title: Spatiotemporal Patterns and Dominant Controls of Riverine Nutrient Export Across Catchments
Creators: name: Ebeling, Pia
Publisher: name: University of Bayreuth
Year: 2023
Subjects: subject: water quality
subject: hydrology
subject: catchments
subject: large-sample studies
subject: major nutrients
subject: spatio-temporal patterns
subject: 500
subject: 550

description: Human activities have affected the quality of water resources globally, threatening both ecosystem and human health. One major threat arises from excess nutrients, largely stemming from agriculture and waste water, causing eutrophication in receiving surface water bodies. For effective water quality management, knowledge of catchment functioning in terms of water and solute dynamics and the interplay of natural and anthropogenic controls is crucial. Catchments are both the focus of management as well as integrators of various interacting hydrological and biogeochemical processes. While most studies focus on specific processes and individual catchments, overarching spatiotemporal patterns of nutrient export from streams and their prevailing control mechanisms still need to be better understood to tailor improved water quality management. The aim of this thesis is to increase the understanding of spatiotemporal patterns of riverine nutrient export at catchment scale and to identify their dominant controls and underlying processes across a wide range of catchments on the basis of a newly assembled data set. Study 1 targets at the spatial patterns and controls of mean riverine nitrate, phosphate and total organic carbon concentrations and export dynamics across 797 German catchments. Study 2 explores spatiotemporal patterns of joint riverine nitrate concentration and discharge seasonality and controls across 290 German and French catchments. Study 3 aims to enable future knowledge gains beyond this thesis by providing the novel large-sample data set of water quality combined with data on water quantity, meteorology, nutrient input and catchment attributes for 1386 German catchments. To compile the consistent large-sample data set, water quality and quantity data from the environmental authorities of the German federal states and France were processed along with various spatial data sets characterizing the catchments. Data-driven analyses were applied to determine catchment archetypes (i.e., to classify catchments) regarding their nutrient export dynamics and identify similarities and differences across catchments. More specifically, catchment responses were characterized from riverine nutrient concentrations and discharge and subsequently linked to catchment characteristics using multivariate methods. In Study 1, mean concentrations and concentration-discharge relationships were determined to classify the catchments. Nutrient source heterogeneity was parameterized from horizontal landscape patterns and vertical nitrate concentration profiles to evaluate their impact on nutrient export dynamics. In Study 2, long-term trajectories of nitrate concentration seasonality were classified, considering seasonally variable response times. This classification allowed to evaluate if concentrations during high-flow season respond faster to management changes as shallow nitrogen sources are affected first and predominantly activated during high flow. The large-sample analysis of nutrient export dynamics across Germany (Study 1) identified regional patterns and nutrient-specific ranges with about 70% of the catchments classified as the respective predominant class. Nitrate and total organic carbon concentrations mostly increased with increasing discharge, while phosphate was predominantly diluted. The variability of both mean nitrate concentrations and export dynamics across the catchments increased with the share of agriculture and was linked to the share of sedimentary aquifers within the catchments. The results suggest that subsurface denitrification can buffer high diffuse inputs, causing vertical concentration heterogeneity across different flow paths. In agricultural lowland catchments with deep sedimentary aquifers, predominant in northern Germany, this leads to low mean riverine nitrate concentrations and strong enrichment patterns. For phosphate, anthropogenic sources dominated, although cycling processes likely caused substantial variability, decoupling the instream concentrations from the catchment source configuration. Phosphate export dynamics were surprisingly strongly controlled by diffuse sources, suggesting that losses from legacy stores in agricultural soils are more dominant than expected and need to be better considered for sustainable phosphorus management. Riverine total organic carbon concentrations were dominated by natural sources from riparian wetlands and by hydrologic controls. Nitrate and discharge varied synchronously in the majority of the German and French catchments (84%) with seasonal maxima during winter (Study 2). The trajectories of nitrate seasonality on the contrary were more diverse, with a similar number of catchments with low-flow and high-flow concentrations responding first to changes in nutrient inputs. The lack of a consistent link between the different trajectories and controls suggests complex underlying processes. In synthesis, consistent spatial patterns of nitrate-discharge relationships and seasonality imply that nitrate export predominantly varies at a seasonal time scale. Although nitrate export dynamics can exhibit significant long-term trends, their spatial variability among catchments was larger and better explained by catchment characteristics, implying persistence of spatial patterns and thus of underlying processes. Overall, this thesis advanced the understanding of spatiotemporal patterns of nutrient export dynamics and their dominant controls across a wide range of different catchments and enables further research by providing data products. Knowledge on regional similarities and differences of catchment archetypes and controls can inform catchment models of water and solute transport, provide context for ungauged basins and future catchment trajectories and help to adapt water quality management.