BACTERIOPLANKTON DYNAMICS IN A LARGE MESOTROPHIC LAKE - I - ABUNDANCE, PRODUCTION AND GROWTH-CONTROL

We studied seasonal and spatial dynamics of bacterioplankton growth to gether with those of chlorophyll, phytoplankton primary production and zooplankton from spring 1990 to December 1991 in mesotrophic Lake Con stance, Germany, on the basis of weekly to biweekly samplings at 7 dep ths between 1 and 50 or 120 m. Both years differed by the thermal stru cture of the lake. Whereas in 1990 a stable stratification established already in May, in 1991 the lake remained weakly stratified until mid -June. As a consequence of this the phytoplankton spring bloom and the dear-water phase in 1990 occurred two weeks earlier than in the follo wing year. In both years the spring maximum of bacterioplankton growth occurred with a delay relative to phytoplankton growth. The maximum o f bacterioplankton production (BP) preceded that of the abundance in b oth years by one week. A regression analysis showed that during the sp ring bloom BP was correlated best with the biomass of ciliates which e xplained 94 and 60% of the variability of BP during this period in 199 0 and 1991, respectively. Significant correlations to phytoplankton pa rameters and temperature also existed in 1990 and 1991 (only to temper ature) but were not as close. In July and August 1991 bacterioplankton abundance and production were 1.5- to 2.5-fold higher than in the pre vious year. This was a result of a much higher biomass and thus grazin g of daphnids yielding a higher release of dissolved organic matter an d substrate transfer to planktonic bacteria. Also abundances of hetero trophic nanoflagellates (HNF) were enhanced during this period. After the decline of the daphnid population the biomass of ciliates increase d and led to a reduction in bacterial abundance and production. This n egative effect was also demonstrated by a significant inverse correlat ion between BP and the biomass of ciliates during the period June to O ctober 1991. Bacterioplankton growth rates varied between <0.1 and 0.7 8 d(-1) with highest values during the spring bloom. As compared to ba cterial abundance and production, growth rates did not continuously de crease with depth indicating that also at greater depth bacterioplankt on growth and mortality were fairly tightly coupled. The abundance of HNF was significantly correlated to the bacterial growth rate in both years but not to BP indicating that the control of BP and the bacteria l growth rate were different. Vertical profiles at all seasonal situat ions usually exhibited much closer correlations among bacterioplankton and phytoplankton parameters than seasonal dynamics.