External Loading of Phosphorus in Deep, Stratified Lake Affected with Drainage Water

Abstract

The Water Framework Directive (2000/60/EC) requires from European countries to achieve environmental objectives. In relation to lakes, it is a good qualitative and quantitative status of all water bodies. Deep and stratified lakes resist anthropopressure for a long time, so they do not seem to require active protection. However, these types of ecosystems are also undergo a degradation processes. This is particularly true for lakes with tributaries - potential outbreaks of nutrients. Even small streams with visually “clean water” can carry loads of pollution exceeding the self-cleaning capacity of water bodies. Our paper presents the impact of anthropogenic watershed (agricultural and urban management) on the environmental conditions in small (deep (15.3m) stratified lake (Lake Święte in Obra, Wielkopolskie Lakeland, Poland). In the research, loads of biogenic pollutants feeding the reservoir from the main external sources were compared: drainage water supply (Pintus watercourse), atmospheric deposition, surface load from direct catchment, angling, and watering place. It has been shown that about 80% of the annual phosphorus load reaches the lake along with an inconspicuous watercourse (average flow 0.04 m3 s−1, phosphorus concentration 0.1 - 0.5 mgP l−1), to which drainage water from the surrounding area is discharged. The second major source of pollution is surface runoff. The total load of phosphorus contaminants was compared with the load discharged outside the ecosystem and the so-called critical level, calculated in accordance with the Vollenweider model (1976).The results of our research indicate that Świçte Lake is a trap for pollution transferred through a drainage and watercourses network. At the same time, the volume of external loading of phosphorus several times exceeds the critical level and indicates the scenario of further eutrophication of the lake. Unexpectedly for the local community and lake users, this pollution is primarily due to the inconspicuous mid-field stream. On the basis of the results obtained, a plan of protective measures was developed, the main element of which is the elimination of this source of pollution. The work also discusses the possible techniques of revitalizing the lake ecosystem.The research indicates the need to appreciate the role of small watercourses in the process of eutrophication of lakes and the role of early conservation measures taking place before the appearance of symptoms of ecosystem degradation, especially before the achievement by lakes of the turbid state - dominated by phytoplankton.