Plankton and water chemistry in Lake Njupfatet before and after liming

Abstract

Moderately acidified Lake Njupfatet was studied during 2 consecutive years, before and after liming (calcite), and compared with corresponding data from six reference lakes. After liming, the concentration of total phosphorus in the lake water decreased by some 30% as did the concentrations of particulate carbon, particulate nitrogen, particulate phosphorus, and phytoplankton biomass. Because of significant increases of inorganic nitrogen and dissolved organic carbon, the concentrations of total nitrogen and total organic carbon remained unchanged after liming. Before liming, there was a close balance between phosphorus and nitrogen limitation of phytoplankton growth, but we conclude that after liming the reduced concentrations of phosphorus induced phosphorus limitation of phytoplankton growth. Liming changed the phytoplankton community structure, most importantly causing the complete loss of the dominant species before liming, the cyanophyte Merismopedia tenuissima. The decrease in total biomass of phytoplankton was not compensated for by a corresponding increase in other species. After liming total biomasses of bacterioplankton and protozoan zooplankton did not change, while total biomass of metazoan zooplankton increased; hence, total plankton community carbon remained unchanged. Phytoplankton, protozoan, and metazoan zooplankton diversity (Shannon index) did not change after liming. Zooplankton biomass remained heavily dominated by calanoid copepods, typical of acidified lakes.