Introduction

The hydroecological conditions in populated river basins are affected by urban and agricultural land uses in complex ways, often resulting in hydro-morphological alterations, poor water quality, and deteriorated ecological status throughout entire river networks (1). Although correlations between land use and aquatic ecosystem health are generally known (2), there is still a poor understanding of the fundamental relationships between ecological water status and land use changes in terms of cause-effect linkages, their spatial dimensions, and the differentiation of overlying effects (3), even in the most densely monitored regions of the world, such as the EU (4, 5) and the US (6). However, the new generation of high-resolution data sets from multiple-scale environmental monitoring (5) provides new opportunities for synthesis, including disentangling the effects of multiple pressures and impacts on aquatic ecosystems (7-9).
Perhaps the globally most dense and most comprehensive environmental data set regarding river networks has been collected over the last two decades across Europe under the regime of the EU-Water Framework Directive (EU-WFD) (4). The ecological status is an assessment procedure based on biological indicators that compares the composition of communities at given sampling sites with reference conditions of low or undisturbed type-specific ecosystems (10, 11). Ecological status is determined for rivers, lakes, transitional waters, and coastal waters based on biological quality elements at multiple trophic levels (algae, macrophytes, benthic invertebrates, fish) and supported by physico-chemical and hydromorphological characteristics. The ecological status for surface water bodies (SWBs) is categorized in the EU-WFD regime as high , good , moderate , poor , orbad applying a ‘one out, all out’ principle, by the biological quality element which has received the worst rating (12).
The most recent assessment by the European Environment Agency {MathWorks, 2019 #17}shows at European scale only around 40% of SWBs in high or good ecological status (13). However, the spatial distribution of ecological status and pressures is not equally distributed, neither over Europe nor across nested catchments (5, 11). For example, the northern Scandinavian region and Scotland, as well as Estonia, Romania, Slovakia and several river basins in the Mediterranean region have a high proportion of water bodies in high orgood ecological status. In contrast, many of the central European regions have the highest proportion of water bodies that are inpoor or even bad ecological status (13). For example, only 8% of SWBs in Germany have good or better ecological status.
This situation is surprising because Germany has the highest wastewater processing rate in Europe (14), with more than 96% of wastewater from private households or public facilities routed to sewage treatment plants. Furthermore, in intensively-managed agricultural regions, which cover 55% of Germany´s land surface, the Common Agricultural Policy (CAP) and supporting agro-environmental measures (15) have been implemented in Germany for more than two decades (16). However, so far these measures have failed to negate adverse ecological impacts (5, 17). In particular, comparison of the first (2010) and second (2016) River Basin Management Plan following the EU-WFD implementation in 2000 revealed that the ecological status of surface waters in Germany has not improved in almost two decades (13). Here, we focus on narrow down our research questions to rivers, which constitute 92% of all SWBs in Germany (18).
What are the reasons for this striking discrepancy between large-scale point and diffuse source control efforts and the failure of streams and rivers to ecologically recover in response to these measures? Our hypothesis is that current river restoration strategies are not effective because they do not account for crucial characteristics of whole river networks, such as ecoregion-dependent susceptibility (8), spatial heterogeneity of both agricultural land use (19) and human populations (20), including the associated impacts of distributed wastewater treatment plants in river networks (21) and the corresponding carry-over effects (22), hydrological convolution of loadings from upstream to downstream (23), and how the temporal evolution of these processes may manifest as time lags of years to decades in receiving water responses (24). If specific relationships for these characteristics can be identified, management programs may be reconsidered to more explicitly account for a spatial prioritization of restoration measures.
In order to test this hypothesis, we evaluated the strength of relationships between ecological status in 6300 natural river water bodies (RWBs) in Germany and two primary pressures - wastewater treatment plants as point sources and agricultural land use as diffuse sources – in an explicit river network approach. We combined information delivered by EU-WFD regarding ecological status with highly resolved spatial information of pressures. The agricultural land use fraction (ALF) is used as proxy for pressures resulting from land cultivation and the intensity of agricultural practices, and the urban discharge fraction (UDF – defined as wastewater-discharge-ratio) is used as proxy for pressures resulting from urban land use. This information was combined based on the river network organization to account for hierarchical structures and connectivity of river segments. The river network structure is described by the Strahler order (ω) (25). We assessed relationships between pressures and ecological status with classifications based on stream order and also major ecoregions in Germany (Alps, elevation > 800 m, Central highlands, 200 m < elevation < 800 m, and central plains, elevation < 200 m). These ecoregions can be interpreted as a slope and run-off gradient extending from south (Alps) to north (central plains).
With the combined data set of proxies for agricultural (ALF) and urban pressures (UDF), and ecological status differentiated for ecoregions, we examined three research questions: 1) How do urban pressures impact ecological status within and between lower- and higher-order streams? 2) What is the corresponding relationship for agricultural pressures? 3) Are urban and agricultural pressures regulated by different hydro-ecological susceptibility among ecoregions?