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.
Plant fragments are commonly noticed in a wide range of freshwater environments. However, data on their further growth remain very scarce. The post-fragmentation growth of Elodea canadensis was analysed in a laboratory experiment in which plants were exposed to different light conditions ranging from 3 to 30 µmol m⁻² s⁻¹. The growth of whole plants (12cm) and fragmented (cut) shoots (apical fragment of 3 cm and middle and lower fragments of 4 and 5 cm respectively) was analysed over 33 days (with measurements of weight and length after 11, 21 and 33 days). In all light treatments both cut and whole plants grew. The growth rates were found to vary greatly over the exposure period. During the first 11 days, whole plants or the fragments thereof exhibited the greatest increases in biomass and length in all light treatments. Following further exposure under no shade and moderate shade, the growth of all plants, although still significant, was visibly more limited. Under conditions of a 90% shade level, 11 days of exposure left both whole and cut plants still alive, but incapable of any further significant increase in length or biomass. Generally, in high light levels cut plants grew more intensively, while in conditions of the most intensive (90%) shade, no differences in growth of these groups of plants were noted or the growth of cut plants was limited to a greater extent. A greater number of new lateral shoots were noted in cut plants than in whole plants. Even in conditions of low light characterized by the poor growth of plant fragments the production of new shoots was still possible. In general, fragments of Elodea canadensis were found to be very efficient at surviving and regenerating under a wide range of light conditions.