Lake Pyhäjärvi phytoplankton Phytoplankton refers to microscopic plants that, in high concentrations, may form so called algal blooms, which are notorious for decreasing water quality. Together with epiphytes and macrophytes, phytoplankton is one of the most productive primary producers in a lake's ecosystem. The concentration and composition of phytoplankton changes according to prevailing water conditions. Phytoplankton utilizes nutrients available in the water. The phosphorus-loading entering the water system increases the concentration of phytoplankton.
In Lake Pyhäjärvi there was a conspicuous increase in the concentration of diatoms and blue-green algae during the 90's. In contrast, a significant decrease has been seen during 2000. Diatoms also thrive in cold waters and they benefit from years when the ice-on period is shorter than normal. Diatoms increase water turbidity, but do not contain toxins or substances harmful to humans. Concentrations of blue-green algae blooms in Lake Pyhäjärvi have been very low. Nevertheless, it is advisable to avoid the use of water containing large amounts of blue-green algae for irrigation, sauna or washing purposes. The colonial blue-green algae Gloeotrichia echinulata has been by far the most abundant blue-green algae species for many summers.
What exactly is a colonial blue-green algae?
Gloeotrichia echinulata is an algal species that forms ball-shaped colonies. The diameter of these colonies is typically 1-2 mm. Because of their large size they are visible to the naked eye. Its life cycle consists of a planktonic growing phase followed by a dormant phase as a resting cell in the sediment at the bottom of the lake.
Figure: the structure of a Gloeotrichia echinulata colony
Resting spores withstand changing environmental conditions well. The spores may survive up to decades embedded in the sediment and act as a “seed bank” for new growth. In the bottom sediments colonies store up phosphorus from the sediment pore water, which they use for growth.
The first sighting of G. echinulata was made in July 1963. However, the species disappeared for three decades only to reappear in 1994. Typically G. echinulata is most abundant in July. According to latest research it does not contain liver toxins or neurotoxins. However, swimming during the densest blooms might cause skin rash. It is possible to prevent G. echinulata by restricting the lake's external and internal loading more stringently.
Lake Pyhäjärvi algae monitoring
The algae situation in Lake Pyhäjärvi is monitored weekly during the open water period. Monitoring points are situated throughout the lake. Monitoring is carried out by trained local surveyors and is coordinated by the Southwest Finland Regional Environment Centre which also gives out information on the current algal situation.