Results for Lake Blankensee
Despite the brief examination period, a definite connection between the growth of algae, fish population and nutrient content in the water could be determined for the Blankensee.
As a result of the filtering activities of the common water flea (daphnia) in spring, Blankensee is very clear and has characteristic underwater vegetation. In the early summer, the lake becomes cloudier and is characterised in summer and autumn by algae. This transition from clear to cloudy water is the result of the great deal of nutrients available to the plant life in the lake and the numerous zooplankton-eating young fish, which prefer to eat daphnia. When the algae and the underwater vegetation die in the summer and autumn, oxygen is consumed in large quantities and sludge forms on the bottom. This results in considerable oxygen deficits at the lake bottom. In addition, the algae blooms cause a shift in the pH values towards alkaline conditions, which in turn has the effect of causing ammonia to occur in greater quantities. This is poisonous to fish and can thus play a role in causing them to die.
Light is prevented from penetrating to the bottom by the algae blooms, which means that only species of taller plants that are tolerant of cloudy conditions find suitable conditions.
The oxygen saturation shows high values during the summer corresponding to the highest primary production and a drop corresponding to the decrease in primary production. While the chlorophyll A content has almost reached its lowest point, relatively high concentrations of filtratable material are still present. This points to a high content of dead, organic, particulate matter, leading to increased oxygen consumption. This is also clearly shown by the oxygen concentrations measured.
A rehabilitation measure for the Blankensee that reduced the high nutrient load would be helpful. The aim is to reduce the density of algae. This must take place in conjunction with increasing the daphnia, which reduce algae by acting as filters. This will allow increased levels of visibility. As a result, the quantity of floating plants can be increased, which can absorb phosphates, but above all can grow on the bottom and not prevent the penetration of light. Great importance is attached to fish as eaters of daphnia. They should be managed above all in such a way that the large predators, such as pike and pike-perch, increase in number. The young European perch and whitefish, which occur in such numbers, and which prefer daphnia, should be reduced in number without delay.
Measures for the Blankensee aimed at improving the water quality would thus be coupled with a process of balancing various interests and usage claims. It should thereby not be forgotten that the lifespan of shallow lakes is limited, the ageing process increasingly accelerates and is increased in relatively densely populated areas by the influence of human beings. Measures can only slow down the ageing process, not reverse it.
The Interreg Project would not only enable constructive cooperation between environmental protection interest groups and the Störströmsamt, but would also result in an exchange of opinions and experiences between lake users, lakeside property owners and the municipal and state authorities. On the basis of the resulting networking and communication, it should be possible to implement long term measures for improving the water quality in a common effort.
