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 diss olved dimethylsulphoniopropionate (DMSPd) and dimethyl sulphide (DMS) was t ested 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 dinoflagellat e Oxyrrhis marina grazed on the prymnesiophyte Isochrysis galbana at a rate equivalent to a turnover of 57 % d(-1) of the standing stock. Three experi ments 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 c ases 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 Ba sin, respectively. Production rates of DMS, DMSPd and DMS+DMSPd due to graz ing 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 laborator y 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 p rocesses are discussed. We suggest that in marine surface waters microzoopl ankton grazing plays an important role in the generation of the dissolved p ool of DMS+DMSPd.