PROPORTION OF PLANKTON BIOMASS IN PARTICULATE ORGANIC-CARBON IN THE NORTHERN BALTIC SEA

In many quantitative studies of the oceanic carbon cycle, measurements of particulate organic carbon (POC) in seawater are used for making budgets of planktonic food webs. In order to evaluate the seasonal changes of the constituents of POC in a temperate sea area, the proportion of autotrophic and heterotrophic plankton biomass in POC was measured for different seasons. At both a coastal and an offshore station in the northern Baltic Sea, POC was dominated by unidentified detritus material. Autotrophic microplankton dominated the plankton biomass in spring, and free-living heterotrophic bacteria in autumn and winter. The remaining plankton groups were of minor importance. The ratio of autotrophic plankton carbon to chlorophyll a (chl a) was determined by frequent sampling, and varied between 5 and 70 (mug l-1/mug l-1), lowest in autumn-winter and highest in spring. Such variability makes it difficult to use constant conversion factors of chl a and POC to quantify plankton biomasses, and emphasizes the need for careful determination of conversion factors for specific environments and seasons.