Is the unsaturated sediment a neglected habitat for riparian arthropods? Evidence from a large gravel-bed river

• Unsaturated river sediments below the surface and above the groundwater are unexplored.
• We developed and deployed novel tube traps to study subsurface arthropod assemblages.
• Abundant and diverse arthropods used the entire unsaturated zone down to 1.1 m depth.
• We hypothesize on possible functions of this zone for arthropod population dynamics.
• Unsaturated sediments are likely the most extensive albeit neglected habitat along braided rivers.

Despite exposed surface sediments of braided, gravel-bed rivers host a diverse and endangered arthropod fauna, the ecology of the unsaturated layers below the surface but above the groundwater is mostly unexplored. Even if only parts of this zone are accessible to arthropods, this could be the most extensive habitat along braided rivers with likely important functions for arthropods’ population dynamics. Across a 200 m-wide gravel bar at the Tagliamento River (Italy), we investigated the abundance, taxon richness, and composition of arthropods at varying sediment depth (0, 0.1, 0.6 and 1.1 m), distance from the channel (1, 5, 20, and 60–100 m), and time of the year (February–November). We used conventional pitfall traps and novel tube traps to sample surface and subsurface sediments comparably. Although abundance and diversity hotspots were located at the sediment surface at the edge of the gravel bar, the subsurface sediments supported an abundant arthropod fauna with similar richness to the sediment surface. We demonstrate that arthropods inhabit unsaturated sediments throughout the year, and speculate on the zone’s role as refugium and/or partial habitat. To ensure the future of this dynamic and diverse habitat we urge science, conservation, and management to include it in future programmes.