A dilution approach to quantify the production of dissolved dimethylsulphoniopropionate and dimethyl sulphide due to microzooplankton herbivory

A dilution approach for quantifying the grazing-mediated production of dissolved dimethylsulphoniopropionate (DMSPd) and dimethyl sulphide (DMS) was tested using laboratory cultures and in natural marine waters. We attempted to make simultaneous estimates of microzooplankton grazing rate and grazing-mediated production of DMS and DMSPd. In the laboratory, the dinoflagellate Oxyrrhis marina grazed on the prymnesiophyte Isochrysis galbana at a rate equivalent to a turnover of 57 % d(-1) of the standing stock. Three experiments using natural waters are presented; 1 in the southern North Sea on 13 April 1998 and 2 in the Iceland Basin on 27 June and 4 July 1998. In all cases there was significant microzooplankton grazing with a turnover of 20% d(-1) of the chlorophyll a in the southern North Sea on 13 April and 19 and 15% d(-1) of the nanophytoplankton on 27 June and 4 July in the Iceland Basin, respectively. Production rates of DMS, DMSPd and DMS+DMSPd due to grazing were calculated from the slope of the regression between algal-specific production and the level of dilution, a proxy of the grazing pressure. DMS+DMSPd production rates due to microzooplankton grazing of 31 +/- 6, 28 +/- 2, 14 +/- 3 and 10 +/- 1 nM d(-1) (+/- SE), were measured in the laboratory and on 13 April, 27 June and 4 July. These values represent a conversion of 19, 14, 15 and 9% d(-1) of the standing stock of DMSPp to DMS+DMSPd. The potential impact of bacterial metabolism of DMS and DMSPd on estimates of grazing mediated production together with other competing production/loss processes are discussed. We suggest that in marine surface waters microzooplankton grazing plays an important role in the generation of the dissolved pool of DMS+DMSPd.