On the use of macrophytes to maintain functionality of overgrown lowland lakes

The presence of macrophytes is a key factor that determines good water quality in eutrophic lakes, and control of their presence is desirable to establish a clean water state. However, the subject is frequently discussed due to the intensive growth of aquatic plants that can restrict functioning of the reservoir (e.g., for swimming, fishing, etc.). Overgrowth of macrophytes also causes terrestrialisation processes of lakes, which occur very intensively in shallow and lowland artificial reservoirs. In order to analyse the contribution of macrophytes to nutrient reduction in lakes and investigate the possibility of using aquatic plants in reservoir recultivation, we investigated the nutrient budget of two artificial reservoirs located in Wielkopolska (Poland) with particular attention on macrophytes and nutrient pollution sources. To determine the loads with internal and external nutrients, we analysed the land use in the respective catchment areas, ground water quality, sewage management, quality and quantity of surface waters supplied to lakes (rivers and ditches), water, macrophytes and sediment quality in the reservoirs. Our results indicate that in both analysed catchments, the major source of nutrients is unregulated sewage systems. This demonstrates why improvement of sewage management is vital for increased water quality and more effective functioning of water reservoirs. We also propose methods to reduce the amount of nutrients directly supplied to lakes from agricultural lands. Our findings suggest that the mowing of macrophytes can reduce nutrient concentrations in the reservoirs. However, its impact is not particularly significant in comparison with the amount of nitrogen and phosphorus supplied from internal sources. Moreover, our study shows that growth limitation of particular species can stimulate growth of a desired species (e.g., charophytes). Controlled removal of macrophytes may also stimulate their intensive growth and possibly give rise to an increase in the amount of nutrients immobilised in the lake.