Increased depth to the water table during river drying decreases the resilience of Gammarus pulex and alters ecosystem function

Abstract

River drying has drastic immediate effects on benthic invertebrates, but their high resilience reduces long-term effects on biodiversity and ecosystem functions (e.g. leaf litter decomposition). The hyporheic zone (saturated sediments below the riverbed) can be a refuge for invertebrates in dry rivers and a primary source of colonists supporting population resilience following flow resumption. However, factors such as the depth to the water table below the riverbed, which could determine the quality of the hyporheic refuge, are not well understood. We explored how depth to the water table (control, À5 cm, À30 cm and completely dry) during a 1-week drying event affected the survival and return to the substrate surface (%RTS) of Gammarus pulex (Crustacea : Amphipoda) using laboratory mesocosms. We measured litter decomposition and Gammarus energy stores to examine effects on ecosystem function and energetic costs for organisms crawling deeper into the hyporheic zone. Two populations, collected from intermittent and perennial rivers, were used to evaluate inter-population variability in response to drying. Survival and %RTS were reduced by ≤39% and 52%, respectively, in the À30 cm and dry treatments, and this had consequences for decomposition (≤46% reduction). Differences between populations in %RTS were high across treatments but did not generally affect survival and decomposition. Our results suggest that increases in depth to the water table during river drying, which often result from longer drying duration and water abstraction, could reduce invertebrate resilience and litter decomposition by diminishing the role of the hyporheic zone as a source of colonists.

Drying duration and stream characteristics influence macroinvertebrate survivorship within the sediments of a temporary channel and exposed gravel bars of a connected perennial stream

“…This suggests that survivorship of individuals within the mesocosms was reduced due to stranding above the free water and supports the findings of other studies (e.g. Navel et al, 2010;Vadher et al, 2015;Vander Vorste et al, 2016b). Vadher et al (2017) demonstrated the importance of sedimentology on the ability of G. pulex to move vertically through sediments.…”

“…Previous research has reported reduced survivorship of common benthic invertebrates such as gammarids due to surface water loss in the surface (e.g. Poznańska et al, 2013) and subsurface (Vander Vorste et al, 2016b) sediments, and field investigations have reported reduced abundance of individuals with increasing intermittence (Clarke et al, 2010;Datry et al, 2014a) and duration of surface drying events (Storey, 2016). The majority of G. pulex individuals survived within the wet subsurface for periods of surface water loss \ 21 days, indicating that wet subsurface sediments can facilitate population persistence during short-term surface drying events.…”

“…Subsurface sediments have been demonstrated to function as a refuge for invertebrate fauna during drying events by both field studies (Hose et al, 2005;Fenoglio et al, 2006;Vander Vorste et al, 2016a) and laboratory investigations (Vadher et al, 2015;Vander Vorste et al, 2016b) encompassing both intermittent streams and those subject to severe low flows where part of the channel bed (e.g. marginal gravel bars) maybe exposed (Holzapfel et al, 2017).…”

Direct observations of the effect of fine sediment deposition on the vertical movement of Gammarus pulex (Amphipoda: Gammaridae) during substratum drying

“…Gammarus pulex is also important due to its role as both a predator and prey for fish and other invertebrate species (MacNeil et al, 1997;Kelly et al, 2002Kelly et al, , 2006. Gammarus pulex have been reported to migrate into subsurface sediments in response to biotic competition (McGrath et al, 2007), elevated temperatures (Wood et al, 2010) and water level reduction (Vander Vorste et al, 2016b;Vadher et al, 2017). Migration has been observed to depths of up to 2 m during adverse environmental conditions (Dole-Olivier et al, 1997).…”

Do benthic invertebrates use hyporheic refuges during streambed drying? A manipulative field experiment in nested hyporheic flowpaths

“…In two karst rivers in the UK, Stubbington et al (2011) observed an increase in hyporheic abundance of Gammarus pulex and total invertebrate abundance during low-flow conditions, specifying that the hyporheic refuge use was more pronounced at sites where downwelling of surface water dominated (which may be interpreted as a major in bed sediments during a disturbance is not sufficient to demonstrate that the HZ is a true refuge (and not, for example, a graveyard), it is also necessary to measure the efficiency of subsequent recolonization from these HZ refuges (Kawanishi, Inoue, Dohi, Fujii, & Miyake, 2013;Vander Vorste, Malard, et al, 2016;Vander Vorste, Mermillod-Blondin, et al, 2016).…”