Results

River network hierarchical structure and ecological status
FIGURE 1
Less than 8% of the 137.000 km total length of German rivers is classified with ecological status better than moderate (Fig. 1A). The dominant status classes are moderate (41%) and poor(36%), followed by bad (15%). Lower-order streams (ω≤3) have the largest proportion of segments (7.5%) with good or better ecological status (Fig. 1B) with a total length of 6026 km. The fraction of lower-order streams (ω≤3) with ecological status high orgood is 8 times larger than for higher-order streams. All ranges of ecological status from high to bad can be found in lower order streams (ω≤3) whereas no segment in larger streams (ω>3) have a high ecological status (Fig. 1C). Based on median values (Fig. 1C), streams in orders ω=1 and ω=2 have a slightly better ecological status (moderate ) than higher order streams (at the boundary between moderate and poor ).
Agricultural land use, urban wastewater, and ecological status across stream orders
FIGURE 2 (8 panels)
Complex and diverse relationships between ecological status, agricultural land use fraction (ALF), and urban discharge fraction (UDF) are evident across stream orders and ecoregions (Fig. 2A through E). For the pooled data across Germany (Fig. 2A), there is a significant relationship between the median ecological status and agricultural land use fraction (p < 0.05, Table 1) that is well described with a quadratic function for lower-order streams (R2 = 0.96, p < 0.05) and with a linear function for higher orders (R2 = 0.97, P < 0.05). In contrast, a significant urban wastewater impacts could only be detected in low stream orders (ω≤3), which virtually disappeared in higher orders (Fig. 2E). In lower-order streams, the ecological status decreased linearly with increasing UDF (Table SI 1). In higher-order streams, there were no statistically significant differences between the 5 groups (UDF for ω > 3, Fig. 2E) such that for these streams, the ecological status is independent from UDF. The median UDF does not exceed 4% for lower order streams and is less than 0.5% for higher order streams (Fig. 2E and Table SI 3). Furthermore, looking at ALF, there is a statistically significant difference between high and goodversus moderate , poor , and bad data clusters. The median ALF ranges from 12% to 71% for lower-order streams, and from 47% to 58% for higher-order streams (Fig. 2A and SI Table 3). Based on median values, the ecological status for lower-order streams isgood or better when median ALF < 32%, and is worse than good for higher-order streams when median ALF > 53% (Fig. 2A).
Agricultural land use, urban wastewater, and ecological status across ecoregions
FIGURE 3(6 panels)
In the Alps, a very clear pattern was detected (Fig. 2B) where ecological status of good or better was observed only for ALF < 29% and < 36% in lower- and higher-order streams, respectively. In the Central plains the median ALF for lower- and higher-order streams was higher than in the Central highlands and Alps (Fig. 2 B, C, D) with extensive forests and less agriculture in the highlands compared to the lowland plains. For the Central plains the median ALF is > 60% in both lower and higher stream orders (Table SI 3). In the Central highlands (Fig. 2 C) ecological status was not significantly different in higher-order streams, but declined in lower-order streams with increasing median ALF except for highecological status.
For each ecoregion, example RWBs with good or better ecological status as well as corresponding cases with a status worse thanmoderate are presented in Fig. 3. These maps show the river reach, land cover, and, where present, the location of WWTPs in the catchment. In each of the example catchments with ecological statusgood or better, ALF was very small (< 10%) (Fig. 3A, C and E). On the opposite side of the distribution, contrasting examples with high ALF (> 90%) consistently showed a badecological status in all three ecoregions (Fig. 3 B, D and F)
The relationships between UDF and ecological status in all three ecoregion are very similar. In lower-order streams the ecological status consistently declined with increasing median UDF (Fig. 2 F, G and H). In higher-order streams, the median UDF is generally lower and the ecological status appears to be independent from UDF.
The 75% percentile of UDF for lower-order streams was larger in the central plains (Fig. 2H) compared to Alps and central highlands. The largest median values for UDF per ecological status group were found in the Central plains (Fig. 2H).