## 1 Introduction  
Allochtonic matter is supplied from a catchment area together with precipitation and surface or subsurface runoff through hydraulic connections, enriching a lake in organic substances [1; 2; 3; 4]. The system of the lake itself develops a mutually regulating mechanism based on close relationships between the biocoenosis and its internal interactions and the biotope, which represents elements of the nonliving environment [2; 5].  
The most important element of limnological status classification is the trophic status of lakes, which describes their abundance in biogenic substances [6]. The level of fertility of a lake is particularly determined by its hydrology and morphometry. The amounts of contaminants supplied to the lake are especially determined by the surface area of the catchment, natural environmental features and land use [7; 8; 9; 10]. Agricultural areas subject to chemisation and mechanisation of production in reference to the typology of soils and hydrological interactions constitute the basic source of biogenic substancesfor water bodies [11; 12; 13]. Moreover, intensive, industrialised agriculture also changes the landscape structure (fragmentation of natural habitats) and chemism of the environment (use of fertilisers and plant protection products). It is mentioned as the primary factor in global assessments of the causes of impoverishment of the environment [14; 15].  
The development of tourism and stationary and unorganised recreation is largely related to both the attractiveness of a given place, particularly determined by environmental and landscape values, and the level of development of tourist and recreation infrastructure. It should be emphasised, however, that a supply of even small amounts of biogenic substances affects water quality and can destabilise the basic components of the lake biocoenosis, i.e., the shore vegetation, macrophytes of the littoral or water invertebrates [16; 17; 18; 19; 10]. Therefore, tourism remains in a close relationship with the natural environment and causes its unfavourable transformations [20; 21].  
The load of biogenic substances is also indirectly determined by modifications regulating water relations in a catchment area. Melioration measures in the form of regulating river channels, digging melioration ditches and incorporation of lakes in, and exclusion of lakes from, the outflow network result in profound changes in the balance of biogenic substances supplied to water bodies [22].  
Shrubs, shore vegetation and lake macrophytes constitute a complex system. It is the last barrier capable of capturing biogenic substances of various origin generated from different land uses in the lake catchment, and is described as the phytogeochemical barrier. The majority of papers discussing the issue of biogeochemical barriers focus on the role of waterholes or field tree stands in the development of the balance of biogenic substances in the agricultural landscape [23; 24; 25; 26; 27; 15], and to a somewhat lower degree typical water ecosystems [28; 29; 30; 31]. Nonetheless, the botanical state of the flora adjacent to the shore and littoral vegetation communities, biocoenotic transformations and anthropogenic conditions in riparian buffer zones can provide preliminary information on environmental pollutants entering water ecosystems. Such data, combined with the analysis of biological indicators of water eutrophication, provide a more comprehensive insight into the functioning of the land-water system without the need for costly analyses of physical-chemical properties of ground and lake waters.  
For the mesotrophic Lake Piaseczno, the objective was to analyse the potential of plant communities along the shore and littoral zone (macrophyytes) to function as a phytogeochemical barrier in reference to the biogenic loads generated from different land uses in the lake catchment and its productivity measured by biological water indicators.