Material and methods
Study areas
The lakes chosen in the study are located in Finland in
Fennoscandia (Fig. 1). The 20 studied medium-sized to
large, non-regulated and regulated lakes are geographically
distributed in the North Boreal, Middle Boreal and
South Boreal climate regions and the duration of the
growing season varies between 100 and 175 days. The
long-term July mean temperature varies approximately
between 14.2 and 16.7 1C, the annual precipitation
between 600 and 750 mm in a year and the duration of
the snow cover on open grounds between 130 and 200
days. A detailed geographical description of the lakes is
presented in Table 1.