Freshwater Macroinvertebrate Community Responses to Drying Reflect Abiotic Stress, Frequency of Prior Drying and Proximity to a Perennial Reach

ABSTRACT Intermittent streams dominate global river networks and are becoming increasingly common due to global change. Understanding the mechanisms by which freshwater biota respond to flow recession, drying, and intermittence is therefore important to our ability to support biodiversity as ecosystems adapt to ongoing stressors. Responses of freshwater communities, including macroinvertebrates, are variable and likely reflect the influence of multiple drivers. We aimed to investigate how flow recession changes abiotic conditions (abiotic drying stress), and if this is affected by how frequently a site dries over the prior two years (frequency of prior drying) or the study stream. We also aimed to untangle how abiotic drying stress (hereafter referred to as drying stress), frequency of prior drying, and distance to a perennial reach interact to influence macroinvertebrate community richness and densities, turnover in taxa between locations with frequent or rare drying, drift composition, macroinvertebrate movement, and life histories. Finally, we aimed to understand whether fish that are visual predators modify macroinvertebrate behavioural responses, and if drying stress increases macroinvertebrate development rate at the cost of body length. We measured abiotic stress (channel area, discharge, temperature and dissolved oxygen) and counted moving macroinvertebrates across five streams with different fish types. We also measured abiotic stress, benthic, and drifting macroinvertebrate communities and counted moving macroinvertebrates in one stream using four sampling campaigns. Each sampling campaign occurred on a different date, and the stream dried at a location with different frequencies of prior drying and distances from a perennial reach. During each sampling campaign, data were collected at sites with different intensities of drying stress. During the sampling campaign when the channel was at its driest, macroinvertebrate drift composition and life histories (body length at each life history stage) were also collected. Temperatures increased and dissolved oxygen decreased with flow reduction, regardless of the frequency of prior drying or study stream. Mayfly development rates and macroinvertebrate movement increased as drying stress increased, but mayflies developed with shorter bodies. A fish visual predator and frequent prior drying were both associated with reduced macroinvertebrate movement. In intermittent reaches, drying stress was associated with reduced macroinvertebrate densities and richness, and taxonomic turnover. In contrast, closer to the perennial reach, there were high densities and relatively little taxonomic change associated with drying stress, possibly due to macroinvertebrate drift from communities unaffected by drying stress. Collectively, our findings suggest that drying stress, the frequency of prior drying, and distance to the nearest perennial reach interact to determine the effects of flow recession on macroinvertebrate communities. Therefore, we need to know the frequency of prior drying in a reach and the location of perennial reaches to predict outcomes of intermittency. Understanding the spatial and temporal context of the drying front is likely key to understanding and predicting community responses, particularly in the context of widespread (and growing) prevalence of intermittent streams with water resource pressures and increases in drought.