Monitoring the hyporheic zone: a global review and strategic directions for improvement

The hyporheic zone (HZ) is a crucial interface in riverbeds that links surface and subterranean waters, supporting a unique ecological community. While recent reviews have explored theoretical and ecological aspects of HZ, this study focuses on evaluating the advances in monitoring the quality and conservation status. We conducted a global systematic review, screening 16,135 articles and retaining a final data pool of 102 peer-reviewed studies published between 1989 and 2024. We address four main areas: (i) spatial and temporal distribution, (ii) monitoring activities, (iii) methodologies employed, and (iv) the impact of legislation. Despite advances in technical knowledge, HZ monitoring efforts remain quantitatively limited. Geographically, research is highly skewed, with Europe (51.0%), Asia (21.6%), and North America (19.6%) dominating the field, while other regions are severely underrepresented. Taxonomically, studies predominantly focus on the entire invertebrate community (35.5%) or microorganisms (30.3%), whereas efficient trait-based approaches remain scarce (6.6%). Methodologically, sampling is largely confined to shallow depths (up to - 50 cm). Furthermore, the enactment of environmental legislation showed a localized, rather than global, stimulus on monitoring outputs. This study emphasizes the urgent need for standardized methodologies, integrated surface-groundwater approaches, and targeted efforts to ensure effective water resource management. Finally, we propose a strategic roadmap with short-, medium-, and long-term actions to guide these improvements.