Two decades of macrophyte expansion on the shores of a large shallow northern temperate lake—A retrospective series of satellite images

We made use of a 22-year (1985-2007) retrospective archive of moderate resolution Landsat TM and ETM+ satellite images to estimate the changes in cover of emergent macrophytes in the large shallow eutrophic Lake Võrtsjärv (270 km²) in Estonia. An original non-parametric image interpretation methodology was applied on late summer images. The combined GIS and statistical analysis of 217 coastal sections showed that the helophytic macrophyte belt, dominated by common reed (Phragmites australis), has rapidly widened during the last two decades, with an average expansion rate of 2.2 m per year. Statistical model revealed that the vicinity of large inflows had the strongest positive effect on the expansion of macrophytes, on average 1.6 times. In some sub-regions of the lake shore, we found the suppressing effect of the presence of small inflows on the change of helophytic belt width. This effect, however, was probably interconnected with the presence of human activity at the shoreline, which itself had statistically near-marginal suppressing effect on the widening of the reed belt.We showed that medium resolution satellite images can successfully be used for the retrospective monitoring of macrophyte vegetation in the littoral zone of large water bodies by applying very simple image classification methodology. As the lake coastal areas showed a tendency to become overgrown with reed and other macrophytes even in conditions of generally reduced agricultural intensity in the catchment area, we hypothesize that the clonal expansion of reed is probably triggered by the complex of drivers: large seasonal fluctuations in the water level create specific low water conditions in estuaries in combination with nutrients supply resolved from lake bottom or brought by rivers. Estuary areas are characterised by mineral sediments suitable for anchoring and protection-provision against destructive wave- and ice-action.Probably the most efficient biodiversity conservation policy to revealed macrophytic expansion is the reactivation of disrupted management activities along the coastline.