Bacterioplankton production and biomass in two lakes of different trophic status : Responses to inorganic nutrients, organic carbon, and other planktonic biota

Sammanfattning: Bacterioplankton growth is affected by physical, chemical, and biological factors. Theavailability of energy and carbon sources, as well as inorganic nutrients, are of specialinterest when studying regulation of heterotrophic bacterioplankton in lakes.Bacterioplankton was studied in two non-humic lakes, oligotrophic Lake Njupfatet andeutrophic Lake Erken. In both lakes, experiments were conducted to test the effects ofinorganic nutrients, organic carbon, and metazoan zooplankton on heterotrophicbacterioplankton growth.Generally, bacterioplankton production was primarily limited by inorganic nutrients inboth lakes. However, there was often a close balance between limitation by inorganicnutrients and organic carbon. Organic carbon alone or in combination with phosphorusbecame limiting during early summer and during the autumn circulation period in LakeErken. In Lake Njupfatet, organic carbon was in short supply during late summer. In bothlakes, metazoan zooplankton stimulated bacterial production, suggesting that processeswithin the water column were important in providing bacteria with inorganic nutrients andorganic compounds. In Lake Njupfatet, bacterial production seemed tightly coupled to theactivity of other planktonic biota, such as grazing and phytoplankton growth. In LakeErken, no evident coupling was found between bacterioplankton and phytoplankton.Despite high production, bacterial biomass and abundance usually remained low in bathlakes, indicating that grazing pressure on bacteria was high. The effect of different forms ofinorganic nitrogen on bacterial growth was studied in Lake Erken. Heterotrophicbacterioplankton production responded more strongly to additions of ammonium thannitrate, suggesting that the inorganic form of nitrogen is important to take intoconsideration when studying regulation of bacterial growth.Altogether the studies show that even in lakes where the input of allochthonous organicmatter is low, as in Lake Njupfatet and Lake Erken, inorganic nutrients and not organic carbon is primarily limiting bacterioplankton growth. This indicates that bacterialproduction in freshwaters may generally be limited by inorganic nutrients, since manylakes have higher concentrations of organic carbon than those investigated in this thesis.