Role of macrophytes in a clay-turbid lake : Implication of different life forms on water quality

The littoral zone is a very important component taking part in forming the food web structure and the prevailing ecology of a lake. Especially in shallow lakes, continuous biotic and abiotic interactions in the littoral also reflect to the pelagic zone, having a bearing on the whole ecosystem. An important structuring force in the littoral is macrovegetation. The role of macrophytes is linked to their spatial distribution and biomass, which in turn is a result of various environmental factors (e.g. trophic status, light, substrate character, competition). Submerged macrophytes have been shown in various studies to be important in lake restorational aspects as they decrease sediment resuspension and remobilisation of nutrients, as well as provide refuge for zooplankton, resulting in decreased turbidity and increased water quality. Hitherto, investigations on the role of macrophytes have been mostly focused on submerged species as they most promptly respond to eutrophication through light limitation and provide profitable refuge for zooplankton due to structural complexity. But in lakes with inherently low submerged vegetation, other functional groups, e.g. emergent and floating-leaved species, may play an important role in determining the ecological status of a lake. In this thesis, the role of the emergent life form was studied in the two important functions of macrophytes: 1) refuge for zooplankton and 2) effect on resuspension. Also, importance of fish herbivory on the restricted submerged flora of a turbid lake was evaluated. This thesis shows that in clay-turbid eutrophicated lakes, emergent vegetation may play an important role in seasonal and diurnal regulation of zooplankton by providing refuge, especially for free swimming cladocerans. In addition, in eutrophic and turbid conditions, lacking submergent flora, emergent and also floating-leaved vegetation may harbour considerable densities of plant-attached cladocerans. These plankters in turn hold considerable phytoplankton filtering capacities, which may result in positive water quality effects through algal control. The results also underline the importance of emergent vegetation in regulating turbidity caused by resuspended sediment. The results demonstrate that during the growing season, sediment resuspension and consequently the increase in inorganic turbidity and internal P-loading may be substantially reduced by emergent, as well as submerged, vegetation. Due to a longer growing season, emergent vegetation may therefore have considerable water quality effects in a shallow lake. This study also revealed that in turbid and eutrophicated conditions with low submerged biomass and few dominating species, herbivory by fish may have an influence on the dominance proportions and species composition of the restricted submerged flora. To conclude, in turbid lakes, other functional forms of macrophytes, besides submergents, should not be overlooked. This thesis has highlighted the importance of emergent macrophytes in the ecosystem of clay-turbid lakes. In the future, more detailed studies on the floating-leaved species, often abundant in turbid circumstances, have to be conducted. In all, more thorough investigations of interactions and processes linked with macrophytes related to phytoplankton-caused and inorganic turbidity should be performed. The establishment of submerged macrophytes is traditionally considered essential in the restoration of eutrophicated lakes. This thesis underlines the importance of other life forms on water quality. In this respect, emergent and floating-leaved species may be a more profitable tool in restoration of lakes not only when removedbut also when retained. In general, the comprehensive function of macrophytes should be taken into more thorough account in lake restoration schemes: recreational expectations and water quality aspects should be weighed – and preferably combined.