Bacterioplankton dynamics in a large mesotrophic lake: I. abundance, production and growth control

Abstract

We studied seasonal and spatial dynamics of bacterioplankton growth together with those of chlorophyll, phytoplankton primary production and zooplankton from spring 1990 to December 1991 in mesotrophic Lake Constance, Germany, on the basis of weekly to biweekly samplings at 7 depths between 1 and 50 or 120m. Both years differed by the thermal structure 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 clear-water phase in 1990 occurred two weeks earlier than in the following year. In both years the spring maximum of bacterioplankton growth occurred with a delay relative to phytoplankton growth. The maximum of bacterioplankton production (BP) preceded that of the abundance in both years by one week. A regression analysis showed that during the spring bloom BP was correlated best with the biomass of ciliates which explained 94 and 60 % of the variability of BP during this period in 1990 and 1991, respectively. Significant correlations to phytoplankton parameters and temperature also existed in 1990 and 1991 (only to temperature) 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 previous year. This was a result of a much higher biomass and thus grazing of daphnids yielding a higher release of dissolved organic matter and substrate transfer to planktonic bacteria. Also abundances of heterotrophic nanoflagellates (HNF) were enhanced during this period. After the decline of the daphnid population the biomass of ciliates increased and led to a reduction in bacterial abundance and production. This negative effect was also demonstrated by a significant inverse correlation between BP and the biomass of ciliates during the period June to October 1991. Bacterioplankton growth rates varied between <0.1 and 0.78 d -1 with highest values during the spring bloom. As compared to bacterial abundance and production, growth rates did not continuously decrease with depth indicating that also at greater depth bacterioplankton 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 bacterial growth rate were different. Vertical profiles at all seasonal situations usually exhibited much closer correlations among bacterioplankton and phytoplankton parameters than seasonal dynamics.