Dimethylsulphide and ammonia in remote marine regions - an Atlantic Meridional Transect study
The NERC Atlantic Meridional Transect (AMT) programme involves a biannual UK-Falklands research transect and has facilitated detailed study of the Atlantic’s oligotrophic gyres. Dimethylsulphide (DMS) and dimethylsulphoniopropionate (dissolved and particulate, DMSPd and DMSPp) were depth profiled on cruises throughout 2001-03 (AMT-12, -13 and -14) and combined with data previously collected by Conor McKee (AMT-5 and -9). Ambient surface DMS concentrations were low (1.0 nM mean; 0.01 – 13.4 nM range), while fluxes were variable (0.4 – 12.2 µmol m-2 day-1). The dataset was interpreted in conjunction with ancillary data such as accessory pigments, carbon fixation rates, bacterial numbers and DMSP-lyase activity as available. As expected, DMS and DMSPp correlated poorly with an indicator of biomass (chlorophyll a). However, significant positive correlations with carbon fixation rates (primary production) were observed. When divided into biogeochemical provinces (Longhurst, 1995) it became clear that such regions are not homogeneous and caution should be exercised when extrapolating measured data to these scales. When compared with values from predictive models, measured surface DMS concentrations were found to be lower. During AMT-14, the in vitro activity of the enzyme responsible for DMS production (DMSP-lyase) was often substantially higher at the surface compared to the chlorophyll maximum. These results provide some support for the proposed protective role of DMS and related parameters during periods of oxidative stress. Atmospheric gas (ammonia, NH3) and aerosol measurements were made throughout AMT-12, -13 and -14. Results suggest that the NH3 flux is finely balanced between in and out of the ocean in low latitudes (flux range between -7.3 and 24.8 µmol m-2 day-1). The potential for a flux out of the ocean is surprising given the low seawater ammonium concentrations in such regions. The submicron ammonium to non-sea-salt sulphate (NH4+:nssSO42-) ratio in remote marine aerosol was consistently greater than 1:1 (i.e. between partially and fully neutralised). However, little correlation was apparent between DMS and NH3 for either sea-to-air fluxes or oceanic concentrations. Studies from around the globe consistently produce a mean NH4+:nssSO42- aerosol ratio between 1:1 and 2:1, but the degree of variation in individual samples suggests that it may be driven purely by variations in source strength of NH3 and nssSO42-.
PhD Thesis
http://web.pml.ac.uk/amt/publications/theses/bell.pdf
Tom_Bell_thesis_email_size.pdf