Dynamics of particulate dimethylsulphoniopropionate during a Lagrangian experiment in the northern North Sea

One of the key steps towards predicting dimethyl sulphide (DMS) emissions to the atmosphere is to understand the production and fate of its precursor, dimethylsulphoniopropionate (DMSP). This study used the framework of a SF6-Lagrangian experiment lasting 6 days, to examine the production and turnover of particulate DMSP (DMSPp) within a developing phytoplankton bloom characterised by abundant Emiliania huxleyi. Detailed information on the composition of the phytoplankton community and literature-derived estimates of cell DMSP content were used to partition DMSPp between 6 taxonomic groups. E. huxleyi was estimated to contribute an average of only 16% and 9% of the DMSPp standing stocks in surface and subsurface layers of the water column, respectively. Other phototrophs, including non-lithed nanoflagellates and dinoflagellates, in particular Prorocentrum minimum, made up a substantial proportion of the DMSPp, especially in the subsurface layer. The Lagrangian approach allowed a direct estimate of the net production of DMSPp, chlorophyll a and the 6 phytoplankton taxonomic groups in the surface layer. The net specific accumulation rate of DMSPp was 0.129 day−1, equivalent to a net production of DMSPp over the 6 days of 37.3 nM averaged through the surface-layer depth. The net phytoplankton growth rate in terms of chlorophyll a was similar, at 0.108 day−1. However, not all the phytoplankton showed a net growth during the 6 days, indicating that only certain taxa were responsible for the increases in DMSP and chlorophyll a. A significant relationship between 14C primary production measurements and the potential production of DMSPp derived from dilution experiments was used to estimate an integrated ‘gross’ production and loss rate of DMSPp for the surface mixed layer. High DMSPp production rates were closely matched by high DMSPp loss rates. Ingestion by microzooplankton appeared to be the major cause of DMSPp loss, accounting for an average of 91% of the DMSPp disappearance. A large proportion of the ingested DMSPp was thought to be released as dissolved DMSP rather than DMS. An estimate of the total DMS flux to the atmosphere was equivalent to only 1.3% of the ‘gross’ DMSPp production in the surface layer during the developing phase of the phytoplankton bloom covered by this experiment. Other processes known to cause DMSP release from phytoplankton, such as viral lysis and senescence, may have become more important if the phytoplankton community had reached higher concentrations and/or encountered more growth-limiting conditions. However, in the present experiment horizontal and vertical mixing appeared to play a major role in determining the fate of the SF6-labelled water, and hence the progression of the phytoplankton community.