Seasonal dynamics of dissolved organic matter and microbial activity in the coastal North Sea

Abstract

The dynamics of dissolved organic matter (DOM) and microbial activity were monitored in the coastal North Sea over an annual cycle. DOM accumulated during the spring bloom towards the summer, associated with high phytoplankton extracellular release. Accumulation of dissolved organic carbon (DOC) occurred from April to June. During this period, based on the bacterial carbon demand (BCD) of the free-living bacteria and the photosynthetic extracellular release (PER), 85 μM C were derived from sources other than direct DOC release by phytoplankton. Thereafter, from the end of August until December, the DOM concentrations decreased by about half. During this period, at least 269 m̈M C was removed from the system via the utilization by the bacterial community and/or sedimentation and export. Overall, our data indicate a pronounced seasonal shift in DOM sources supporting BCD. From spring to summer, BCD is almost fully supported by PER alone while in fall and winter, BCD is supported about equally by PER and by the DOM accumulated in the spring-summer period and originating presumably from a variety of DOM production mechanisms. © Inter-Research 2010.