Abstract

Estimations of bacterioplankton production and biomass were carried out in enclosure experiments during two consecutive years (1989 and 1990) in oligotrophic clearwater Lake Njupfatet. The lake was limed in November 1989, and the experiments were carried out both in 1989 (unlimed) and in 1990 (limed). Bags (∼3001) were manipulated with inorganic phosphorus and nitrogen, organic carbon, and metazoan zooplankton abundance. Both years, bacterial production was stimulated by inorganic nutrients alone and in combination with organic carbon. However, the increase in bacterial production when inorganic nutrients were added alone was much stronger in 1990 than in 1989. In 1989. bacterial production increased strongly only when inorganic nutrients and organic carbon were added together. The phytoplankton community was dominated by the cyanobacterium Merismopedia tenuis-sima during 1989, and the phytoplankton biomass increased only slightly when receiving inorganic nutrients. In 1990, when the lake had been limed. M.tenuissima had completely disappeared and the phytoplankton community, dominated by Chrysophyceae and Chlorophyceae, responded strongly to additions of inorganic nutrients. The increased phytoplankton productivity in 1990 may have resulted in increased release of organic carbon, and this in turn that the carbon limitation of bacterioplankton production decreased from 1989 to 1990. Zooplankton had a positive effect on bacterioplankton production in 1989, but no effect in 1990. The loss of bacterial biomass approximated 60% of the bacterial production in 1989, while in 1990 it almost equalled the bacterioplankton production.