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

Abstract

Microinvertebrates play a critical role in riverine food webs, and recent studies have hypothesized that slackwaters, non-flowing regions associated with the main channel, are important for their reproduction and recruitment. However, little is known regarding the population and community dynamics of microinvertebrate communities in slackwater regions, or how they compare with those in mid-channel regions. This study examined microinvertebrate communities in the epibenthic and pelagic zones of slackwater and mid-channel regions (i.e. four habitats) of an Australian floodplain river in relation to physico-chemical parameters and food availability (as estimated by chlorophyll-a concentration) between September 2005 and November 2006. Results from this study indicate that microinvertebrate abundance and diversity were greater in slackwater habitats than midchannel habitats overall, corresponding with the slower current velocities associated with the former. Nevertheless, communities in all four habitats were most abundant and diverse in late spring (coinciding with an increase in water temperature), and followed similar seasonal trajectories in terms of density, taxon richness and community structure. These findings support the view that slackwaters are important for in-channel microinvertebrate production, and suggest that animals frequently disperse (either actively or passively) among slackwater and other main channel regions year round. Given the critical importance of microinvertebrates in riverine food webs, rivers should be managed with a view to maintaining a natural variety of accessible slackwater regions in order to support the production and survival of microinvertebrate communities.

Impacts of flow regulation on slackwaters in river channels

“…Hydraulic criteria are commonly species specific and life stage dependent [Hancock and Bunn, 1999;Schiemer et al, 2001]; however, low velocities are clearly an important consideration with respect to mechanisms such as swimming ability of fishes [Koehn et al, 1994;Bowen et al, 1998;Price, 2007] and hydraulic residence time for invertebrates [Schiemer et al, 2001; King and Crook, 2002] and phytoplankton [Hein et al, 2004]. Mechanistic relationships cannot be discounted when supported by overwhelming empirical evidence [Ning et al, 2010].…”

“…[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].…”

“…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].…”

“…Survival and mortality can be closely related to spatial and temporal variabilities in the hydraulic environment where biota have limited swimming abilities, such as larval or juvenile fish [Price, 2007] or shrimp larvae [Ning et al, 2010], or are affected by stream forces, such as algae [Humphries et al, 2002]. Favorable conditions are particularly important during sensitive life stages [Schiemer et al, 2001, Price, 2007.…”

“…[2] Slackwaters in rivers have been described as ''nonflowing regions associated with the main channel'' [Ning et al, 2010], ''nearly continuously wetted habitats where directional water currents are minimal or non-existent except during high water'' [Thorp and Casper, 2002], and locations where ''no discernible flow or back eddies occurred'' . Terms including ''backwater,'' ''still littoral habitats'' [King, 2004;Price, 2007], ''inshore nursery zone,'' ''dead zone'' [Reynolds et al, 1991;Schiemer et al, 2001], and ''still-water patches'' [Price, 2007] are used to describe similar hydraulic environments characterized by low velocity and often shallow depths, irrespective of the international context [Schiemer et al, 2001;Bowen et al, 2003;King, 2004].…”