| Popis: |
Littoral zones with emergent vegetation are very narrow or even lacking in Dutch shallow lakes due to a combination of changed water level regime and unfavorable shore morphometry. These zones are important as a habitat for plants and animals, increasing species diversity. It has also been demonstrated that littoral zones can act as a nutrient sink to improve water quality. Therefore, it is important to understand how these zones can be restored or created and if they can be effective in this sense. The results presented in this thesis give more insight in the functioning of littoral zones at different water level regimes and in the quantative aspects of several nutrient retention processes. Furthermore, the research revealed that it is possible for artificial littoral zones to become comparable to more mature zones in nutrient cycling and nutrient retention within 10-15 years. Two important factors were identified as crucial in determining nutrient cycling in the littoral zone. Reed vegetation strongly affects nutrient storage during the growing season and was shown to decrease nutrient concentrations in the soil. Furthermore, in vegetated sites nutrient accumulation in the soil was higher compared to sites devoid of reed. The second steering factor in a littoral zone is water level (regime). Processes like reed growth, organic matter accumulation and denitrification were influenced by water level. Reed biomass was stimulated at high spring water levels, more organic matter was accumulated under flooded conditions and denitrification rate was positively correlated to water level. In formerly saline lakes, like lakes in the IJsselmeer area and Lake Volkerak, reduced plant growth is likely to occur when salinity reaches high levels after drawdown. In that situation reed biomass will mainly grow in the deeper parts of the littoral zone. From these results it is concluded that relatively high water levels will increase nutrient retention processes in the littoral zone. At a more natural water level regime with a large amplitude, (temporarily) flooded conditions will prevail in a more extensive area. At local scale the results imply a significant nutrient retention in the vegetated part of the littoral zone. However, a modelling study pointed out that in a lake with high nutrient loading, a relatively large marsh surface area is necessary to reduce nutrient concentrations effectively at the whole-lake scale. In lakes where nutrient concentrations are reduced to around critical values or in lakes where concentrations are already low, littoral zones are able to make the difference between a turbid and a clear lake. From this study I can conclude that a more natural water level regime is favorable for the expansion of helophyte zones and that these zones play a role in improving water quality in lakes were nutrient concentrations are not excessively high. At the same time, expanding these zones will contribute to the ecological status of the lake and will help to comply to the requirements of the EU Water Framework Directive. |
