Macrophytes typically form an important biotic component of shallow lakes. They considerably influence physico-chemical environment, trophic structure and nutrient cycling. Increasing lake trophy is manifested by significant changes in macrophyte cover, biomass and species composition with a very abundant vegetation and high species richness at meso-eutrophic stage of eutrophication. During the further eutrophication of lakes there is usually a decline of macrophytes accompanied by an increase of biomass of phytoplankton and loosely aggregated metaphytonic filamentous algae. Much of recent discussion on eutrophication of shallow lakes is directed at the relationship between submerged macrophytes and phytoplankton. In the light ofthe concept of alternative stable states, at low nutrient concentrations only the clear water state dominated by macrophytes will be stable, and at high nutrient concentrations - only the turbid state with phytoplankton domination. At a wide range of intermediate values of nutrient level the clear and turbid states may exist as alternatives (Fig. 1). Both states can be maintained by a number of buffering mechanisms which tend to preserve them against changing from one to another. In the clear water state macrophytes reduce phytoplankton biomass through shading, reduction of nutrient availability and releasing of suppressant substances. Slowing down of water movement in dense macrophyte beds results in increased sedimentation rate. Macrophytes also provide a refuge for pelagic zooplankton against planktivorous fish resulting in an increased zooplankton grazing on phytoplankton. In the turbid state, on the contrary, the growth of macrophytes is prevented primarily by a low light intensity resulting from a high phytoplankton biomass and resuspension of sediments unprotected by rooted plants. Shallow lakes may shift between the state of dominance by submerged macrophtytes and the dominance of phytoplankton. Data from a number of lakes show that the switch may be triggered by changes in nutrient loading above the critical level, disturbances from extreme meteorological conditions or various management practices. In many cases it is not quite clear what causes the changes and the multiple effect from both abiotic and biotic control factors are suggested. As submerged macrophytes are able to stabilize the clear water state, their re-establishment is essential for an effective restoration of eutrophic turbid shallow lakes. The recolonization rate of plants differs greatly among lakes. It can be hampered or delayed due to limited bank of propagules, sediment resuspension, heavy growth of filamentous algae or herbivore grazing. Thus, in order to accelerate macrophyte recovery, additional measures may be needed. They may involve planting new plants or protection of germinating ones against unfavorable demands.
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.