Littoral zone — GIS of Lake Constance: a useful tool in lake monitoring and autecological studies with submersed macrophytes

In 1993, the submersed vegetation of the whole littoral zone of Lake Constance was mapped by boat on the basis of aerial photographs. The survey was the third of its kind after 1967 and 1978, including a period of fast eutrophication in the 1960s and 1970s (mainly caused by increasing phosphate concentrations) and a subsequent period of decreasing trophy, due to the international efforts of the countries and states in the catchment area in eliminating phosphate from waste water. The change in distribution of most aquatic macrophytes shows a high relation to the changes in trophic status of the lake. The oligotraphentic species Chara contraria Kützing decreased in the 1960s and 1970s and increased in the 1980s and 1990s, the eutraphentic species Zannichellia palustris L., a very rare species before the 1970s, increased to one of the most dominant species until the late 1970s, but decreased in the 1980s and was rarely found in 1993. The use of GIS for data storage and analyses enabled to plot geographically referenced maps of single species distribution areas, to analyse changes of distribution area of species based on data from the whole littoral zone of the lake and to calculate summarising indices like a macrophyte index of nutrient load in the littoral zone. GIS-based analyses of whole lake above ground biomass and depth niches of selected species for the mappings of 1978 and 1993 show, that the difference in aboveground biomass of Potamogeton pectinatus L. is small, but distribution area and depth distribution are wider, whereas density of stands is lower in 1993. Potamogeton perfoliatus L. in 1993 has a higher aboveground biomass, a wider distribution area and depth distribution and a deeper depth optimum than in 1978