Climate Change and the European Water Dimension - Agri ...

VI.A.3. Limnology of Lake Erken
Lake Erken is a mesotrophic lake in eastern Sweden 10 km north of Norrtälje and 70
km northeast of Stockholm. Granites and gneisses dominate the bedrock of the area,
while the overlaying soils are rich in calcareous material. The lake is stratified during
summer and winter (dimictic) and has a relatively small inflow of humic matter from
the catchment. The mean Secchi depth is 4.5 m and the theoretical lake water
retention time 7.4 years. The catchment area (140 km 2 ) consists of 70% forest, 10%
agricultural land and 20% lakes. The lake is always ice-covered in winter. The ice
break-up occurs between March and early May Dominant phytoplankton species are:
Stephanodiscus hantschii var. pusillus, Asterionella formosa, Chrysochromulina
parva, Rhodomonas lacustris, and Gloeotrichia echinulata. The latter causes water
blooms in the lake in July and August (Pettersson et al., 1993).
VI.A.2. Data availability and investigations
The ice break-up has been registered since 1954. Also water samples have been
taken since 1954 with more intensive sampling both in the 1970s and from 1993
onwards. Thus, at least 20 years of data from Lake Erken are available. Climate
impacts on Erken have previously been analysed by Weyhenmeyer et al. (1999),
Blenckner et al. (2002) and Pettersson and Grust (2002). These analysis indicate
that this mesoeutrophic lake is a very good ecosystem to analyse climatic impacts on
lake processes, as the land use in the small catchment has not been change over
time. This unique study environment offers therefore the possibility to analyse direct
and indirect effects on climatic change, as the observed changes are not due to
changes in the catchment.
Observed changes
The studies so far performed document a one month earlier ice break-up during
the later period, compared with the 1960s, and a decrease of springtime snow
cover. These changes, in turn, can explain a shift in the timing of the spring
phytoplankton bloom, which now occurs one month earlier than in the 1960s and
1970s. Additionally, warmer winters seem to shift the phytoplankton species
composition towards diatom dominance. A longer ice-free period at present has led
to an increase in water temperature, especially in May, which may stimulate
bacterial activity (Goedkoop and Törnblom, 1996). The resulting enhanced
remineralization of nutrients combined with the longer ice-free period has enhanced
the availability of nutrients to algae. Indeed, a significantly higher summer
phytoplankton biomass has occurred during the late 1990s (Blenckner et al., 2002)
(Figure VI.A.7).
Chlorophyll a (µg/l)
20
16
12
8
4
0
1 3 5 7 9 11 13 15 17 19 21
Week of the year
160
70s
90s
Figure VI.A.7 The comparison of phytoplankton biomass (Chl a, lines) and the ice
cover periods (bars) between the 70s (blue) and 90s (brown) in Lake Erken.