Hydrological and biotic forces constrain brown trout (Salmo trutta) population dynamics, but tests of their role across numerous streams are uncommon. In 30 French stream reaches, using 5–8 samples (1 per year) each, we investigated whether the year-to-year seasonal hydrology influenced annual trout densities within reaches, and whether the relationships were shared by all reaches. We also searched for intraspecific interactions between and within cohorts. Trout data were age class (0+, 1+, and adults) densities. For each year, hydrology was described using 13 variables, each computed for a reproduction, emergence, and growth period related to the biological cycle of trout. We used analyses of covariance (ANCOVA) to test how trout densities at year n – 1 and hydrology at year n influenced trout densities at year n. High flows during emergence significantly reduced the 0+ densities, consistently across the 30 reaches. Then, 1+ and adult densities were linked, respectively, to 0+ and 1+ densities from the previous year. Analyses also revealed density-dependent survival mechanisms for the 0+ cohort, suggesting intracohort competition. Therefore, hydrology constrains trout dynamics only during the critical emergence period, after which intracohort interactions regulate the 0+ density. Such mechanisms, validated across 30 environmentally different reaches, seem to be fundamental to trout population dynamics.

Fifty years after the hyporheic zone was first defined (), there are still gaps in the knowledge regarding the role of biodiversity in hyporheic processes. First, some methodological questions remained unanswered regarding the interactions between biodiversity and physical processes, both for the study of habitat characteristics and interactions at different scales. Furthermore, many questions remain to be addressed to help inform our understanding of invertebrate community dynamics, especially regarding the trophic niches of organisms, the functional groups present within sediment, and their temporal changes. Understanding microbial community dynamics would require investigations about their relationship with the physical characteristics of the sediment, their diversity, their relationship with metabolic pathways, their interactions with invertebrates, and their response to environmental stress. Another fundamental research question is that of the importance of the hyporheic zone in the global metabolism of the river, which must be explored in relation to organic matter recycling, the effects of disturbances, and the degradation of contaminants. Finally, the application of this knowledge requires the development of methods for the estimation of hydrological exchanges, especially for the management of sediment clogging, the optimization of self-purification, and the integration of climate change in environmental policies. The development of descriptors of hyporheic zone health and of new metrology is also crucial to include specific targets in water policies for the long-term management of the system and a clear evaluation of restoration strategies.