Discussion

Proxies for urban and agricultural pressures
The anthropogenic pressures from agricultural land use and urban settlements have been identified as significant stressors on the ecological status of watercourses (26). Agricultural land use is a proxy for various changes in habitat conditions in watercourses, in particular through runoff regulation, watercourse straightening, loss of bank and floodplain vegetation, increased nutrient inputs from fertilization, increased soil erosion, and pesticide inputs (27). Urban settlements are characterised by far-reaching changes in the water environment due to paving of land surfaces, alteration of flow paths and water balances, watercourse straightening, stormwater runoff, and discharges of municipal or industrial wastewater (27). Because of the inherent complexities in the differential mapping of all resulting impact factors, relationships, and hierarchies, robust proxies are needed.
Median relationships between urban and agricultural pressures and ecological status across stream orders
According to our results, urban wastewater discharge impacts on ecological status are significant only for lower-order streams (Fig. 2 E). This is consistent with higher relative flow contributions from WWTPs in low-order streams because of low local dilution (28), with high dilution from upstream tributary flow convolution in higher stream orders. This pattern is consistent for the aggregated total data set (Fig. 2 E) and the subsets differentiated by ecoregions (Fig. 2 F, G, H).
The median UDF thresholds for good ecological status differ significantly between the ecological regions studied (Alps 0.4 %, central highland ranges 0.9 % and central plains 3.2%; see Table SI 3). Alpine water bodies and their biota react most sensitively to wastewater discharges, in part because steep gradients induce comparatively short water residence times in hillslopes. Lowland waters in the central plains and their biota appear comparatively more robust and tolerate much higher wastewater percentages in a goodecological status. This higher resilience is presumably based on the higher buffering capacities of lowland waters, supported by naturally higher nutrient levels, higher temperature means and variances, and longer residence times with correspondingly stronger self-purification capacities. These relationships need further testing based on the general findings of this study.
The UDF interquartile ranges increase with declining ecological status (Fig. 2 E, F, G, H) for all river orders ω ≤ 3. Thus, increasing loads of wastewater are associated with more variable ecological responses of the water bodies such that other co-variables become increasingly important. It is noteworthy that in the central plains, water bodies with a high percentage of wastewater (UDF 75thpercentile = 12%; Fig. 2 H) can still sustain a good ecological status. This corresponds to a threshold value that is 10 times higher than that of waters in low mountain ranges and the Alps.
In contrast to UDF, increasing ALF impairs the ecological status for river reaches across all stream orders. With regards to median trends for all small streams (ω ≤ 3) in Germany, an ecological status not better than moderate can be expected if ALF exceeds one third of the catchment area. If ALF exceeds 56% of land cover, then ecological status of no better than moderate can be expected for all stream orders in all ecoregions examined (Fig. 2A, SI Table 3).
However, there are considerable differences between the ecoregions. In fact, the median ALF surrounding water bodies with goodecological status is lowest in Alpine regions (29% for ω ≤ 3; 36% for ω > 3), increases for the Central highlands (31% for ω ≤ 3; 47% for ω > 3), and is highest in the Central plains (61% for ω ≤ 3; 69% for ω > 3). The relative sensitivity of water bodies and their biota to stressors from agricultural land use is generally similar to that of wastewater-related impacts. The fundamental difference, however, is that this effect persists for the agricultural land-use fraction in the higher stream order sections (ω > 3).
A systematic directed increase in interquartile ranges between ecological status classes is not discernible for any of the ALF groups and we hypothesize that different sets of co-variables control the dependence of ecological status on agricultural land use.
It is evident that good ecological status can be maintained for all water body classes even with very high percentages of agricultural land use (Alps: 55 - 65%, Central highlands 45 - 62 %, and Central plains 82 - 84 %; see Table SI 3). These values are much higher compared to studies without differentiation of river systems and ecoregions (29).
