Are Ullfjärdarna hotspots for dispersal of Planktothrix to the rest of Mälaren? (Master)

Last changed: 10 June 2021

Lilla Ullfjärden (part of Lake Mälaren) was a former drinking water source for the city of Bålsta. After having problems with toxic cyanobacteria that likes cold conditions, they had to stop taking drinking water from the lake.

This cyanobacterium is a red form of the genus Planktothrix that blooms during under the ice in late winter and then stays in colder deep parts of the lake during summer, depth that are convenient as a drinking water source.

The question is, how prevalent is Planktothrix in the rest of Mälaren? Is there a dispersal from Lilla to Stora Ullfjärden and further to the basin of Ekoln?

At regular sampling, it risks being undetected, since phytoplankton is normally only sampled at lower depth and only during summer. Late winter or early spring sampling of Planktothrix just below the ice or summer sampling in the metalimnion during summer is needed to obtain a data for this project, where existing environmental monitoring data can also be evaluated on the presence of this cyanobacterium.


  • Main subject area: Biology or Environmental Science
  • Level: Advanced (Master)
  • Length: 30 credits/20 weeks