Organizational Principles of Hyporheic Exchange Flow and Biogeochemical Cycling in River Networks Across Scales

Abstract

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface‐subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio‐temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local‐to‐regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies.

Microplastics in Freshwater Sediments Impact the Role of a Main Bioturbator in Ecosystem Functioning

“…The sediment–water interface is a hotspot of biological activity in streams and rivers, which play a fundamental role in ecosystem functioning through biogeochemical processes involved in carbon (C) and nitrogen (N) cycles. − Although these biogeochemical processes are driven mainly by microbial activities, ,, benthic invertebrates acting as bioturbators can significantly influence these microbial processes by sediment reworking, biogenic structure building (e.g., burrows), and bioirrigation (i.e., the action of benthic organisms flushing their burrows with overlying water). These processes can modify pore water chemistry, which will in turn have consequences on microbial communities and associated biogeochemical processes. − Chironomid larvae (Diptera, Chironomidae) and tubificid worms (Oligochaeta, Tubificidae) have been identified as important bioturbators in freshwater ecosystems. − Despite this recognized importance of bioturbators on organic matter processing and nutrient recycling at sediment-water interfaces of freshwater ecosystems, the consequences of microplastics exposure of sediments on these bioturbation-driven processes remain unexplored.…”

“…12,14,15 The sediment−water interface is a hotspot of biological activity in streams and rivers, which play a fundamental role in ecosystem functioning through biogeochemical processes involved in carbon (C) and nitrogen (N) cycles. 16−18 Although these biogeochemical processes are driven mainly by microbial activities, 17,19,20 benthic invertebrates acting as bioturbators can significantly influence these microbial processes 21 by sediment reworking, biogenic structure building (e.g., burrows), and bioirrigation (i.e., the action of benthic organisms flushing their burrows with overlying water). These processes can modify pore water chemistry, which will in turn have consequences on microbial communities and associated biogeochemical processes.…”

Combined Surface-Subsurface Stream Restoration Structures Can Optimize Hyporheic Attenuation of Stream Water Contaminants

“…Rigorously optimizing hyporheic structures requires the selection of an objective function or performance metric. There is widespread agreement that hyporheic fluxes and transit times must be considered jointly to predict contaminant attenuation outcomes. ,,,, However, several different metrics are commonly used to evaluate designs, each placing a different emphasis on flux versus transit time. First, the distribution of hyporheic transit times and Damköhler numbers (the product of transit time, t , and first-order reaction rate, k ) can both be flux-weighted to generate closely related metrics: flux-weighted transit time distributions (FW-TTD) and reaction significance factors (RSFs), respectively.…”

The importance of explicitly representing the streambed in watershed models

“…Even in the same river reach, streambeds can exhibit strong heterogeneity in measured K that varies several orders of magnitude (Datry et al, 2015). In addition to K , streambeds often vary in thickness and width, but the roles of those properties on exchange fluxes have received limited attention (Ghysels et al, 2018; Krause et al, 2022).…”