Submerged macrophytes in shallow eutrophic lakes - regulating factors and ecosystem effects

Sammanfattning: In this thesis I have studied factors that regulate submerged macrophyte abundance, morphology and distribution in shallow eutrophic lakes, and how submerged macrophytes may affect the ecosystem. Light attenuation through the water column (turbidity and water depth) is probably not the most important factor regulating the distribution of submerged vegetation in macrophyte dominated shallow eutrophic lakes, e.g. due to clear water and the plants capability of morphological responses. However, during plant establishment and re-colonisation of vegetation (natural or induced shifts between turbid and clear-water states) turbidity and water depth are likely to be of greater importance. It was also found that periphyton may be a ?double-edged sword? in shallow lake ecosystems. Increased nutrient uptake by periphyton due to increased available surface area, when submerged macrophytes are abundant, is an important mechanism stabilising the lake in a clear-water state. However, periphyton affect submerged macrophytes negatively, and wave exposure dependent periphyton growth might initiate shifts from clear-water to turbid states due to the loss of submerged vegetation at sheltered sites. Grazing by crayfish affected seedlings but not mature plants, indicating that plants can outgrow grazers, but also that the time of grazing (in the life-history of the plant) is important for the ecological implications. Waterfowl grazing was higher at sheltered sites and may also contribute to initiate shifts from clear-water to turbid states. Submerged macrophyte seed bank is highly variable within and between lakes, and the importance of the seed bank for the vegetation dynamics seems to vary depending on lake history, wave exposure and the species involved. Submerged macrophyte re-colonisation is important for successful biomanipulation. Important factors for re-colonisation are: sufficient Secchi depth improvment, lake morphometry, species composition of the remnant populations, and low grazing pressure. Viable seed banks do not seem to be necessary but are likely to increase the probability of re-colonisation, and diversity.