Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 53

break-up to occur about 17 days earlier in the south of Sweden, i.e. south of
60°N, but only 4 days earlier in the northern part of the country.
An altitudinal gradient of ice phenology was studied in the Tatra Mountains of
Poland and Slovakia (Šporka et al . 2006). Mini-thermistors with data loggers
measured surface water temperatures in 19 morphologically different lakes, which
covered an elevation range of almost 600m (1580-2157m a.s.l.). For practical
reasons, freezing date was defined as the calendar date on which the measured
lake surface water temperature decreased to 0°C, and the date on which it
increased above 0°C again, represented melting date. Freeze-up dates spanned a
period of 52 days, but exhibited no detectable dependence on altitude. Break-up
dates, occurring between beginning of May and end of June, however, depended
strongly on altitude. The average gradient in the timing of melting was about
9 days per 100m and explained more than 60% of the variability. Ice-cover
duration varied from 136 to 232 days and exhibited a significant linear dependence
on altitude, at a rate of about 10 days per 100m. In contrast to ice break-up,
which depends strongly on altitude (as a proxy for air temperature), freezing
appears to be governed not only by air temperature, but to a considerable extent
by local factors like lake morphometry, exposure to radiation and wind, or inflows,
which influence lake surface water temperature in autumn and early winter.
Chemical impacts
Climate not only has an impact on the physical characteristics of surface waters,
but also is a master variable for ecologically important chemical processes. Here,
we discuss two examples of how climate may directly or indirectly affect surface
water chemistry, first with respect to changes in concentrations of dissolved
organic carbon (DOC) and secondly related to increases in the release of major
ions and heavy metals from active rock glaciers in high mountain regions.
Dissolved organic carbon in surface waters
DOC is an important constituent of many natural waters. It is generated by the
partial decomposition of organic matter and may be stored in soils for varying
lengths of time before transport to surface waters. The humic substances generated
by organic matter decomposition impart a characteristic brown colour to the
water due to the absorption of visible light by these compounds. DOC thus
influences light penetration into surface waters, as well as their acidity, nutrient
availability, metal transport and toxicity. During the past two decades, rising DOC
concentrations have been observed across much of the British Isles (Fig. 3.11),
large areas of Fennoscandia, parts of Central Europe, and northeastern North
America (e.g. Freeman et al . 2001; Evans et al . 2005, 2006; Vuorenmaa et al .
2006; Monteith et al . 2007). When first observed, these increases were widely
interpreted as evidence of climate-change impacts on terrestrial carbon stores due
to rising temperatures and the increasing frequency and severity of summer
droughts (e.g. Freeman et al . 2001; Hejzlar et al . 2003; Worrall et al . 2004).
Increasing precipitation could also lead to increasing DOC concentrations, first