No abstract

Submerged macrophytes play an important role in maintaining good water quality in shallow lakes. Yet extensive stands easily interfere with various services provided by these lakes, and harvesting is increasingly applied as a management measure. Because shallow lakes may possess alternative stable states over a wide range of environmental conditions, designing a successful mowing strategy is challenging, given the important role of macrophytes in stabilizing the clear water state. In this study, the integrated ecosystem model PCLake is used to explore the consequences of mowing, in terms of reducing nuisance and ecosystem stability, for a wide range of external nutrient loadings, mowing intensities and timings. Elodea is used as a model species. Additionally, we use PCLake to estimate how much phosphorus is removed with the harvested biomass, and evaluate the long-term effect of harvesting. Our model indicates that mowing can temporarily reduce nuisance caused by submerged plants in the first weeks after cutting, particularly when external nutrient loading is fairly low. The risk of instigating a regime shift can be tempered by mowing halfway the growing season when the resilience of the system is highest, as our model showed. Up to half of the phosphorus entering the system can potentially be removed along with the harvested biomass. As a result, prolonged mowing can prevent an oligo—to mesotrophic lake from becoming eutrophic to a certain extent, as our model shows that the critical nutrient loading, where the lake shifts to the turbid phytoplankton-dominated state, can be slightly increased.

Mass development of monospecific submerged macrophyte vegetation after the restoration of shallow lakes: roles of light, sediment nutrient levels, and propagule density.Aquatic Botany

Throughout the world, mass development of native and non-native submerged macrophytes leads to nuisance problems for humans. However, often neither the type of nuisance nor the characteristics of nuisance vegetation have been uniformly quantified, leaving nuisance vegetation as a largely unsubstantiated qualification. The lack of a consensus about when submerged plants cause nuisance hampers comparative research on the environmental conditions leading to nuisance. Furthermore, defining and evaluating management goals to reduce nuisance caused by submerged plants are not possible when characteristics of the nuisance vegetation remain unquantified. In this study, we performed a literature review and gathered stakeholder information to identify (1) which problems are caused by nuisance submerged macrophytes, (2) which plant characteristics underlie 'nuisance' and (3) and which species cause nuisance. We (4) synthesised this information into a framework to classify submerged vegetation as either 'nuisance' or 'non-nuisance' using a case study to illustrate the principles. We found that most nuisance problems that affect human use of the ecosystem can be grouped into problems for boat traffic, swimming, fishing and hydrological functioning of the system. Additionally, a multitude of ecological effects have also been reported, but these were outside of the scope of this study. Vegetation cover and canopy depth below the water surface are the main determinant characters for nuisance. Therefore, both invasive and native eutrophilic species with a vertical growth strategy are particularly problematic, but other species can also cause nuisance.

The quagga mussel (Dreissena rostriformis bugensis) and zebra mussel (Dreissena polymorpha) are invasive freshwater bivalves in Europe and North America. The distribution range of both Dreissena species is still expanding and both species cause major biofouling and ecological effects, in particular when they invade new areas. In order to assess the effect of temperature, salinity and light on the initial byssogenesis of both species, 24 h re-attachment experiments in standing water were conducted. At a water temperature of 25°C and a salinity of 0.2 psu, the rate of byssogenesis of D. polymorpha was significantly higher than that of D. rostriformis bugensis. In addition, byssal thread production by the latter levelled out between 15°C and 25°C. The rate of byssogenesis at temperatures<25°C was similar for both species. Neither species produced any byssal threads at salinities of 4 psu or higher. At a salinity of 1 psu and a water temperature of 15°C, D. polymorpha produced significantly more byssal threads than D. rostriformis bugensis. There was no significant effect of the length of illumination on the byssogenesis of either species. Overall, D. polymorpha produced slightly more byssal threads than D. rostriformis bugensis at almost all experimental conditions in 24 h re-attachment experiments, but both species had essentially similar initial re-attachment abilities. The data imply that D. rostriformis bugensis causes biofouling problems identical to those of D. polymorpha.