Variability around median trends and diversity of spatial settings
FIGURE 4 (12 panels)
While significant trends were found for median values, there is high variability in the relationships between ecological status, agricultural land use and urban impacts (Fig. 2). This is illustrated and discussed by selected extreme cases with counter-intuitive combinations of land use and ecological status at comparable spatial scales (Fig. 4, SI Table 4). There were catchments in all three ecoregions with water body ecological status of good or better but also with very high proportions of agricultural land use (ALF > 90%, Fig. 4 A, B, C). In the examples described here, there are conspicuous features of the water bodies that support inferences about hydro-morphological status, the location of extensively used or natural areas, and the characteristics of neighbouring water bodies. The case study from the Alpine ecoregion is a water body whose headwater is located in forested or semi-natural areas (Fig. 4 A). The course of the water body is curved and meandering, which indicates a near-natural hydro-morphological status. In addition, there are numerous first order tributaries, some of which are located in natural areas such as wetlands. All these characteristics contribute to a corresponding resilience towards the otherwise dominant agricultural use. Similar conditions are shown by the case studies in the Central highlands (Fig. 4 B) and Central plains (Fig. 4 C). Both watercourses originate in forest areas or near-natural areas, show pronounced longitudinal profile developments, and, in the case of the Central Highlands, regularly follow near-natural areas in the immediate watercourse corridor.
By contrast, there were also catchments in all three ecoregions with water body ecological status of poor or bad but also with very low proportions of agricultural land use (ALF < 10%, Fig. 4D, E, F). The example case in the Alpine ecoregion represents a network of 1st and 2nd order headwaters upstream of a larger settlement in a closed forest area (Fig. 4 D). In mountain regions, watercourses above settlement areas with steep gradients are sources of danger from flooding and bed load transport. In Germany and other mountainous regions in Europe, running waters in those settings are typically developed for flood protection, with heavily modified hydromorphology that constrains connectivity and habitat for biota. The poor ecological status in the absence of agricultural land-use in this example is most likely due to these changes. In each of the two other cases, high proportions of urban areas with discharges from sewage treatment plants are found in the water body itself and in the neighbouring subcatchment areas (Fig. 4 E), or the catchment area is completely urbanised (Fig. 4 F). All water courses are comparatively elongated, which indicates a high degree of hydraulic engineering interventions, as a result of which the hydromorphological conditions and habitats have been degraded.
Similarly, we also found cases in all three ecoregions with goodor better ecological status but with high wastewater contents (UDF > 10% (Fig. 4 G, H, I). The case study from the alpine ecoregion is a water body into which an isolated settlement area discharges wastewater (Fig. 4 G). The course of the water body is highly curved and meandering, which indicates a near-natural hydro-morphological status, and the water corridor is forested over long stretches in the wastewater-polluted section of the river. The adjacent watercourse sections show similar spatial land use patterns and hydro-morphological characteristics. All these factors likely contribute to a corresponding resilience of the ecological status against the relatively high wastewater load. The land use configuration is even more pronounced in the case study for the Central Highlands (Fig. 4 H). Here, two wastewater treatment plants discharge wastewater, but the entire water corridor and the direct watercourse environment is formed by forest and near-natural areas. In addition, there is a first-order inflow from a sub-catchment area with neither agricultural nor urban land uses. The watercourse is curved and meandering, which indicates near-natural hydro-morphological conditions and potentially high habitat diversity. The case study from the lowland ecoregion (Fig. 4 I) is characterized by a single settlement area, but here, too, there are extensive areas above and below the wastewater discharge location that are either forested or near natural according to the land-use classification. The watercourse itself only touches the settlement area at the edge, is clearly curved and meandering, and is likely subject to little hydro-morphological changes with correspondingly high habitat diversity.
Finally, we also show cases for each of the three ecoregions in which the wastewater fraction is low (UDF < 1%) and yet the ecological status is poor or bad (Fig. 4 J, K, L). In each of these cases, the urban areas and the wastewater discharges are found in the headwaters, and agricultural land uses are predominant in the remainder of the catchment. The longitudinal courses of the water bodies are conspicuously elongated everywhere, indicating intense hydraulic engineering changes and likely degraded habitat conditions. In none of these case studies are there inflows from tributaries that are either slightly or not at all anthropogenically altered.
From the analysis of these extreme cases, it can be concluded that the spatial arrangement of anthropogenic stressors from agricultural land use and urban settlements in relation to natural system properties (minimally-impacted tributaries, connectivity, hydro-morphological settings) are important systematic factors that determine the extent of the ecological response to anthropogenic stressors.
