Plant and zooplankton in six lakes in Stockholm County, 2021

The 2021 survey covered a total of six lakes in Stockholm County. Of these, one lake was given high status (Stora Skogssjön), three good statuses (Garnsviken, Orlången and Turingen) and two unsatisfactory status (Norrviken and Vällingen) with regard to phytoplankton. Five of the lakes could also be assessed for the three-year period 2019-2021, which gave good status for Garnsviken, moderate status for Orlången, Turingen and Vällingen, and unsatisfactory status for Norrviken. Orlången and Norrviken have been treated with aluminium, in order to trap phosphorus and thus improve water quality, in 2019 and 2020. In Orlången, both phosphorus concentrations and phytoplankton biomass have fallen sharply and the phytoplankton status is now good, unlike unsatisfactory or poor before treatment. In Norrviken, the sharply reduced phosphorus concentrations have not yet been reflected in improved phytoplankton status (unsatisfactory) but biomass has decreased after treatment and the proportion of cyanobacteria has decreased. Four of the lakes surveyed in 2021 also took zooplankton samples. The zooplankton community in all lakes was dominated by small wheeled animals followed by nauplia from cyclopoida jump crayfish. Indicator species and the composition of the zooplankton community indicate eutrophy in Garnsviken and Norrviken. Some eutrophication can be interpreted even in Turingen, while Orlången’s zooplankton community indicates balanced nutritional conditions.