The effect of increasing fine sediment load and drying duration on the re-emergence of Gammarus pulex (Amphipoda: Gammaridae) from the subsurface following flow resumption

1. Environmental change and growing anthropogenic pressure on water resources is increasing the duration and intensity of drying events in streams in many geographical locations. Favourable sediment characteristics (e.g. high porosity and low fine sediment load within the substrate matrix) may facilitate benthic macroinvertebrate use of subsurface sediments in response to drying. However, the influence of sedimentary characteristics on the use and subsequent recovery of macroinvertebrates from initial vertical migration into, survival during unfavourable conditions within and subsequent re-emergence from subsurface sediments has not been directly observed. 2. Transparent mesocosm tanks were used to directly observe the vertical movement and subsequent re-emergence of G. pulex from subsurface sediments in response to increasing dry period (1 day, 7 days or 21 days) and fine sediment load (0.5–1 mm particle diameter used for light and heavy sediment treatment) and following rehydration and resumption of flowing conditions. 3. Increasing volumes of fine sediment addition limited the ability of G. pulex to access subsurface sediment in response to drying and re-emerge following rehydration. The longest dry period (21 days) reduced the ability of G. pulex to re-emerge from the subsurface sediments following rehydration and flow resumption. 4. Increasing fine sediment load negatively affects taxa using subsurface sediments as a refuge. Increased fine sediment deposition has the potential to reduce both access to the sub-surface and re-emergence once surface flow resumes. 5. As many rivers are beginning to dry out, or are showing prolonged drying due to global warming, it is increasingly important that river management reduces the input of fine sediment into rivers and increase sediment porosity of riverbeds to facilitate access into the subsurface refuge by benthic fauna.