When more soil becomes impermeable, less water seeps away, and more dirt and nutrients are washed into freshwater ecosystems. Especially near water bodies, soil sealing is therefore a key factor for water quality and freshwater organisms.
The team, led by Peter Haase of the SGN and Markus Venohr of the IGB, examined the spatio-temporal trends of urbanization using high-resolution impervious surface data from urban and non-urban areas in Germany and assessed the effects of urbanization on changes in runoff, nutrient emissions, and communities of invertebrates.
Sealing is not only an urban phenomenon
Using mathematical models, the researchers found that urbanization increased by 3.2 percent nationwide between 2006 and 2015. Non-urban areas contributed nearly 20 percent to the expansion of impervious surfaces. "The urbanization process of rural areas is not accounted for in previous calculations of water pollution. That is why consequences such as nutrient discharges are not sufficiently recorded," explained first author Hong Hanh Nguyen, a researcher at the SGN and IGB. In numbers, this means that 15 percent of the runoff, 10 percent of the total nitrogen and about 13 percent of the total phosphorus that end up in water bodies from sealed areas are not considered.
Invasive species increase, sensitive species decrease
These high emissions from sealed non-urban and urban areas are having a significant impact on the organisms living in them. In the study, the authors looked in particular at the populations of invertebrates. "Pollution-resistant and non-native species such as basket shells or killer shrimps are increasing, especially in large rivers. Sensitive species such as mayflies and stoneflies are decreasing," explained Peter Haase.
Use data on sealed surfaces instead of “urban land” of land use maps
"Our results show that the proportion of sealed areas is a much better parameter than the ‘urban land’ of land use maps, since urban land includes urban forest on the one hand, but ignores sealed rural areas on the other hand. In the future, this will enable to better record the effects of urbanization processes on freshwater ecosystems and to consider the urbanization effects in a more targeted manner in the long-term planning and management of freshwater ecosystems," concluded Markus Venohr.