Assessment of the role of copepods and ciliates in the release to solution of particulate DMSP

The exchanges between pools of partlculate DMSP (pDMSP) and the total dissolved pool of DMSP and DMS [dDMS(P)], as well as dDMS(P) removal rates were investigated in 4 shipboard incubations of amended and slze-fractionated natural planktonic assemblages of the Med~terranean Sea in spring 1995 and in a laboratory expenment wlth cultured populations. In the shipboard experiments, the effects of different concentrations of copepods and the presence/absence of micrograzers were assessed. Removal rates of dDMS[P), obtained from seawater samples spiked with dDMSP (dissolved DMSP), were linearly correlated with dDMS(P) levels in the range 10 to 50 nM and were unrelated to size-fractlonation treatments. The blologlcal turnover rate constant of dDMS(P) was 0 5 d.' In most of thv experiments, production of dDMS(P) was independent of copepod concentration and lowest in waters from which both copepods and micrograzers > l 0 pm had been removed. Overall, the results of the sh~pboard expenments suggested that (1) dDMS(P) production occurred in the microplanktonic food web, probably because pDMSP occurred predorn~nately In the slze fraction c10 pm and was unavailable for direct copepod consurnptlon, and (2) dDMS(P) removal rates were likely due to organisms < l 0 pm in size, probably bacteria. The laboratory experiment lnvolved a common Med~terranean microplanktonic ciliate specles (Strombid~um s~rlcaturn) grazing on a DMSP-containing Pryrrlnesiophyceae (Isochrysis gaibana) The rate of dDMS[P) release from algal DMSP was greatly increased when the phytoplankton was subjected to grazing by ciliates. The majority of prey pDMSP ( > M % ) was released to solution.