## 5 Discussion
### Role of the Flora of Buffer Zones in the Bioretention of Environmental Pollutants
Tree and shrub species are of particular importance to the functioning of buffer zones because they considerably reduce loads of biogenic substances. It is particularly evident in the transitional zones between arable land and water ecosystems. A buffer zone with a width of 16 m and an admixture of such flora, with a contribution from grasses, can even assimilate 94% N and 91% P from the environment [64]. It is worth emphasising that with age and biomass growth, tree species increase their bioassimilation capacity [72]. The effective functioning of buffer zones of the shore flora of a lake should, therefore, cover diverse flora and their habitat preferences and tolerance for the variable hydrological conditions that occur in catchments of water bodies [31]. Macrophytes of the littoral, which constitute a deeper zone of the biogeochemical barrier, also have a positive effect on the ecological state of water bodies. The most important benefits include competition for phytoplankton in absorbing biogas that are accumulated in plants for a considerably longer time (measured in years) than phytoplankton (measured in days), a haven for zooplankton and other invertebrates feeding on filtered seston, and feeding, hiding and reproduction grounds for invertebrates. Macrophytes are also involved in the intensive process of matter circulation and participate in the oxygenation and solidification of bottom sediments by developing a compact structure on the bottom. Macrophytes in larger clusters fulfil the role of a buffer that protects waters from runoff of harmful compounds, and some species produce metabolites, some of which have the ability to hinder the development of plankton [50; 73; 74; 75]. The amounts of biogas that plants are able to absorb and incorporate into their mass depend, in particular, on their species, rate of growth, range of ecological tolerance and general condition. Abiotic factors, such as temperature, concentration of particular ions, reaction (pH) and interactions between elements, are also important, as are biotic factors, such as competition, pathogens and herbivores [76]. In the littoral zone, the greatest amounts of nitrogen are incorporated into the tissues of _Glyceria maxima_, the greatest amounts of phosphorus and potassium are incorporated into the tissues of _Acorus calamus_, and the greatest amounts of sodium are incorporated into the tissues of _Typha latifolia_. The lowest amounts of nitrogen are incorporated into the tissues of _Schoenoplectus lacatstris_, and the lowest amounts of phosphorus, potassium and sodium are incorporated into the tissues of _Phragmites australis_[77]. The accumulation of elements in plants does not occur in a continuous way. The period of retention of accumulated elements in a plant depends on its phenological cycle and the impact of abiotic and biotic factors. Macrophyres have the ability to release different compounds, but in very small amounts. The highest amounts of biogenic elements are accumulated by macrophytes in the beginning of the vegetation period. At the end of the period and after its termination, plants return the main biogen load to the environment. The longest vegetation period in the climatic zone of Poland concerns _Typha latifolia_ and _Phragmites australis_, as well as _Glyceria maxima_ and _Schoenoplectus lacatstris_. The duration of the vegetation period can, however, be modified by climatic factors, and can have different courses in different years [76]. Macrophyres also have the ability to actively (selectively) absorb heavy metals. The most actively absorbed heavy metals are usually those that are necessary for the development of a given plant (e.g., copper and zinc). Other factors that determine their uptake include the form in which they occur in the environment (degree of oxygenation, type of chemical compound in which they occur) and whether they belong to a type of metal that shows synergistic (intensifying the uptake of other metals) or antagonistic properties (the hindering uptake of other metals).