Changes in the main habitats over time and the effects on fauna
The construction of the new dam and the rise of the lake water level by at least three metres compared with the earlier situation brought about various changes in the main habitats, as well as effects on the fauna and the flora of the area. In the last 30 years, several national and European research projects have taken place in the area with the participation of significant scientists on subjects regarding the flora and fauna of the area, water management, the aggradation of the lake as a result of sedimentary materials, etc., as well as on the effects incurred on ecosystems and organisms living in these habitats.
The reedbeds basically consist of common reeds (Phragmites australis), cattails (Typha sp.) and the species of Scirpus lacustris. In 1981 they covered an area of about 1,000 hectares at a height of 29 5 to 31 m a.s.L The annual flooding days were 200-310 (1980-1991) and the maximum depth, where they grew, was 3.2 m, while the average depth during the two months with the maximum water height in summer was 2.8 m. In 1984, two years after the function of the new dam and the new hydrological conditions created, the reeds disappeared. They remained only in small groups in various parts of the lake, especially at the mouth of the river and in the northeast part of the lake, which totally died out after a few years. Today there are small groups of reeds along the north shoreline of the lake and its southwest part (in the area of Koryfoudi). The reeds in Kerkini did not grow again because of the large fluctuation of the water level throughout the year and the grazing in the
In former hydrological conditions (before the construction of the new dam in 1982), there were a few white water lilies (Nymphaea alba) among the reeds on the northwest end of the lake, at a height of 30 4- 31.5 m. The wateriilies were benefited by the new
10 hectares in 2002, less than 0,5 hectares in 2003 and they finally died out.
This decrease is due to:
Other aquatic macrophytes had also a similar development such us Polygonum amphibium, which also occupied large areas in the river delta and the western part.
The riparian forest consists of white willows (Salix alba), almond willows (Salix triandra) and willow hybrids (Salix sp). Until the late 1980s there were also individual trees of white poplar (Populus alba), tamarisk (Tamarix sp.), ash (Fraxinus sp.) and extended rows of Amorpha fruticosa. The latter is a bush imported from South America in Europe that has expanded along the rivers.
Between 1980 and 1990 the forest area was restricted from 670 to 350 hectares. From 1991 up to 2000, the forest area was further reduced by around 50% and this decrease continued over the next years. Large openings were created and big parts of the forest died out. The first species that were affected was Amorpha fruticosa, which almost disappeared from the lake (only individual trees remained in higher spots) and the second largely affected was the willow. The flooding period in the forest varies throughout the year from 50 to 190 days according to altitude.
The situation became worse given the fact that the forest is not naturally regenerated. This is due to the flooding of the ground (at lower altitudes) on one hand and the pressure of grazing in the area because of the reduction of available grasslands (at higher altitudes) on the other hand.
The riparian forest lay at an altitude of 31.4-34.7 m a.s.l. before the new dam. These areas were flooded up to an altitude of 32.4m. The maximum flood duration was 99 days with a average water depth of 90cm for the two months of maximum flooding. In the new conditions and the first years of the function of the
new dam, the forest was restricted to an altitude of 32.5-34.7m. Over the years the area occupied by the forest has declined further as vveli the limits within which it spreads. In the late 90s, the riparian forest expanded at an altitude of about 33.0m up to an altitude of about 34.7m. The average duration of flooding until 1991 was 191 days with an average depth of 3.1m at the end of spring. After 1991 the average depth and the average flood duration were increased, thus resulting in the acceleration of the degradation rate of the forest and the creation of large openings, as well as in the reduction of its density because of drying lower branches, lower trees and bushes.
Before 1982, meadows covered areas located at an altitude of 2.9.5-35.5 m (areas between 29.5 and 32.7 m altitude were flooded). The new conditions limited these areas to a minimum altitude of 31.2 m. All areas were flooded in spring, thus being unavailable for foraging and nesting.
Before 1982 there were lots of islets in various parts of the lake that were not flooded in spring (even in the most southern parts of the lake near Lithotopos). In the new hydrological conditions, ail islets are flooded (most of them in March-April) and they are not available for bird nesting or resting.
Old bed of Strymonas
The river diversion and the encapsulation of the riverbed took place along with the function of the new dam. During these works, bird colonies in the old riverbed were destroyed twice. There were inaccessible islets and thickets that used to be a refuge for birds and mammals not only during breeding season but throughout the year.
The old bed of the river was the main spawning grounds for weld catfish and northern pike because of the suitable habitats of the area. After the river diversion and encapsulation, these areas went lost incurring direct effects on the populations of these species.