Abstract
Benthic macroinvertebrates inhabit the streambed sediments of temporary streams during drying events. Fine sediment (< 2 mm in diameter) deposition and clogging of interstitial pathways reduces the connectivity between benthic and subsurface habitats, potentially inhibiting macroinvertebrate vertical movements. Direct observations within subsurface sediments are, however, inherently difficult. As a result, confirmation of macroinvertebrate vertical movement, and the effect of fine sediment, is limited. We used laboratory mesocosms containing transparent gravel sized particles (10–15 mm) to facilitate the direct observation and tracking of vertical movements by Gammarus pulex in response to water level reduction and sedimentation. Seven sediment treatments comprised two fine sediment fractions (small: 0.125–0.5 mm, coarse sand: 0.5–1 mm) deposited onto the surface of the substrate, and a control treatment where no fine sediment was applied. We found that G. pulex moved into the subsurface gravel sediments in response to drying, but their ability to remain submerged during water level reduction was impeded by fine sediment deposition. In particular deposition of the coarser sand fraction clogged the sediment surface, limiting vertical movements. Our results highlight the potential effect of sedimentation on G. pulex resistance to drying events in streams.
Highlights
Streambed drying as a result of climate variability and anthropogenic pressures on water resources is an increasing global phenomenon (Acuna et al, 2014; Leigh et al, 2016), even in historically perennial systems (Datry et al, 2014; Pyne & Poff, 2017)
Sediment treatments comprising greater proportions of coarser sand particles reduced the percentage of G. pulex submerged at the end of experiments (GLM, F6, 42 = 17.061, P \ 0.001; Fig. 4), fewer G. pulex remained submerged at the end of experiments in treatment 2 compared to treatment 3 (Fig. 4)
Some G. pulex individuals were unable to remain submerged during dewatering, supporting our first hypothesis. These results support the observations of Stumpp & Hose (2013) and Vadher et al (2017) that reducing water level in artificial mesocosm experiments resulted in the stranding of individual invertebrates within subsurface sediments
Summary
Streambed drying as a result of climate variability and anthropogenic pressures on water resources is an increasing global phenomenon (Acuna et al, 2014; Leigh et al, 2016), even in historically perennial systems (Datry et al, 2014; Pyne & Poff, 2017). Many macroinvertebrate populations persist during dry events by employing a range of survival strategies including behavioural adaptations, such as vertical movement into the saturated riverbed sediments (Stubbington, 2012; Vander Vorste et al, 2016a; Vadher et al, 2017), or physiological adaptations to desiccation (Strachan et al, 2015; Stubbington et al, 2016) and declining water quality (van Vliet & Zwolsman, 2008). Lotic macroinvertebrate taxa have been recorded and observed in the saturated subsurface sediments of drying streams, indicating that they may serve as a habitat where fauna may persist (Hose et al, 2005; Fenoglio et al, 2006), and from which populations may recolonize waterbodies following the resumption of flow (Vander Vorste et al, 2016a). Confirmation of the vertical movement and the tracking of individuals have been hampered by the inherent difficulties associated with making direct observations within subsurface sediments (Vadher et al, 2017)
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