DMSP and DMS dynamics and microzooplankton grazing in the Labrador Sea: application of the dilution technique

We adapted the dilution technique to study microzooplankton grazing of alga l dimethylsulfoniopropionate (DMSP) vs. Chl a, and to estimate the impact o f microzooplankton grazing on dimethyl sulfide (DMS) production in the Labr ador Sea. Phytoplankton numbers were dominated by autotrophic nanoflagellat es in the Labrador basin, but diatoms and colonial Phaeocystis pouchetii co ntributed significantly to phytomass at several high chlorophyll stations a nd on the Newfoundland and Greenland shelfs. Throughout the region, growth of algal Chl a and DMSP was generally high (0.2-l d(-1)), but grazing rates were lower and more variable, characteristic of the early spring bloom per iod. Production and consumption of Chl a vs. DMSP followed no clear pattern , and sometimes diverged greatly, likely because of their differing distrib utions among algal prey taxa and size class. In several experiments where P haeocystis was abundant, we observed DMS production proportional to grazing rate, and we found clear evidence of DMS production by this haptophyte fol lowing physical stress such as sparging or filtration, It is possible that grazing-activated DMSP cleavage by Phaeocystis contributes to grazer deterr ence: protozoa and copepods apparently avoided healthy colonies (as judged by relative growth and grazing rates of Chl a and DMSP), and grazing of Pha eocystis was significant only at one station where cells were in poor condi tion. Although we hoped to examine selective grazing on or against DMSP-con taining algal prey, the dilution technique cannot differentiate selective i ngestion and varying digestion rates of Chl a and DM SP. We also found that the dilution method alone was poorly suited for assessing the impact of gr azing on dissolved sulfur pools, because of rapid microbial consumption and the artifactual release of DMSP and DMS during filtration. Measuring and u nderstanding the many processes affecting organosulfur cycling by the micro bial food web in natural populations remain a technical challenge that will likely require a combination of techniques to address. (C) Crown copyright 2000 Published by Elsevier Science Ltd. All rights reserved.