Factors limiting the vertical movement and use of subsurface sediments by lotic macroinvertebrates in response to dewatering and surface drying

Climate change projections suggest increased frequency and duration of streambed drying in many regions across the globe. These predictions even apply to streams in temperate environments, which are typically characterised by perennial river flow. The subsurface sediments of streams are an important refuge for benthic invertebrates during streambed drying; this role may become more important given the predicted increased future extent and duration of stream drying. However the vertical movement of organisms into subsurface and hyporheic sediments, and factors affecting this movement, has received limited attention historically. The research in this thesis employed a series of laboratory experiments to examine the impact of sedimentological characteristics (particle size, porosity, sedimentation) on the vertical movement of macroinvertebrates. This research also used tightly controlled flume and field experiments to examine how the duration of drying events affected macroinvertebrate survival. Specifically, the laboratory studies used direct observation in transparent artificial mesocosms containing different sediment sizes and interstitial pore space volumes to examine the subsurface vertical movements of five lotic taxa displaying different biotic traits in response to water level reduction. The results indicated that lotic macroinvertebrates actively moved into subsurface sediments in response to water level reduction. The extent to which taxa moved vertically through sediments was: i) species-specific, and ii) restricted by sediment porosity. In addition, the research examined the effect of varying loads of fine sediment (particles < 2 mm) on the vertical movement of the freshwater shrimp, Gammarus pulex, through transparent mesocosms during water level reduction. The results demonstrate the limiting effect of fine sediment load and size on the ability of G. pulex to remain submerged as sand particles (0.5 - 1 mm) capable of bridging interstitial pathways into the subsurface impeded and prevented the movement of individuals into the subsurface. Finally, this thesis presents the results of flume and field mesocosm studies which examine the effect of surface water loss duration on the survivorship of G. pulex within the subsurface. These experiments demonstrate how increasing drying duration and variability in water quality can significantly reduce faunal survival within subsurface sediments. The results and synthesis illustrate the value of laboratory / mesocosm-based research and effective riverbed management to ensure instream ecology can access and utilise the vital hyporheic refuge in the face of increased drying due to climate change and anthropogenic management.