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

1. Streambed drying is becoming more common due to climate change and increas-ing anthropogenic water resource pressures. Subsurface sediments are a poten-tial refuge for benthic macroinvertebrates during drying events in temporarystreams.2. Sediment characteristics are important controls on the distribution of macroin-vertebrates in subsurface habitats, but difficulties making observations impedesquantification of vertical movements. Species traits (e.g. subsurface habitat affin-ity) also influence vertical movements of macroinvertebrates into the subsurfacesediments, but most species-specific responses remain uncharacterised.3. Transparent artificial mesocosms were used to directly observe the verticalmovements of individuals of three aquatic insect nymphs and two crustaceans.Mixtures of three types of transparent sediment of varying particle size werecombined to produce six treatments with differing interstitial pore volumes and,hence, differing subsurface porosity. Macroinvertebrate vertical movements weremeasured during incremental reductions in water level from 5 cm above to20 cm below the sediment surface. These species comprised a variety of traitcategories including feeding group, species affinity to temporary streams andsubsurface habitats. Active and passive vertical movements were determined byconducting experiments with both live individuals and their cadavers.4. Sediment treatment influenced the vertical movements of individuals as reducingsubsurface porosity decreased vertical movements for most species. Verticalmovement into subsurface sediments in response to water level reduction wasthe result of active, not passive, movements for all species.5. Species identity influenced the vertical movements made by individuals. Nemouracambrica had the highest affinity for temporary streams and subsurface habitatsand its vertical movements were unaffected by sediment treatment, generallyreaching depths between 20 and 25 cm. Most individuals of species with aweaker subsurface affinity (i.e. the benthic grazer Heptagenia sulphurea and thefilter-feeder Hydropsyche siltalai) became stranded as water levels were reducedin all sediment treatments. Vertical movements of Gammarus pulex and Asellus aquaticus were restricted primarily by pore volume, these taxa becomingstranded most commonly in sediments with smaller interstitial volumes.6. Our results highlight the need for the development and implementation of rivermanagement strategies that increase streambed porosity, allowing macroinverte-brates to access to the saturated subsurface habitat during stream drying.No Full Tex