Macrophytes, a key to understanding changes caused by eutrophication in shallow freshwater ecosystems

Eutrophication of shallow waters has often been associated with the decline of submerged macrophytes. In this study, a conceptual model as a working hypothesis for the mechanism responsible for this decline was used to generate the research topics. Aims of the study were to better understand the mechanism by testing parts of the hypothesis, and to compilate the results in a simulation model, a.o. for management purposes.The model hypothesis used suggests that periphyton development acted as a trigger for macrophyte decline, with a postponed phytoplankton reaction due to allelopathic growth limitation by substances excreted by the macrophytes.We selected one abundant macrophyte species, Potamogeton pectinatus L., and studied growth, photosynthesis and plasticity. Tuber size appeared to be an important factor for the plant that developed from it. Temperature and light influenced growth nonlinearly and interactively. With increasing age, maximal photosynthetic rate and respiration decreased. Light level during gro wth also affected photosynthesis. With respect to photosynthetic performance and stem elongation capacity, P. pectinatus can be characterized as 'average' when compared to other macrophyte species.Periphyton development on glass slides under eutrophic conditions in the laboratory was comparable to that in the field as found by other authors. Temperature distinctly affected the growth curves, whilst light did so for the attenuance-density curves of the experimental communities. In the studied light and temperature range, periphyton attenuated about 50% of the light within 3-4 weeks.All four tested freshwater snail species significantly removed periphyton from glass slides, whilst the two tested crustaceans did not. Differences in removal between and within species could be explained largely by differences in snail activity, snail size and taxonomic composition of the periphyton. Temperature effects were rarely found. From the plant's point of view, a population of Lymnaea peregra (Müll.) appears to be the best solution against periphyton accumulation. It has one of the highest removal rates and a constant, high activity and presence on macrophytes during the season.The existence of allelopathic effects of macrophyte exudates on phytoplankton growth was demonstrated clearly. When present, it was reflected in a growth limitation after one week of on average 10-15%. It could be different for different times of the season, different planktonic algae and different macrophyte species.In enclosures with different types of wall material (gauze and polythene), we found that polythene had the highest macrophyte biomass, the lake had the lowest and gauze was intermediate. This could be explained by improved light conditions in the polythene enclosures due to reduced seston and periphyton density, and by the absence of sediment-disturbing fish in both enclosure types. In another enclosure experiment, sticklebacks (Gasterosteus aculeatus L.) in field densities had a similar effect on zoo- and phytoplankton as the bream-dominated (Abramis brama L.) fish community in Lake Veluwe.The simulation model SAGA1 could reasonably well describe the seasonal vegetation development in Lake Veluwe under various light conditions. Simulations pointed out that in shallow water both periphyton shading and water turbidity are important in limiting plant development. In deeper waters the relative importance of periphyton is reduced. The present, seemingly stable situation in Lake Veluwe actually may well be very labile, since small deviations from the present conditions in the model resulted in large changes in simulated macrophyte biomass.For our conceptual model, we may conclude that the supposed role of allelopathy is now experimentally supported. Periphyton removal by freshwater snails, as well as the role of benthivorous and piscivorous fish, and possible effects of pesticides on periphyton grazers and zooplankton appear components worthwhile to incorporate. Finally, some illustrations are given of the use of the present study for the management of shallow lakes and submerged macrophyte beds.