Regulation of bacterioplankton production and biomass in an oligotrophic cleanvater lake--the importance of the phytoplankton community

Estimations of bacterioplankton production and biomass werecarried out in enclosure experiments during two consecutiveyears (1989 and 1990) in oligotrophic clearwater Lake Njupfatet.The lake was limed in November 1989, and the experiments werecarried 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 aloneand in combination with organic carbon. However, the increasein bacterial production when inorganic nutrients were addedalone was much stronger in 1990 than in 1989. In 1989. bacterialproduction increased strongly only when inorganic nutrientsand organic carbon were added together. The phytoplankton communitywas dominated by the cyanobacterium Merismopedia tenuis-simaduring 1989, and the phytoplankton biomass increased only slightlywhen receiving inorganic nutrients. In 1990, when the lake hadbeen limed. M.tenuissima had completely disappeared and thephytoplankton community, dominated by Chrysophyceae and Chlorophyceae,responded strongly to additions of inorganic nutrients. Theincreased phytoplankton productivity in 1990 may have resultedin increased release of organic carbon, and this in turn thatthe carbon limitation of bacterioplankton production decreasedfrom 1989 to 1990. Zooplankton had a positive effect on bacterioplanktonproduction in 1989, but no effect in 1990. The loss of bacterialbiomass approximated 60% of the bacterial production in 1989,while in 1990 it almost equalled the bacterioplankton production.