) cycles. Here we review the current DMS modeling approaches, outline the parameterization of key processes, and identify areas where our knowledge is poor and improvements should be made. Model skill can only be assessed against detailed regional and global data sets, however data have not always been collected in a form suitable for model parameter estimation or model calibration/validation. DMS time series, which are essential for calibration of seasonal or multi-annual simulations, are rare. We discuss the minimum requirements for a successful future integration of observational and theoretical efforts.

Potential Feedbacks Between Pacific Ocean Ecosystems and Interdecadal Climate Variations.

Oceanic ecosystems altered by interdecadal climate variability may provide a feedback to the physical climate by phytoplankton affecting heat fluxes into the upper ocean and dimethylsulfide fluxes into the atmosphere

Coupling between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact?

[1] Two hypotheses that postulate interactions between ocean biota and aerosols in the atmosphere have generated substantial research into marine systems. The stimulation of phytoplankton photosynthesis by the provision of iron, a micronutrient contained in deposited aeolian dust (the Iron Hypothesis), and the contribution of dimethylsulphide (DMS) produced by marine ecosystems to the atmospheric burden of aerosols (the CLAW Hypothesis) have been the focus of much research. Satellite sensors, such as the Seaviewing Wide Field-of-view Sensor (SeaWiFS) now provide moderate-resolution time series of measurements of the optical properties of the oceans and atmosphere over most of the Earth’s surface. These data provide an unprecedented opportunity to investigate the ubiquity of biotic linkages between the ocean and atmosphere at the global scale. We analyzed 5 years of SeaWiFS 8-day fields of two variables, chlorophyll concentration and aerosol optical depth, for the global oceans. This first global, multiyear approach does not yet allow unequivocal conclusions, as satellite measurements of chlorophyll can be influenced by aerosol properties of the atmosphere and several variables we do not yet examine are likely to play a role. We find correlation between optical properties of the ocean and atmosphere over much of the globe, in particular the midlatitudes. While some regional analyses indicate that SeaWiFS chlorophyll retrievals are biased by dust in the atmosphere, our results do not support the existence of widespread bias in the SeaWiFS products, but are consistent with global-scale couplings posited by the Iron and CLAW hypotheses.

Independent marine and atmospheric model estimates of the sea‐air flux of dimethylsulfide in the Southern Ocean

Marine and atmospheric models have been combined with data collected at the Cape Grim atmospheric baseline station, to estimate the flux of dimethylsulfide to the atmosphere during the spring-summer period in the Subantarctic Southern Ocean in the vicinity of the Cape Grim baseline atmospheric monitoring station. The marine model predicts that production of phytoplankton and of dissolved DMS will increase during spring to reach a maximum in summer consistent with the data on atmospheric DMS. Local wind and sea temperature data have been used to calculate the DMS transfer velocity which was used to compute the sea-to-air flux of DMS. Independent predictions of the DMS flux using an atmospheric model and Cape Grim data are in excellent agreement with the marine based prediction.

Bioaccumulation of lipophilic compounds from sediments by oligochaetes

Abstract The bioaccumulation of liphophilic compounds from sediments by polychaetes has been considered as two partition processes at equilibrium. The first partition was considered to be between sediment and interstitial water and the second between interstitial water and the polychaete. Theoretical derivations indicated that the bioaccumulation factor (concentration in biota/concentration in sediment) should be weakly dependent on the Kow value of the compound involved and primarily dependent on the organic carbon content of the sediment and the lipid content of the polychaete. However, the bioconcentration factor (concentration in biota/concentration in interstitial water KB) should be strongly dependent on Kow. Actual results from laboratory experiments on bioaccumulation of a range of chloro-hydrocarbons by Tubifex tubifex and Limnodilus hoffmeisteri from sediments were investigated using actual observed interstitial water concentrations, observed interstitial water concentrations corrected for the presence of colloids and interstitial water concentrations calculated from sediment concentrations. The KB values were calculated using the observed biotic concentrations together with the interstitial water concentrations calculated as above except for the last situation listed where the biotic concentrations were corrected for not reaching equilibrium. The relationships between log KB and log Kow obtained were in general accord with the deductions from theory outlined above. The relationship obtained using interstitial water concentrations calculated from sediment concentrations and the corrected biota concentrations was log K B = 1.11 log K ow − 1.0. This is believed to be the most accurate representation of the bioconcentration step and is in accord with that expected theoretically. It is suggested that this theoretical treatment and the currently available experimental data are in accord with a partition bioaccumulation process