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).