Växtplankton i Storvänern 2022

The seasonal average phytoplankton biovolumes were on a higher level than normal for the three sampling sites in Lake Vänern. As typical for the lake, the phytoplankton community was dominated by diatoms both in April and in May, although cyanobacteria and chryptophytes made comparatively large parts of the totals in May. In June and August the total biovolumes were higher than normal for all three sites, and especially at the Northernmost sampling site Tärnan the total biovolume was markedly higher than the long-term average 1979-2022. In June the species composition was dominated by chrysophytes, whereas in August the diversity was higher also including diatoms and several other phytoplankton groups. The ecological status is high according to the average total biovolumes in August 2020-2022. This is also true for the whole period from 1979, even though the total biovolumes have slowly increased over time. Assessments using the phytoplankton trophic index PTI indicates in general a slightly lower water quality than for the biovolumes and chlorophyll. This is most likely due to a large proportion of cyanobacteria in the phytoplankton community, a proportion that is slightly increasing over time, although the total amount of cyanobacteria is comparatively low.