Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 50

thermocline in Lake Hälsjärvi was a change in the oxygen stratification during
the summers of 2005 and 2006. While the minimum oxygenated layer was only
1 m deep in 2004, mixing increased its depth to 3.5 m in the experimental years
2005 and 2006. The manipulation experiment also affected nutrient levels in
Lake Hälsjärvi, causing a statistically significant decline in total nitrogen and
ammonia (M. Forsius unpublished data, 2009). Changes in lake temperature,
oxygen distribution and nutrient levels in the lake also affected the phytoplankton
(L. Arvola unpublished data, 2009). The biomass of diatoms and non-flagellated
green algae increased as well as the rate of change of the phytoplankton
community, while the zooplankton and fish communities remained steady.
Another whole-lake manipulation experiment conducted in the deeper,
oligotrophic clear-water lake Breisjon in Norway increased the thermocline depth
(from 6 to 20 m) and the mean temperature at the time of maximum heat content
(from 10.7°C to 17.4°C) and delayed freezing by about 20 days (Lydersen et al .
2008). During the experimental manipulation, only minor changes in water
chemistry, nutrients and water transparency occurred, but planktonic chlorophytes
and diatoms decreased, mixotrophic dinoflagellates increased and periphyton
biomass increased. The manipulation did not affect zooplankton biodiversity and
did not affect fish populations (i.e. size, condition factor or population density of
perch and brown trout).
Water temperature in Swiss rivers and streams
Rivers and streams have also warmed during the past few decades, and stream
water temperatures are projected to increase further in a future warmer climate
(e.g. Stefan & Sinokrot 1993; Webb 1996). Many studies have used linear
regression models of stream temperature versus air temperature to explain the
variance in water temperature (cf. Mohensi & Stefan 1999 and references therein).
However, in addition to meteorological variables, the energy budget and thermal
capacity of rivers and streams may be heavily affected by human activities such as
the increased input of heated cooling waters from power plants.
Within Euro-limpacs, water temperatures in rivers and streams across
Switzerland, covering an altitudinal range of more than 4000 m, were studied by
Hari et al . (2006). The study showed parallel warming at all altitudes, reflecting
the changes in regional air temperature during the past 40 years (Fig. 3.10).
Regional coherence (i.e. spatial correlation between time series within a region)
was high for rivers and streams on the Swiss Plateau and in the foothills of the
Alps, but decreased with increasing altitude. In catchments containing glaciers or
hydroelectric power stations, the warming was strongly reduced, presumably
owing to the influence of inflowing melt water from glaciers and deep-water
from reservoirs.
The effects of warming on populations of cold-water fish, such as brown trout,
are expected to be deleterious at the warmer boundaries of their habitat and
positive at the cooler boundaries. On the Swiss Plateau, brown trout populations
inhabit the upper limit of their range of temperature tolerance. There, a potential
upward migration of fish due to increasing water temperatures is often impeded
by natural and artificial physical barriers, and a climatically driven upward habitat
shift increases the likelihood of a population decrease. Hari et al . (2006)