Vertical movements through subsurface stream sediments by benthic macroinvertebrates during experimental drying are influenced by sediment characteristics and species traits

Streambed drying is becoming more common due to climate change and increasing anthropogenic water resource pressures. Subsurface sediments are a potential refuge for benthic macroinvertebrates during drying events in temporary streams. Sediment characteristics are important controls on the distribution of macroinvertebrates in subsurface habitats, but difficulties making observations impedes quantification of vertical movements. Species traits (e.g. subsurface habitat affinity) also influence vertical movements of macroinvertebrates into the subsurface sediments, but most species-specific responses remain uncharacterised. Transparent artificial mesocosms were used to directly observe the vertical movements of individuals of three aquatic insect nymphs and two crustaceans. Mixtures of three types of transparent sediment of varying particle size were combined to produce six treatments with differing interstitial pore volumes and, hence, differing subsurface porosity. Macroinvertebrate vertical movements were measured during incremental reductions in water level from 5 cm above to 20 cm below the sediment surface. These species comprised a variety of trait categories including feeding group, species affinity to temporary streams and subsurface habitats. Active and passive vertical movements were determined by conducting experiments with both live individuals and their cadavers. Sediment treatment influenced the vertical movements of individuals as reducing subsurface porosity decreased vertical movements for most species. Vertical movement into subsurface sediments in response to water level reduction was the result of active, not passive, movements for all species. Species identity influenced the vertical movements made by individuals. Nemoura cambrica had the highest affinity for temporary streams and subsurface habitats and its vertical movements were unaffected by sediment treatment, generally reaching depths between 20 and 25 cm. Most individuals of species with a weaker subsurface affinity (i.e. the benthic grazer Heptagenia sulphurea and the filter-feeder Hydropsyche siltalai) became stranded as water levels were reduced in all sediment treatments. Vertical movements of Gammarus pulex and Asellus aquaticus were restricted primarily by pore volume, these taxa becoming stranded most commonly in sediments with smaller interstitial volumes. Our results highlight the need for the development and implementation of river management strategies that increase streambed porosity, allowing macroinvertebrates to access to the saturated subsurface habitat during stream drying.