Invertebrates and sestonic matter in an advancing wetted front travelling down a dry river bed (Albarine, France)

Temporary rivers are shifting mosaics of aquatic and terrestrial habitat driven by hydrologic variability. Advancing wetted fronts (AWFs) that rewet dry river beds are unpredictable events, and knowledge about their composition and role in habitat mosaics is scarce. We collected dead and living terrestrial invertebrates, aquatic invertebrates, and sestonic matter (i.e., suspended sediments and organic matter) entrained by flow in an AWF travelling downstream over a 7-km-long dry reach. We collected samples at 12 sites along the rewetting reach and at 3 sites in the upstream perennial reach of the Albarine River, France. Invertebrates in the AWF were mainly of terrestrial origin and organic matter was essentially coarse (>5 mm). Terrestrial invertebrate density and taxonomic richness and sestonic matter concentration were several orders of magnitude higher in the AWF than in the perennial reach. However, only terrestrial invertebrate taxonomic richness increased longitudinally. At least ⅓ of terrestrial taxa could have survived submersion, and the density and taxonomic richness of these taxa decreased downstream. These results indicate that terrestrially derived material is stored downstream during rewetting where it could greatly stimulate in-stream aquatic productivity and succession in terrestrial invertebrates along the riparian zone. In contrast, entrained aquatic taxa represented ∼15% of the benthic taxa collected 1 mo after rewetting, indicating a low contribution of AWFs to benthic invertebrate succession. In the context of global change and increasing appropriation of water resources by humans, our results suggest that conceptual models of invertebrate dynamic and organic matter processing in rivers should account for dry phases and transitional periods from dry to wet conditions.