Phytoplankton primary production and its utilization by the pelagic community in the coastal zone of the Gulf of Gdansk (southern Baltic)

In this study we estimated the amount and fate of phytoplankton primary production in the coastal zone of the Gulf of Gdansk, Poland, an area exposed to nutrient enrichment from the Vistula River and nearby municipal agglomeration. The investigations were carried out at 2 sites during 5 months in 1993 (February, April, May, August and October). A prolonged bloom period occurred in the coastal zone, as compared to the open Gulf and the open sea waters. From April until October most values of gross primary production in the near-surface layer were in the range 100 to 500 mg C m(-3) d(-1) Phytoplankton net exudate release constituted on average 5% of the gross primary production; total exudate release was estimated to be about 2 times higher. Bacterial production in the growth season was relatively low (the mean value lying between 5 and 9 % of gross primary production); nevertheless, the microbial community (bacteria and protozoans) utilized a large proportion of primary production (from about 50% in April and May to 16% in October). Usually direct protozoan grazing on phytoplankton exceeded bacterial uptake of the phytoplankton exudates. In winter, summer and autumn community respiration exceeded depth-averaged primary production, indicating that external energy sources (sediment resuspension, allochthonous organic matter) play a substantial role in community metabolism.