Lake Constance-Untersee Research Articles

Effects of mooring management on submerged vegetation, sediments and macro-invertebrates in Lake Constance, Germany

Mooring areas are a common form of berthing in many of Germany’s inland waters for the growing number of sporting boats. A circular swinging zone around the anchor zone is formed, in which the submerged macrophyte vegetation is destroyed to a large extent, and the sediment surface is eroded. We investigated the effects of two types of buoy (conventional and so-called hook-buoys) in comparison with undisturbed reference sites nearby. The aim of the study was to identify possible harmful consequences of mooring sites to lake littoral habitats in Lake Constance-Untersee, and to provide information to managers to aid them in the formulation of mooring management plans with the least ecological impact. The study focused on submerged vegetation, surface sediments and macro-invertebrate colonisation. In the swinging circle of conventional buoys (87 m2) we observed a significant sediment matter erosion of 0.9 tonnes and a reduction of organic matter amount by 4.5% of the undisturbed reference. The vegetation free area increased by 122%, and the phytomass (mainly Chara div. spp.) was reduced by 14.6% per berth. The psammophilous macro-invertebrate numbers were not significantly affected (−3%) in contrast to the phytophilous taxa which were reduced by 12.7% per berth. The mayfly larvae were the most negatively affected taxon. Oligochaetes profited from the clearing of the sediment surface in the swinging circles. The ecological effects of the hook-buoys were more minor, mainly due to the smaller swinging circle (6 m2). We concluded that the detrimental effects of mooring can be significantly reduced by mooring systems, e. g. the hook-buoy system as it was used in this study, which reduce the area disturbed and cleared by the anchor chain. We argue that these results can be generalised to mooring areas with soft bottom and dense macrophyte vegetation in Lake Constance and other large lakes.

Disturbance and rehabilitation of lakeside Phragmites reeds following an extreme flood in Lake Constance (Germany)

After an extreme flood in Lake Constance in 1999 the Phragmites australis belt showed a severe decline in area and vitality. A three year monitoring project was installed in 2000 to document the die-back and rehabilitation process immediately afterwards, to identify the co-factors of the damage, and to find out significant stressors that may impede the recovery of the reeds. The monitoring is based on CIR aerial photo interpretation, quantitative GIS analyses and field investigations on shoot density, stand structure and biomass production in 50 monitoring plots, grouped in five degrees of damage. In result we found that c. 0.306 km 2 of aquatic reed bed area died back at Lake Constance-Untersee (i.e. 23% of the former area in 1998). Among the stands which had survived the severely damaged stands were mainly composed of secondary shoots, whereas primary and insect infested shoots dominated in less damaged stands. The development from 2000 to 2001 was characterized by an overall decrease in shoot density, a change in the composition of the shoot population in favour of primary shoots, and in a recovery in culm stature. All variables depended on the degree of initial damage by the extreme flood. A conceptual model is proposed to assess the future development of Lake Constance reeds.

Auswirkungen des Extremhochwassers 1999 auf die Flächendynamik und Bestandsstruktur der Uferröhrichte des Bodensees

In consequence of the extreme flood at Lake Constance in 1999 the reed belts showed a severe loss in vitality so that a permanent damage could be expected. The monitoring project presented here focuses the documentation of the damage following the disturbance event, the identification of co-factors, and is designed to find out significant stressors that may impede the recovery of the reeds. The monitoring is based on CIR aerial photo interpretation, quantitative GIS-analyses and field data. The design of the study implies the definition of five degrees of damage using the interpretation of CIR aerial photo series from July 2000. The aquatic reed beds were digitised and areal sums were calculated. These data were compared with the corresponding data from 1999 and 1993. Stand structural and biomass attributes were investigated in 50 randomly distributed permanent plots covering all site types and damage degrees. The results showed a loss of about 24 hectares of aquatic reeds at Lake Constance Untersee alone. The stands which had survived showed a great patchiness in terms of culm density, shoot population composition, culm architecture, and aboveground biomass production. Severely damaged stands were mainly composed of secondary shoots, whereas primary and insect infested shoots dominated in less damaged stands. One co-factor which explained the patchiness was the vertical position of the stand, i.e. the probability of being severely damaged increased with increasing distance below mean water level. The monitoring programme will be continued till 2003.

