The Effect of Changes in the Thermal Stratification Time on the Eutrophication Processes in Lakes

In this study, the effect of changes in the thermal stratification time of lakes on the eutrophication processes was determined. The study included four lakes of Suwalsko-Augustowskie Lake District, i.e. Necko, Rospuda Augustowska, Biale Augustowskie i Studzieniczne, which are localized in north-eastern Poland. In investigation the model of lakes eutrophication, defined in the WASP (Water Analysis Simulation Program) and developed by the U.S. Environmental Protection Agency (U.S. EPA) was used. Model was developed for the one-year period from 1 May 2006 till 30 April 2007, on the second level of complexity of the eutrophication process in EUTRO module. This allows the prediction of the concentration of: ammonia nitrogen, nitrate (V) nitrogen, organic nitrogen as well as phosphate phosphorus (V), the organic phosphorus concentration, dissolved oxygen, carbon biochemical oxygen demand and chlorophyll a. Then, the model was used to simulate taking into account the changes of factors, which determine the quality of water and processes progress associated with eutrophication. This paper presents the results of prognoses taking into account the extention and shortening the time of thermal stratification in analyzed lakes by one month. Simulations started up at the beginning of the spring season. It was assumed that stratification occurs one month earlier or later in comparison to thermal conditions observed in 2006 and 2007. The simulation indicates that, in some cases, small changes in the time of stratification may result in noticeable change in the value of water quality indicators connected with eutrophication. In eutrophic lakes, ex end or shortening period in which the hypolimnion water are deoxygenated is directly related to the intensity of the internal nutrients loads. In this case, the extend the period by one month caused the increase of maximal forecasted annual chlorophyll a concentration (in the Biale Augustowskie Lake by 17%). The increase of concentration nitrogen and phosphorus components was also noted. In the lakes, in which the main nutrient source are inflows (for example Rospuda Augustowska and Necko) the length of period with the limnetic layers has a less impact on mentioned indicators.