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

Deep water warming observed in peri-alpine and alpine lakes may lead to less
upward mixing of nutrients, which accumulated in the hypolimnion during the
previous stratification period. In this way, climate variability influences both the
causes and symptoms of trophic changes in lakes (Straile et al., 2003).
Very little is known about the way in which year-to-year variations in the weather
influence the flux of nutrients from the shallow littoral and the bottom sediments of
shallow lakes. Day to day variations in the wind speed are known to influence the
transfer of nutrients from the bottom sediments and short-lived, thermally induced
currents may also increase the rate at which nutrients are transported from the littoral
zone into the open-water (George, 2000). Large internal phosphorus loading may
occur when wind-induced resuspension reaches deeper, anoxic sediments with high
P concentration in pore-water. In shallow Lake Võrtsjärv the daily release of SRP
(45.2*10 3 kg) during the storm exceeded the annual external phosphorus load to the
lake (Nõges and Kisand, 1999).
Salinity
Higher evaporation rates may increase lake water salinity levels in endorheic basins
of semi-arid and arid regions. Lower flows in rivers as they enter the Mediterranean,
coupled with sea level rise would accelerate saltwater intrusion into estuaries and
coastal aquifers (Haas, 2002).
Plyas in Spain are mainly small closed-basin saline lakes, which respond to changes
in precipitation/evaporation balance with changes in lake depth and salinity. These
changes are, in their turn, reflected in changes in biota, lithology and geochemistry.
IV.B.8. Dissolved Gas Regime
Most of the lakes in the boreal region are supersaturated with CO2 in relation to the
atmosphere. Here, the pCO2 is closely related to the DOC concentration in lakes,
which, in turn, is often regulated by the catchment characteristics (Sobek et al. 2003).
In a future climate precipitation will direct the discharge of DOC into the lakes and,
thereby, on pCO2. Most boreal lakes are, therefore, net sources of greenhouse gases
(GHG) such as CO2 and methane.
A study in Finland (Juutinen et al. 2001) analysed the methane fluxes in relation to
temperature and water level changes. Seasonal methane flux dynamics were found
to affect changes in lake water level, which directly responds to variations in
precipitation. Additionally, methane fluxes were highest during spring flood,
illustrating the strong climate feedback mechanisms in boreal lakes.
Even a productive lake such as Esthwaite Water in the English Lake District is a
source of CO 2 to the atmosphere over a year despite substantial under-saturation
during much of the summer (Talling, 1976; Maberly, 1996). High concentrations of
CO 2 in lakes is input of CO 2 -rich water from streams and rivers (Cole and Caraco,
2001) and the heterotrophic breakdown of terrestrial organic carbon within the lake
(del Giorgio and Peters, 1994).
The emission of GHG from reservoirs due to rotting vegetation and carbon inflows
from the catchment is a recently identified ecosystem impact (of climate) of storage
dams. Reservoirs interrupt the downstream flow of organic carbon, leading to
emissions of GHG that contribute to climate change (Dams and Development, 2000).
A first estimate suggests that the gross emissions from reservoirs may account for 1
to 28% of the global warming potential of GHG emissions (St. Louis et al., in press).
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