Susceptibility of lakeside phragmites reeds to environmental stresses: Examples from Lake Constance-Untersee (SW-Germany)

The lakeside border of Phragmites reeds is under the control of many environmental factors, among which the flooding height and the degree of mechanical load by waves and washes are the most prominent. Examples from Lake Constance-Untersee show that the reed front retreats under enhanced stresses. Models are presented that describe the susceptibility of lakeside reeds to mechanical damage and to flooding events. A monitoring programme which ran from 1984/85 to 1992/93 showed a slow lakeward progression of the reeds in a decade of abnormally low water conditions.

Impact of winter reed harvesting and burning on the nutrient economy of reed beds

The fringing reeds (Phragmites australis (Cav.) Trin. ex Steud., Poaceae) at Lake Constance-Untersee were mown or burnt in winter thereby drastically reducing the addition of decomposable organic matter to the sediment. The purpose of this study was to test whether or not this management significantly decreased the contents of organic matter and nutrients in the surface sediment layer and if the oxygen conditions in the withinreed water body improved. Hypotheses were tested by monitoring 6 treated reed beds and 3 reference fields over a period of up to 4 years. The sediment/water system of reed beds was found to be well buffered against the removal of the current year's crop of dead Phragmites straw, because only slight, and mostly insignificant, differences between treated and untreated reeds were detected. Hence, the benefit of winter reed harvesting to reduce nutrient overloading of the reed-belts and the ‘die-back’ of reeds remains dubious.

Lake shore deterioration, reed management and bank restoration in some Central European lakes

The causes of lakeshore deterioration and reed decline are concisely summarised, and demonstrated with examples of seven Central European lakes (Germany: Havel lakes, Lake Constance-Untersee, Lake Constance-Obersee; Switzerland: Lake Zürich, Lake Biel, Lake Neuchâtel, Lake Geneva). The main causes are assumed to be bank erosion, lake eutrophication, mechanical damage to the reeds, and recreational activities. Countermeasures can be grouped into: reed protection against mechanical load (fences), wave dissipative constructions (refilling of substrate, brushwood fascine), nutrient export from the reeds (winter mowing), and supplementary measures (reed plantations, prohibition of access for the public). Favourable issues and undesired results are discussed. It is stated that there is a need for fundamental research and scientific surveillance of the restoration measures to avoid ill effects in the future.

Shoreline algal wash as a factor in reed decline in Lake Constance-Untersee

This paper assesses the chemical and mechanical impact of algal wash (Cladophora, Spirogyra, Chara) upon the lakeside reed belt (Phragmites australis) using field mapping methods, bioassays with Scenedesmus acutus in batch culture, and field experiments. Heavy mats of filamentous algae are correlated with a reduction in number of the outermost reed stalks. The water pressed from decaying heaps of Cladophora and Spirogyra reduced the growth rate of Scenedesmus significantly, but mats from Chara did not. It is assumed that the toxic substance is an organic compound. In field experiments the detrimental effect could not be clearly evidenced. The reasons for this are discussed. It is concluded that mechanical impact is of major importance.

Damage by episodic flooding to Phragmites reeds in a prealpine lake: proposal of a model.

The development of lakeside fringes of Phragmites reeds can be affected by high water levels, leading in extreme cases to die-back. A model is presented that allows estimation of the proportions of the damage. The model-formulated for 1 year and one Phragmites stalk-is based on the growth curve of the stalk, the altitude of the growth site, and the water level curve for the year. Applied to a reed stand over a series of years, the model is based upon the frequency distribution of the critical stalk length and the chance that the water level will be below a given value, each parameter being time dependent. Three case studies from Lake Constance-Untersee provide an example of the significance of episodic high water levels. Hence, aestival high water levels are expected to be one of the main factors controlling the lakeside frontline of the reedbelt. The model can be used in waterworks engineering, where reed plantations are to be established on the shores of lakes and reservoirs with strongly fluctuating water levels.

EM-algorithm as a tool for structure analysis of stands of the common reed ( Phragmites australis)

The frequency distributions of morphometric and mechanical properties of randomly sampled stalks of seven reed stands at Lake Constance-Untersee are analysed by the Estimation-maximisation Algorithm. The aim was to look for composed (bimodal) distribution indicating at least two different stem types. In each stand, between one and nine parameters are found to give a bimodal distribution. The two stem types are ‘flowering shoots’ and ‘non-flowering shoots’. The means of nearly all parameters investigated are different at the 1% level of significance. The flowerless shoots are identical with ‘secondary shoots’ and the flowering shoots refer to terminal (= ‘primary’) shoots. The separation of stem types allows investigation of the stand structure and the influence upon it by ‘rhizome-internal’ and ‘external’ factors in more detail.