Fate of dimethylsulfoniopropionate (DMSP) during the decline of the northwest Atlantic Ocean spring diatom bloom

Abstract

A 7 d Lagrangian process study of the biogeochemical cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) was conducted within a decaying diatom bloom in the northwest Atlantic Ocean in spring 2003. Ambient profiles of DMSP and DMS were surveyed daily in the water column and were used to estimate in situ net transformation rates. Phytoplankton and bacterioplankton abundance were determined within the surface mixed layer (SML) as well as at the deep chlorophyll maximum (DCM), and sinking fluxes of particulate DMSP (DMSP p ) below 75 to 100 m were assessed using free-drifting particle interceptor traps. Chlorophyll a (chl a) concentration and diatom abundance declined in the SML over the course of the study period, and the phytoplankton chl a biomass progressively settled above the nitracline forming the DCM. The decline of the diatom bloom coincided with the settling of DMSP p out of the SML and the formation of a DMSP-rich layer at the DCM. The low daily sinking loss rate of DMSP p at 75 m (< 2% d -1 ) provided confirmation of the efficient retention of DMSP p at the DCM. The decaying bloom gave rise to an initial release of dissolved DMSP (DMSP d ) in the upper water column, which was rapidly consumed by the growing bacterial community. The rapid loss of DMSP d was accompanied by significant increases in net production of DMS in the SML and fluxes of DMS to the atmosphere. Despite this increase in DMS dynamics, overall in situ net production rates remained fairly low during the 7 d period (0.4 nmol DMS l -1 d -1 ), suggesting that demethylation by the developing bacterial community dominated DMSP d -consuming processes.