PHYSICOCHEMICAL ENVIRONMENT, PHYTOPLANKTON BIOMASS AND PRODUCTION IN OLIGOTROPHIC, SOFTWATER LAKE KALGAARD, DENMARK

Water chemistry of Lake Kalgaard in 1976-77 was characterized by low concentrations of total-CO2 and inorganic nutrients. The ionic composition resembled that of precipitation (Na>Ca>Mg >K and Cl>SO4>HCO3). The seasonal pattern of total-CO2 and PO4 was regulated by internal processes and maximum concentrations as a result of decomposition processes occurred during summer stagnation. NO3 concentrations showed the opposite pattern and were relatively high from late autumn through spring and were extremely low during summer. Total-P and PO4 increased during summer due to release from the sediment. The phytoplankton biomass of surface water was low. The water chemistry suggested a shift from N-limitation of phytoplankton during summer to P-limitation at other seasons. Maximum algal concentrations occurred at 6 m during summer, probably due to a supply of nutrients (especially NH4) from deeper layers. Phytoplankton productivity was often bimodal, with an upper maximum at depths of 0 or 2 m and a second maximum at 6 m. © 1979 Dr. W. Junk b.v. Publishers.