Abstract
Surface and subsurface sediments in river ecosystems are recognized as refuges that may promote invertebrate survival during disturbances such as floods and streambed drying. Refuge use is spatiotemporally variable, with environmental factors including substrate composition, in particular the proportion of fine sediment (FS), affecting the ability of organisms to move through interstitial spaces. We conducted a laboratory experiment to examine the effects of FS on the movement of Gammarus pulex Linnaeus (Crustacea: Amphipoda) into subsurface sediments in response to surface water loss. We hypothesized that increasing volumes of FS would impede and ultimately prevent individuals from migrating into the sediments. To test this hypothesis, the proportion of FS (1–2 mm diameter) present within an open gravel matrix (4–16 mm diameter) was varied from 10 to 20% by volume in 2.5% increments. Under control conditions (0% FS), 93% of individuals moved into subsurface sediments as the water level was reduced. The proportion of individuals moving into the subsurface decreased to 74% at 10% FS, and at 20% FS no individuals entered the sediments, supporting our hypothesis. These results demonstrate the importance of reducing FS inputs into river ecosystems and restoring FS-clogged riverbeds, to promote refuge use during increasingly common instream disturbances.
Highlights
The bed sediments of river ecosystems are recognized as an important habitat and refuge for benthic macroinvertebrates during adverse conditions in the surface stream (Williams & Hynes, 1974; Stubbington, 2012)
The point at which a significant proportion of the G. pulex population had migrated into the subsurface sediments differed between fine sediment (FS) treatments
While previous research into the use of sedimentary refuges has focused on the subsurface hyporheic zone (Dole-Olivier, 2011; Stubbington, 2012), implicit within these studies is the ability of organisms to migrate through the surficial benthic zone into the contiguous subsurface sediments
Summary
The bed sediments of river ecosystems are recognized as an important habitat and refuge for benthic macroinvertebrates during adverse conditions in the surface stream (Williams & Hynes, 1974; Stubbington, 2012). Many studies have reported significantly greater invertebrate abundance and diversity in benthic compared to hyporheic sediments during disturbances such as severe flow reductions (James et al, 2008), while other studies have indicated that invertebrates may not even migrate into surficial sediments when a streambed dries (Extence, 1981) This contrasting evidence demonstrates that bed sediments are a patchy refuge (Dole-Olivier et al, 1997), with environmental conditions including hydrological exchange, dissolved oxygen availability and sediment composition determining their potential for invertebrate use and persistence (Stubbington, 2012). Porosity and permeability influence faunal movements through interstitial spaces (Fowler & Death, 2001; Descloux et al, 2013), and where invertebrates are unable to enter interstices, they become stranded on the riverbed and may be exposed to adverse conditions such as emersion during streambed drying (Perry & Perry, 1986; Stubbington et al, 2009)
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