Responsiveness to surface–groundwater interactions by spawning salmonids: site selection patterns and control mechanisms in lotic ecosystems by Oncorhynchus, Salmo and Salvelinus genera

Surface–groundwater interactions play a significant role in the selection of spawning areas by salmonids, but the degree of reliance on their presence and quality is species- and habitat-specific. We surveyed the degree of influence of surface–groundwater interactions in spawning site selection by Oncorhynchus, Salmo and Salvelinus genera in riverine systems. The diversity of the sources is framed with respect to the spatiotemporal scale of occurrence, life-history traits, physicochemical quality, sympatry and intraspecific competition. Such variables are also intended when considering the main constraints of exploiting different surface–groundwater interaction regimes. Salvelinus and Oncorhynchus genera rely most on groundwater-dominated conditions linked to marginal habitats inside expansive floodplains. Salmo genus mostly prefers spawning in surface water-dominated conditions advecting through pool-riffle sections. Primary ecosystem services provided by surface–groundwater interactions are connected to improved intragravel water circulation, benign temperature regimes and adequate dissolved oxygen supply rates. Diversity in surface–groundwater interactions inside complex habitats may lead to different spawning strategies. Such a process generates a variety of life-history traits connected to run time and targeted spawning niches. The main constraints related to the surface–groundwater interactions are connected to the limitation in the space of groundwater-fed patches, which can lead to inter- and intraspecific competition phenomena. In addition, percolation through organic-rich layers can provoke oxygen deprivation, making groundwater-fed patches unsuitable for spawning. Finally, most of the reviewed studies evaluated the presence of surface and groundwater interactions on a local level without considering the hydrogeological processes at the watershed scale. Indeed, the usage of surface water-dominated areas in connection with geomorphic bed features on a local scale of investigation can be nested with groundwater seeps if the regional scale is considered. To this purpose, the importance of contextualizing the scale of investigation is emphasized. This will avoid apparent contradictions in the ubiquitous usage of surface or groundwater areas by the same species.