Limitations of this study
Of course our study has inherent limitations with regard to the data basis and the derivation of proxies for the ecologically effective pressures from urban and agricultural land uses. Moreover, important determinants for ecological system properties of watercourses could not be mapped explicitly. This includes in particular the discharge regime with respect to magnitude, frequency, duration, and timing (30) or fragmentation (31). The correlation of ecological status versus UDF and ALF represent temporal averaging periods of one to six years (SI Table 2). The proxies for our study had to be derived from the routine monitoring carried out by environmental agencies, which is designed to record the state of the environment rather than to analyse causal relationships or understand the systemic relationships between environmental changes and ecological impacts. Inevitably, routine monitoring only covers a part of the essential variables. Alternative strategies have been proposed for the next generation of ecological monitoring systems (32). While each of these factors includes clear limitations for this study, the results indicate promising starting points for further work.
An important direction for future work is to differentiate the components from which ecological status is determined. This concerns the stressor-specific differentiation of the individual biotic indicators algae, macrophytes, macroinvertebrates and fish, the ”one out-all out” principle versus alternative determinations, such as max-min, average or median indicator values. A complementary attempt may be made to further differentiate the proxies for the pressures resulting from agriculture and urban settlements.
Further research may follow our approach and expand across wider natural and anthropogenic impact gradients. A next step could be the extension of the analysis to other European countries aiming for a comparison of ecological status relationships with ALF between highly industrialized countries and less developed countries.
Environmental implications
With these limitations in mind we suggest a reconsideration of receiving water-oriented catchment management with regard to agricultural and urban pressures and impacts. Ecological protection measures can be more effectively allocated when targeting context-specific pressure and impact relations in a river network perspective. Starting in the early 20th century, large scale urban drainage systems were implemented across Germany to tackle the worst water-related problems originating from urban emissions (33). However, our results show surprisingly clearly that the impact of urban emissions on the ecological status of small watercourses (ω≤3) is still severe. The pervasive and persistent effect of urban emissions on small streams is initially surprising because headwaters of river networks are predominantly located in rural, sparsely populated landscapes (Fang et al., 2018) where the amount of wastewater generated is correspondingly low. Ultimately, for this reason, low-tech wastewater treatment processes are more commonly used in rural areas and the permissible discharge limits according to the emission principle are less stringent than for large urban wastewater treatment plants (EU, 1992). The underlying pragmatic assumption has been that improved wastewater treatment is cost-effective to yield better receiving water quality, and that improvement of the ecological status can best be achieved by means of uniformly applied end-of-pipe measures in wastewater treatment and stormwater management. Against the background of our results, this may have been a costly misjudgment. And if investments continue to focus on larger wastewater treatment plants, as currently proposed to manage micropollutants (34), we will continue to miss the environmental targets for the vast majority of water bodies despite great expense.
Our approach and results help to address this problem, emphasizing the need to scale down efforts for protecting the ecological health of our receiving waters with regard to urban emissions, and the need to improve quantitative cause-effect relationships in the receiving water system for operational application. Highly developed societies today have reached a high efficiency with respect to physical-chemical purification of the large wastewater volumes in cities (35), however it is debatable whether we should extensively expand traditional treatment approaches to small streams. Suggested alternative approaches include more efficient source control (36, 37) combined with physico-chemical pollution abatement employing enhanced nature-based solutions (38), hydrologic management measures of stormwater runoff (39, 40), and morphologic restoration (41). Such integrated approaches would yield higher ecological quality throughout the receiving water network.
Policies for environmentally compatible agriculture and agronomic management also must be devised accordingly. To our surprise, we found good ecological status in water bodies where the predominant catchment land use is agricultural (median up to 60% in Central plains at stream orders ω≤3, in extreme cases even at agricultural land-use fractions larger than 90% in all three ecoregions). This is an indication that the relationship between agricultural land use and ecological status of water bodies depends on not just the proportion of land use but also the type and intensity of agricultural activities, as well as the spatial location and configuration in the river network, and the presence of additional pressures from urban areas. It is therefore a question of water-sensitive agriculture, which limits its unavoidable influences (e.g. discharge regulation and drainage, morphological changes, loss of bank and floodplain vegetation, nutrient inputs, soil erosion, pesticide inputs) to a compatible level for aquatic ecosystems locally and at catchment levels. The type and intensity of agricultural land use needs to be differentiated according to its location in the catchment area and, in particular, consistently and comprehensively protect low-order watercourses (ω≤3).