AME 18:133-144 (1999)  -  doi:10.3354/ame018133

Effects of inorganic nutrients and zooplankton on the growth of heterotrophic bacterioplankton--enclosure experiments in an oligotrophic clearwater lake

Katarina Vrede*

Department of Limnology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 20, S-752 36 Uppsala, Sweden

ABSTRACT: Bacterioplankton production and abundance were studied in the oligotrophic clearwater Lake Njupfatet (Sweden) during 2 consecutive years (1989 and 1990). The lake was slightly acidic in 1989 and was limed during the autumn-circulation period that year. In June of both years, experiments were carried out in 300 l bags immersed in the lake. Amounts of inorganic nutrients (PO4 and NO3) and metazoan zooplankton in the bags were manipulated. In both years, additions of inorganic nutrients stimulated phytoplankton growth, and bacterial abundance, production and cell-specific production. Since phytoplankton growth was also stimulated, it cannot be concluded whether bacterioplankton growth was limited by inorganic nutrients alone or in combination with organic carbon provided by phytoplankton. The increase in bacterial biomass was smaller than expected based on the production measurements, especially in fertilized bags, indicating that there was strong grazing pressure on bacteria. In 1989, bacterial cell-specific production was enhanced by adding metazoan zooplankton, probably because zooplankton released nutrients that bacteria could utilize. Furthermore, the results indicate that the species composition of phytoplankton can affect the growth of bacterioplankton.

KEY WORDS: Bacterioplankton · Inorganic nutrients · Phosphorus · Nitrogen · Zooplankton · Enclosure experiments · Liming

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