Abstract
Hypoxia represents a growing threat to biodiversity in freshwater ecosystems. Here, aquatic surface respiration (ASR) and oxygen thresholds required for survival in freshwater and simulated blackwater are evaluated for four lowland river fishes native to the Murray-Darling Basin (MDB), Australia. Juvenile stages of predatory species including golden perch Macquaria ambigua, silver perch Bidyanus bidyanus, Murray cod Maccullochella peelii, and eel-tailed catfish Tandanus tandanus were exposed to experimental conditions of nitrogen-induced hypoxia in freshwater and hypoxic blackwater simulations using dried river red gum Eucalyptus camaldulensis leaf litter. Australia's largest freshwater fish, M. peelii, was the most sensitive to hypoxia but given that we evaluated tolerances of juveniles (0.99±0.04 g; mean mass ±SE), the low tolerance of this species could not be attributed to its large maximum attainable body mass (>100,000 g). Concentrations of dissolved oxygen causing 50% mortality (LC50) in freshwater ranged from 0.25±0.06 mg l−1 in T. tandanus to 1.58±0.01 mg l−1 in M. peelii over 48 h at 25–26°C. Logistic models predicted that first mortalities may start at oxygen concentrations ranging from 2.4 mg l−1 to 3.1 mg l−1 in T. tandanus and M. peelii respectively within blackwater simulations. Aquatic surface respiration preceded mortality and this behaviour is documented here for the first time in juveniles of all four species. Despite the natural occurrence of hypoxia and blackwater events in lowland rivers of the MDB, juvenile stages of these large-bodied predators are vulnerable to mortality induced by low oxygen concentration and water chemistry changes associated with the decomposition of organic material. Given the extent of natural flow regime alteration and climate change predictions of rising temperatures and more severe drought and flooding, acute episodes of hypoxia may represent an underappreciated risk to riverine fish communities.
Highlights
Hypoxia and blackwater events occur in lowland rivers and wetlands world-wide and can be a natural phenomenon associated with rising temperature, floodplain inundation and the subsequent decomposition of organic material [1,2,3]
The oxygen concentration which caused 50% mortality in M. peelii within hypoxic freshwater experiments resulted in 4%, 9% and 4% percent mortality in M. ambigua, B. bidyanus and T. tandanus respectively
The hypoxia tolerances of the freshwater fishes examined here were comparable to other temperate stream fishes world-wide [24,25,26], whereby T. tandanus was the most tolerant and M. peelii amongst the most sensitive
Summary
Hypoxia and blackwater events occur in lowland rivers and wetlands world-wide and can be a natural phenomenon associated with rising temperature, floodplain inundation and the subsequent decomposition of organic material [1,2,3]. Fish inhabiting rivers and wetlands where hypoxia occurs may be adapted to tolerate low dissolved oxygen concentrations and variable water quality using a variety of mechanisms ranging from air-breathing to behavioural avoidance [6]. Using behaviour referred to as aquatic surface respiration (ASR), many species of fish will rise to the surface when exposed to hypoxia, in an attempt to extract oxygen from the thin surface layer of water that is contact with the atmosphere (Figure 1). Developing an understanding of low dissolved oxygen tolerances and sub-lethal effects on aquatic animals will be essential in developing natural resource management guidelines to mitigate the effects of humaninduced hypoxia
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