Persistence of aquatic refugia between flow pulses in a dryland river system(Cooper Creek, Australia)

Abstract

In many dryland rivers with intermittent flow, relatively deep segments of the river channel serve as refugia for aquatic life during protracted intervals between flows. Semipermanent channel segments are known as waterholes in the semiarid Cooper Creek system of western Queensland. Fractional water loss by evaporation was estimated in 15 Cooper waterholes from the increase in conservative ion (Na ϩ and Cl Ϫ ) concentrations and independently from evaporative fractionation of oxygen and hydrogen isotopes in water. The major solute chemistry and isotope results indicated that evaporative water loss controlled the water levels between flows and that the surface waters were effectively isolated from underlying groundwater that had a distinctive chemical and isotopic composition. Fractional water loss rates combined with stage-volume relationships for each basin showed a mean evaporative loss rate of 2.1 m yr Ϫ1 ; during that time (April-October 2002), pan evaporation averaged 2.5 m yr Ϫ1. Site-specific extrapolation of those estimated evaporative loss rates indicated that the waterholes would dry to 10% of their bankfull volumes in 6-23 months, although those estimates were based on sampling in 2002, when pan evaporation rates were 18% higher than the long-term mean. These persistence times show the importance of occasional, irregular flow pulses in sustaining these aquatic refugia; the desiccation of waterholes could become more common if future water withdrawals reduce the frequency and intensity of river flows to the point where they occur less often than annually.

An arid zone awash with diversity: patterns in the distribution of aquatic invertebrates in the Pilbara region of Western Australia

“…In the river pools of some arid zones, water quality deteriorates as water volume declines during drought periods, even in permanent wetlands, and especially where water levels are controlled by surface water flows and/or where saline groundwater intrudes when water levels are low (Costelloe et al 2005;Hamilton et al 2005). Such pools can experience raised salinity and temperature, concentration of nutrients and increased solar radiation as pools contract away from banks, with subsequent problems associated with algal blooms (Lake 2003).…”

“…These impacts have well-known effects on aquatic invertebrates, including reduced species richness (reviewed in Lake 2003). Hamilton et al (2005) suggested that waterholes maintained by fresh groundwater would be 'exceptionally important' refugia in the Cooper Creek system of the Lake Eyre Basin. By contrast, in the Pilbara, pools maintained by fresh hyporheic or spring flows are not unusual and water quality tends to remain high during dry periods.…”

Small environmental flows, drought and the role of refugia for freshwater fish in the Macquarie Marshes, arid Australia

“…Here, biotic interactions, including competition for limited food resources and predation from waterbirds, are concentrated. Species become locally extinct as their ecological tolerances are exceeded, with final population persistence determined by the habitat characteristics of an individual refuge, the rate of evaporative water loss, the resilience of the taxa present and disturbance intensity (Closs and Lake, 1996;Arthington et al, 2005;Hamilton et al, 2005). If a drought is particularly severe or protracted, loss of entire fish communities may occur at large spatial scales (e.g., river reaches).…”

A review of groundwater–surface water interactions in arid/semi‐arid wetlands and the consequences of salinity for wetland ecology

“…Due to the temporary nature of many wetlands, and the resultant variability of physico-chemical factors, the biota have evolved unique features and life-cycle adaptations that enable them to persist over dry intervals (e.g. use of deep channel segments as aquatic refugia, Walker et al, 1995;Sheldon et al, 2002;Hamilton et al, 2005). Arid/semi-arid wetlands show faunal diversity as high as, or greater than that in many temperate and tropical wetlands (Williams, 1998a,b).…”