A comparison of temperature regimes in dry-season waterholes in the Flinders and Gilbert catchments in northern Australia

Abstract

Dry-season waterholes in ephemeral rivers provide vital habitat for aquatic biota, whose survival is dependent on the waterholes lasting throughout the dry season with temperatures that are not lethal. To examine this in the Flinders and Gilbert Rivers, 20-min temperature measurements were taken during the 2012-2013 dry season in 10 waterholes in each catchment. These data were used to derive thermal-frequency curves that quantify how often waterhole temperature exceeds thresholds for (1) the optimum growth of tropical fish and (2) their lethal temperature. Waterholes that remained deeper than ,0.5 m throughout the dry season provided thermally suitable fish refugia, especially if they were turbid, because this decreased the risk of exposure to undesirable temperatures at the bottom of the waterhole. However, surface temperatures in these waterholes often exceeded optimal and even potentially lethal temperatures, so fish may have had to move to cooler water at the bottom of these waterholes. The risk to aquatic species in waterhole refugia is, therefore, primarily determined by depth, with shallow waterholes presenting the greatest risk because they become thermally unsuitable well before (1-2 months) they fully dry out.

Bund removal to re-establish tidal flow, remove aquatic weeds and restore coastal wetland services – North Queensland, Australia

“…Water was warmer below the bund during this period, reaching 35°C. The exceedance of the preferred temperature threshold for some tropical freshwater fish (see [62]), T pref = 31°C, was much higher below the bund (23% of the time) compared with above the bund (6.9% of the time). Comparison of 2013 and 2014 thermal frequency curves (Fig 6b) shows that bund removal per se did not markedly affect the wetland thermal regime.…”

Thermal and asphyxia exposure risk to freshwater fish in feral‐pig‐damaged tropical wetlands

“…Calibrated Hydrolabs (OTT Hydromet GmbH; http://www.ott.com) were deployed in wetlands (0.2 m depth) in the catchment for between 2 and 4 days to record water temperature, dissolved oxygen (%), electrical conductivity and pH every 20 min (electrical conductivity and pH are not analysed here); logging at this frequency provides explicit insight into diel changes in environmental water processes (Wallace et al ., , ; Waltham et al ., ; Waltham & Fixler, ) (Table S1 in Supporting information). Weather was fine with wetlands on the falling stage of the hydrograph, representing the most extreme time of year for fauna occupying these types of wetlands.…”

Feral pig exclusion fencing provides limited fish conservation value on tropical floodplains

“…In each wetland, a calibrated high frequency Hydrolab multi-parameter logger (OTT Hydromet USA) was deployed (0.2m depth) for between 2 and 4 days to record epilimnion (0.2m) water temperature, dissolved oxygen (%), electrical conductivity and pH every 20mins; logging at this frequency provides explicit insight into diel changes in environmental water processes [20, 22]. Weather conditions were fine with wetlands surveyed on the falling limb of the hydrograph.…”

“…In tropical north Australia, seasonal off channel wetlands are more pronounced owing to high evaporation rates, loss to groundwater [19], and in many situations waters quickly retract away from the banks and riparian shade [16]. At that point, it is thought that they become more prone to reduced water quality conditions - most notably reduced water depth [18], and high water temperatures [10, 20]. This increases aquatic fauna exposure risks to acute and chronic thresholds [21, 22].…”

“…In the late-dry season, fish confined to isolated wetlands on floodplains therefore have very limited avoidance options [10], unlike other fauna such as the freshwater crab also occupying seasonal tropical rivers in northern Australia, that will employ terrestrial re-location and access burrows when confronted with thermoregulation [23]. Fish must exploit available ephemeral aquatic habitats [24, 25], which can be specific to each wetland depending on orientation and location [26], depth and vegetation cover in the landscape [20], in order to survive until monsoonal rain reconnects overbank river networks again.…”