Long-term Monitoring Shows that Climatic Variability and Riparian Management Control the Coupling of Hydrology and Nutrient Concentrations in a Mixed Land Use Lowland Catchment

Abstract:

Nutrient fluxes from fertilizer applications are a major source of freshwater pollution in many catchments world-wide and often show a strong coupling to hydrology. However, open questions still remain in hydrochemistry interpretation due to the structural complexity of catchments and external forcings such as climatic variability and water management. Here, we analysed hydrological and biogeochemical data over a 30-year period from the long-term experimental catchment Demnitzer Mill Creek (66 km²) in Northern Germany to explore the associated implications of long-term hydroclimate and hydrological variability on nutrient dynamics. The catchment underwent extensive artificial drainage (before 1990), wetland restoration (2000) and beaver re-colonization (after 2007) along the river network.

Our results indicated the inherent characteristics of individual solutes as a dominating factor of concentration-discharge (C-Q) modalities, as DOC and nutrients (NO₃-N and SRP) showed chemostatic and moderate chemodynamic behaviors throughout the research period. Stronger C-Q correlations and more chemodynamic behaviors were observed during lower discharge for all solutes, revealing the importance of hydrological conditions in determining the relative dominance of hydrologic and biogeochemical processes. According to event-based hydrograph separation, elevated stormflow was the major driver of nutrient export. Supply limitation was common for phosphorus while nitrate exhaustion only occurred during extreme storm events. We further saw the substantial impacts of wetland restoration on biogeochemical processes, which constrained the nutrient responsiveness and strengthened the overall C-Q correlations from extensive wetlands. The interaction between climatic variability and biogeochemical processes was also evidenced by C-Q correlations becoming weaker during wetter periods but stronger during drier and warmer periods. These long-term changes resulting from riparian rehabilitation and climatic variability provide invaluable insights into fundamental hydrochemical functioning and a potential evidence for future local management.