Dimethylsulphoniopropionate (DMSP) synthesised by phytoplankton is the principal precursor of the climatically active gas dimethyl sulphide (DMS). The rates of production of particulate DMSP (DMSPp) and turnover by microzooplankton were determined in surface waters of the northern North Sea, using a dilution approach. The phytoplankton communities were characterised by DMSP-rich taxa including Emiliania huxleyi and Prorocentrum minimum and DMSPp:chlorophyll a (chl a) ratios of 64 to 162 nM ?g-1. Microzooplankton biomass varied from 25.5 to 56.7 ?g C l-1 and was dominated by oligotrich ciliates and heterotrophic dinoflagellates. DMSPp production rates ranged from 14.8 to 45.6 nM d-1 and represent a doubling time of the ambient DMSPp pool of between 1.2 and 3.1 d. Consumption rates of DMSPp by microzooplankton varied between 11.4 and 59.9 nM d-1 and were equivalent to turnover rates of the ambient DMSPp pool of between 16 and 43% d-1. In general, production rates of DMSPp were lower than those of chl a and E. huxleyi, with respective mean doubling times of 1.9, 1.5 and 1.3 d. Loss rates due to grazing were similar for DMSPp and E. huxleyi but generally significantly lower than those of the bulk phytoplankton, with mean turnover rates of 31, 30 and 40% d-1 of the standing stock of DMSPp, E. huxleyi and chl a, respectively. E. huxleyi contributed an estimated 2 to 25% of the total DMSPp production and 6 to 23% of the DMSPp ingested by microzooplankton, indicating the importance of other phytoplankton to DMSPp dynamics in ŒE. huxleyi blooms¹. At the depths sampled, DMSPp production was closely coupled to primary production and was equivalent to approximately 11% of the carbon fixation. DMSPp may be an important component of the diets of microzooplankton. Ingested DMSPp could have provided 2 to 3% of the microzooplankton carbon demand and 26 to 44% of their sulphur demand. DMSPp production and turnover rates were closely matched and suggest that in these waters microzooplankton grazing may be the principal determinant of the fate of DMSPp. The quantity of ŒDMSP¹ excreted by microzooplankton, calculated from ingestion rates, biomass and assumed growth rates and growth efficiencies, ranged from 8.0 to 41.9 nM d-1. This is equivalent to 13 to 35% d-1 of the DMSPp standing stocks and sufficient to support a daily turnover of the DMS plus dissolved DMSP (DMS+DMSPd) pool
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