Macroinvertebrate communities and their use of refuge in intermittent and perennial streams

Stream hydrology is predicted to become more variable and intermittent due to increased water withdrawals, dam construction, and climate change. Duration of stream drying is considered the primary hydrologic variable influencing biotic communities. Longitudinal distance to perennial refuge as well as quality and quantity of refuge within a stream reach has been shown to mediate macroinvertebrate community structure in response to stream drying. We compared macroinvertebrate density, biomass, richness and diversity in four perennial reaches and five intermittent reaches in autumn after streams resumed flow, and in spring during stream drying. We also assessed the importance of the hyporheic zone as a refuge from stream drying by comparing macroinvertebrate hyporheic communities in a perennial and intermittent stream as streams dried. We identified distinct macroinvertebrate community assemblages across a gradient of flow duration. We were better able to distinguish differences in macroinvertebrate community structure when we assessed taxon-specific biomass across macroinvertebrate communities grouped by flow duration. Macroinvertebrate community structure depended on flow duration, seasonal changes in habitat (e.g. loss of riffle habitat) and resources (e.g. prey and benthic organic matter). Species traits reflected changes in environmental conditions as predicted by the habitat template hypothesis but we did not see greater adaptations to drying in the macroinvertebrate communities of intermittent streams. Isolated pools and the hyporheic provided important refuge for macroinvertebrates in both intermittent and perennial streams. Macroinvertebrate biomass increased in the hyporheic of both perennial and intermittent streams in spring suggesting the hyporheic zone is an important refuge from increasing temperatures and decreasing dissolve oxygen concentrations as streams dry. Macroinvertebrate richness did not increase in the hyporheic as streams dried suggesting only specific taxa, adapted to living in hyporheic habitats, descended into the hyporheic. Taxa within the hyporheic of both streams were generally depositional, burrowing taxa tolerant of low dissolved oxygen in spring. Loss of refuge habitat when streams dry may result in a smaller pool of colonists available for macroinvertebrate reassembly when streams rewet, altering macroinvertebrate community composition over time and potentially lowering community biomass.