Abstract
This resampling study in 338 semi-per- manent plots analyses changes in river macrophyte diversity in 70 water courses (small streams to medium-sized rivers) from four regions of the north- west German lowlands during the last six decades. The total macrophyte species pool decreased between the 1950s and 2010/2011 by 28% (from 51 to 37 species), mean plot-level species richness by 19% (from 4.7 to 3.8 species per releve ´) and the number of red-listed species by 40% (from 30 to 18 species). Species loss was associated with marked change in species traits: species with presumably higher mechanical stress tolerance (indicated by low specific leaf area and short leaf longevity) are more abundant today. Nearly, half of the species present in the 1950s had either disappeared or been replaced by other species in the recent releves. The dramatic impoverishment is likely a consequence of continued nutrient input that drove oligo- and mesotraphent species to extinction, and of restructuring and maintenance works in the water courses that reduced stagnant and undisturbed river habitats, where stress-intolerant species can persist. Efficient measures to reduce the nutrient load and to re-naturalise stream and river beds are urgently needed to halt and reverse the loss of macrophyte diversity.
Highlights
With less than 100 species, the hydrophytic macrophyte flora
With rapidly increasing amounts of fertilizers used in the agricultural landscape and manifold hydro-engineering measures completed, the biota of running waters have been exposed to an ever-increasing pressure in the last century with consequences for macrophyte diversity and community composition (Phillips et al, 1978; Robach et al, 1996; Smith et al, 1999; Egertson et al, 2004; Hilton et al, 2006; Kozlowski & Vallelian, 2009)
The diversity decline in the macrophyte vegetation of north-west German lowland water courses over six decades has to be attributed to a variety of causes
Summary
As an outcome of macrophyte monitoring, plenty of information exists on the ecological conditions of streams and rivers in many European regions (Pott, 1980; Wiegleb, 1981; Riis et al, 2000; Kuhar et al, 2007; Grinberga, 2011). This information is predominantly of qualitative nature and does not provide reliable information on long-term changes in the diversity and composition of the vegetation or alteration in environmental conditions that affect community composition. A limited number of studies on long-term change in river macrophyte communities covering several decades exists (Whitton & Dalpra, 1967; Holmes & Whitton, 1977; Herr et al, 1989; Mesters, 1995; Whitton et al, 1998; Riis & SandJensen, 2001; Schwieger, 2002; Schutz et al, 2008) and an over-regional picture of diversity loss has not yet emerged
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