Abstract
Elevated nutrient inputs and reduced riverine concentration variability
challenge the health and functioning of aquatic ecosystems. To improve
riverine water quality management, it is necessary to understand the
underlying biogeochemical and physical processes and their interactions
at catchment scale. We hypothesize that spatial heterogeneity of
nutrient sources dominantly controls the variability of instream
concentrations among different catchments. Therefore, we investigated
controls of mean nitrate (NO), phosphate (PO), and total organic carbon
(TOC) concentrations and concentration-discharge (C-Q) relationships
from observations in 787 German catchments covering a wide range of
physiographic and anthropogenic settings. Using partial least square
regressions and random forests we linked water quality metrics to
catchment characteristics. We found archetypal C-Q patterns with
enrichment dominating NO and TOC, and dilution dominating PO export.
Across the catchments, we found a positive but heteroscedastic relation
between mean NO concentrations and agricultural land use. We argue that
denitrification, particularly pronounced in sedimentary aquifers,
buffers high inputs and causes a decline in concentration with depth
resulting in chemodynamic, strongly positive C-Q patterns. Consequently,
chemodynamic NO enrichment patterns could indicate effective subsurface
denitrification. Mean PO concentrations were related to point sources
though the low predictive power suggests effects of unaccounted
processes. In contrast, diffuse inputs better explained the spatial
differences in PO C-Q patterns. TOC levels were positively linked to the
abundance of riparian wetlands as well as negatively to NO
concentrations suggesting interacting processes. This study shows that
vertical concentration heterogeneity mainly drives nutrient export
dynamics, partially modified by interactions with other controls.