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?