Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 46

Analyses of long-term data series demonstrate that such a change has already
occurred in recent decades. One of the first studies on increasing water
temperatures was on boreal soft water lakes in the Experimental Lakes Area of
north-western Ontario (Canada) by Schindler et al . (1990). The authors showed
that these lakes experienced an increase in water temperature of c. 2°C between
1969 and 1988 and that water renewal rates decreased as a result of higher-than-
normal evaporation and lower-than-average precipitation. At Lake Tahoe in the
south-west USA, volume-weighted lake temperature increased by about 0.15°C
per decade between 1970 and 2002 with a concomitant increase in lake thermal
stability (Coats et al . 2006). At Lake Baikal, the world's largest lake, with a
maximum depth of 1600 m, surface waters have warmed at a rate of 0.2°C per
decade over the past 60 years (Hampton et al . 2008). Lake Baikal was expected
to be rather resistant to climate change due to its enormous volume, but even
here, increasing water temperatures and a longer ice-free season are having major
implications for nutrient cycling and food-web structure. In Lake Constance,
a warm monomictic lake in Central Europe, the mean annual water temperature
has increased by 0.17°C per decade since the 1960s (Straile et al . 2003). This
warming is strongly related to increasing winter air temperatures and has affected
the duration and extent of winter lake mixing, the heat content of the lake and the
vertical distribution of oxygen and nutrients. Reduced winter cooling favours the
persistence of small temperature gradients and may result in an incomplete
mixing of the lake.
Fluctuations in lake surface water temperatures are transported downwards by
vertical mixing, and can reach the deep waters when the thermal stratification is
weak. In particular, the hypolimnetic temperatures of deep lakes, which are
determined by winter meteorological conditions and the amount of heat reaching
deep-water layers before the onset of thermal stratification may act as a 'climate
memory'. Increasing air temperatures may thus lead to a progressive rise in deep-
water temperatures, as found, for instance, by Ambrosetti & Barbanti (1999) for
lakes in Northern Italy.
Dokulil et al . (2006) reported a coherent warming in the hypolimnia of 12
deep lakes across Europe. Annual mean hypolimnetic temperatures increased by
about 0.1°C-0.2°C per decade during the past 20-50 years, despite differences
between lakes and years. Hypolimnetic temperatures in most lakes tended
to reflect fluctuations in the North Atlantic Oscillation (NAO) with the
winter-to-spring NAO index explaining 20%-60% of the inter-annual variability
in deep-water temperatures. In particular, winter-to-spring periods with a high
positive NAO index were associated with high hypolimnetic water temperatures,
when warmer-than-average surface water was transported downwards during
spring overturn. However, the strength and persistence of the climate signal with
time and depth are determined by the lakes' geographical location, landscape
topography, mixing conditions and lake morphometry.
The thermal regime of Lake Zurich
Lake Zurich, a 136-m deep peri-alpine lake in Switzerland, has one of the longest
temperature data series in Europe with water temperature profiles measured at
approximately monthly intervals since the 1940s (Livingstone 1993). This lake