82
A. Choi´ nski and R. Skowron
by emergent vegetation also decreased by 0.27% (from 11 219.0 to 10 637.2 ha).
The surface area of lakes occupied by emergent vegetation averages 7.69%. In the
case of lakes with small (below 80 ha) and medium surface areas (80 ÷ 200 ha), the
degree of overgrowing was the highest, and equalled, respectively: 14.3 and 9.6%.
Fast rate of overgrowing of the lakes is related to the amount and quality of biogenic
substances migrating to lakes from the catchment area, the resistance of lakes to
degradation, the contribution of the littoral zone in the surface area of lakes, and age
of the lake.
A tangible effect of the changes in the lake basin was usually a shift of isobaths
from the shores to the middle of the lake, and development of new peninsulas, islands,
and in some cases the division of the original lake into several smaller ones [32]. An
example is Lake Jamno where in the period 1889–1960 a decrease in the surface area
by 6.5% and volume by 22.7% was determined [15]. Similar situations of changes
were observed in other regions of the Polish Lowland ([25, 33–36]).
Similar changes also occur in lakes in Lithuania, Latvia, Estonia, and Finland. In
Lake Luupuvesi (central Finland), the range of macrophytes increased from 96 ha in
1953 to 355 ha in 1996 [37]. Research conducted in shallow lakes: Engure (Latvia)
and Võrtsjärv (South Estonia) confirm an increase in the range of macrophytes in the
second half of the twentieth century [38].
The range of changes in water resources accumulated in lake basins, tarns, and
artificial reservoirs can be insubstantial, which is undoubtedly particularly related to
the process of decline of lakes. The parameter largely reflecting the predisposition
of particular lakes to decline is mean depth. The value of the parameter for lakes in
Poland is very variable, and in the Pomeranian Lake Districts it equals 6.84 m, in the
Masurian Lake District – 7.45 m, in the Wielkopolska-Kujawy Lake District – 5.70 m,
and in lakes south of the range of the last glaciation – 4.34 m [8]. Assuming that the
mean depth of Polish lakes is 7.02 m, and mean annual accumulation of sediments
is approximately 1 mm, the range of loss of water resources can be concluded.
Considering only the sedimentation process in lake basins, the loss of resources is
0.014% annually, which permits the estimation of the prospective age of lakes for
approximately 7 thousand years [8].
5.6 Conclusions
The total water resources comprised of lakes in Poland were estimated based on
the Catalogue of Polish Lakes (Choi´ nski 2006a) including information concerning
surface area for 7081 lakes (above 1 ha). The calculation procedure involved
summating known lake water volumes for more than 30% of lakes, and adding
the total water volume of unmeasured lakes. This was calculated as a product of their
surface area and mean depth equalling to 19.7349 km
3 . Water resources within particular lake districts are as follows: Masurian Lake District – 10.1183 km
3 (51.27%),
Pomeranian Lake District – 7.1292 km
3 (36.12%), Greater Poland-Kujavian Lake
A. Choi´ nski and R. Skowron
by emergent vegetation also decreased by 0.27% (from 11 219.0 to 10 637.2 ha).
The surface area of lakes occupied by emergent vegetation averages 7.69%. In the
case of lakes with small (below 80 ha) and medium surface areas (80 ÷ 200 ha), the
degree of overgrowing was the highest, and equalled, respectively: 14.3 and 9.6%.
Fast rate of overgrowing of the lakes is related to the amount and quality of biogenic
substances migrating to lakes from the catchment area, the resistance of lakes to
degradation, the contribution of the littoral zone in the surface area of lakes, and age
of the lake.
A tangible effect of the changes in the lake basin was usually a shift of isobaths
from the shores to the middle of the lake, and development of new peninsulas, islands,
and in some cases the division of the original lake into several smaller ones [32]. An
example is Lake Jamno where in the period 1889–1960 a decrease in the surface area
by 6.5% and volume by 22.7% was determined [15]. Similar situations of changes
were observed in other regions of the Polish Lowland ([25, 33–36]).
Similar changes also occur in lakes in Lithuania, Latvia, Estonia, and Finland. In
Lake Luupuvesi (central Finland), the range of macrophytes increased from 96 ha in
1953 to 355 ha in 1996 [37]. Research conducted in shallow lakes: Engure (Latvia)
and Võrtsjärv (South Estonia) confirm an increase in the range of macrophytes in the
second half of the twentieth century [38].
The range of changes in water resources accumulated in lake basins, tarns, and
artificial reservoirs can be insubstantial, which is undoubtedly particularly related to
the process of decline of lakes. The parameter largely reflecting the predisposition
of particular lakes to decline is mean depth. The value of the parameter for lakes in
Poland is very variable, and in the Pomeranian Lake Districts it equals 6.84 m, in the
Masurian Lake District – 7.45 m, in the Wielkopolska-Kujawy Lake District – 5.70 m,
and in lakes south of the range of the last glaciation – 4.34 m [8]. Assuming that the
mean depth of Polish lakes is 7.02 m, and mean annual accumulation of sediments
is approximately 1 mm, the range of loss of water resources can be concluded.
Considering only the sedimentation process in lake basins, the loss of resources is
0.014% annually, which permits the estimation of the prospective age of lakes for
approximately 7 thousand years [8].
5.6 Conclusions
The total water resources comprised of lakes in Poland were estimated based on
the Catalogue of Polish Lakes (Choi´ nski 2006a) including information concerning
surface area for 7081 lakes (above 1 ha). The calculation procedure involved
summating known lake water volumes for more than 30% of lakes, and adding
the total water volume of unmeasured lakes. This was calculated as a product of their
surface area and mean depth equalling to 19.7349 km
3 . Water resources within particular lake districts are as follows: Masurian Lake District – 10.1183 km
3 (51.27%),
Pomeranian Lake District – 7.1292 km
3 (36.12%), Greater Poland-Kujavian Lake