Creating a disturbance: manipulating slackwaters in a lowland river

Abstract

The dynamic nature of habitat patches in rivers is driven primarily by flow regime. Altered hydrology, through river regulation, can limit the size and distribution of slackwater patches; important areas for the development of young fish and for shrimp in lowland rivers. Between late October 2002 and late January 2003, we investigated responses of fish, shrimp and their potential prey to the experimental creation of slackwaters and, conversely, to the experimental creation of flowing patches, by diverting water away from flowing patches and into slackwater patches, respectively. A pre-experimental survey indicated that slackwaters contained many more fish than flowing patches, and fish larvae were flushed out of slackwaters during the construction of flowing patches. Creation of slackwaters resulted in increased abundance of fish and shrimp, with the opposite occurring when slackwaters were changed into flowing patches. Converting slackwaters into flowing patches, and vice versa, altered the species composition of zooplankton and microbenthic assemblages but did not change their densities. Thus, standing crop of potential prey alone could not explain the differences in fish or shrimp abundance found between patch types. We hypothesize that slackwaters primarily act as refuges from current and provide energetic advantages to the young stages of fish and to shrimp. River regulation has the potential to affect the recruitment success of fish and shrimp by affecting the size, arrangement and availability of slackwater patches.

Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river

“…Fluvial dynamics create a mosaic of habitats within river-floodplain systems including off-channel habitats such as oxbow lakes, sloughs and other slack water areas (Amoros and Bornette, 2002). These habitats serve a variety of ecological functions including spawning and nursery areas and refuge from high flows in the main channel (Sabo and Kelso, 1991;Humphries et al, 2006;Pease et al, 2006). Physicochemical attributes of different habitat units have a strong influence on local species assemblages (Tejerina-Garro et al, 1998;Winemiller et al, 2000;Suarez et al, 2004) and interactions between flood dynamics and habitat characteristics influence the value of different habitats for spawning, feeding or refuge (Feyrer et al, 2006).…”

Microinvertebrate dynamics in riverine slackwater and mid‐channel habitats in relation to physico‐chemical parameters and food availability

“…Hydrodynamic forces often constrain these microinvertebrates, directly or indirectly, by affecting their life history patterns (Rzoska, 1978;Saunders and Lewis, 1989), habitat characteristics (Humphries et al, 2006), dispersal patterns (Speas, 2000), resource acquisition (Guelda et al, 2005) and biotic interactions (Baranyi et al, 2002;Keckeis et al, 2003). As a result, communities occurring in the main channel flow are often of relatively low abundance and/or biomass (Basu and Pick, 1996;Ietswaart et al, 1999), and are typically comprised of small-bodied taxa with short generation times (Shiel et al, 1982;Pace et al, 1992;but still experience some form of mixing or exchange with the main flow (Lancaster and Hildrew, 1993).…”

Impacts of flow regulation on slackwaters in river channels

“…[3] Slackwaters have been found to contain significantly greater numbers of fish, shrimp, and zooplankton than midchannel habitats in lowland rivers [Bowen et al, 2003;Richardson et al, 2004;Nielsen and Watson, 2008;Ning et al, 2010] with an order of magnitude more fish and shrimp observed in one study [Humphries et al, 2006]. They have also been found to provide refuge from excessively high-water velocities, which stress many biota [Schiemer et al, 2001;Maddock et al, 2004] enabling recolonization of the channel following flow recession [Lancaster and Belyea, 1997;Speirs and Gurney, 2001].…”

“…[40] While slackwaters represent only one key habitat within the complexity of natural rivers [Cooper et al, 1997], they are empirically associated with significantly greater numbers of fish, shrimp, and zooplankton than in fast-flowing waters [Bowen et al, 2003;Richardson et al, 2004;Humphries et al, 2006;Nielsen and Watson, 2008] and physically associated with processes such as organic matter retention [Schiemer et al, 2001]. Therefore, it is probable that changes to slackwaters resulting from regulation are likely to influence biota that use slackwaters at some or multiple stages in their life cycle.…”

“…Among the major impacts are reductions in physical habitat [Lloyd et al, 2002] and higher rates of mortality [Bowen et al, 2003;Humphries et al, 2006]. It has been suggested that regulation alters the distribution, total area, and patch area of slackwaters through altered flow regimes [Bowen et al, 2003;Humphries et al, 2006;Ning et al, 2010] and altered channel morphology [Milhous, 1998;Maddock et al, 2004;Jacobson and Galat, 2006]. Irrigation represents 80% of the total human freshwater use worldwide [Thenkabail et al, 2009] and is a significant factor in global patterns of water scarcity and freshwater biodiversity threats [Vörösmarty et al, 2010].…”