Phytoplankton responses to biomanipulated grazing pressure and nutrient additions enclosure studies in unlimed and limed Lake Njupfatet, central Sweden

Enclosure experiments aimed to assess the role of Chaoborus larvae in regulating lower trophic levels under natural and nutrient-enriched conditions were performed in situ in the dimictic, oligotrophic clearwater Lake Njupfatet, central Sweden. One experiment was performed before, and one after, whole-lake calcite treatment. In the acidic system, total planktonic biomass (TPB; the sum of phyto-, bacterio-, protozoo-, and metazooplankton), dominated by the cyanobacterium Merismopedia, was only weakly stimulated by the nutrients while the predator had no effect. In the limed environment, TPB increased strongly in response to the nutrients in combination with a negative response to the predator. Principally phytoplankton, but also bacterioplankton and protozooplankton (ciliates) were stimulated by the nutrients. It is argued that Merismopedia acted as a dead-end in the energy flow in the acidic system. Liming resulted in the disappearance of Merismopedia and this opened the flow of material to higher trophic levels. Since liming also led to a significant reduction in the concentrations of phosphorus in the lake water, it is proposed that a combination of liming and gentle fertilization might be used to restore the productivity of acidified lakes.