The quest for understanding the organisational principles of hyporheic exchange flow and biogeochemical cycling across scales

Abstract:

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualisation of hyporheic exchange fluxes, residence time distributions and associated biogeochemical cycling in streambed sediments do not fully encompass the complexity of streams and rivers that are observed in the field. Specifically, the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in the streambed, and the feedbacks between ecological processes are not included presently in conceptual models. While simplification is justifiable and necessary for scaling, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions, limiting our understanding and management of hyporheic zones. This is particularly true at the landscape scale, where organisational principles that influence spatial patterns and temporal dynamics of the drivers and controls of hyporheic exchange flow and biogeochemical cycling along a catchment continuum are largely unknown.

We here synthesise the results of recent field experimental and modelling studies to identify the most important drivers and controls of hyporheic exchange flow and biogeochemical cycling. Our analysis is integrating results from a wide range of case studies that in their complexity challenge current paradigms and conceptual model frameworks. We discuss the interactions of local-to-regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning in the landscape context. Based on this, we develop a conceptual model of the overarching landscape organizational principles of hyporheic exchange flow and biogeochemical cycling from reach to catchment scale to direct future river research and watershed management.