Oxygen concentrations in our waters are declining. Since biological and geochemical processes depend on oxygen this decline is concerning. Much of the research has focused on the low oxygen zones (oxygen minimum zones) in our oceans, which are increasing in areal extent, and on coastal sites (Breitburg et al. 2018). The oxygen concentrations in some of these areas are now so low that they are affecting organisms. Yet here in Australia our major river system, the Murray-Darling, also experiences low oxygen levels. This occurs when there have been major flood events, which export carbon from the flood plains to the river – the breakdown of the carbon uses more oxygen than can be replenished in the system leading to reduced oxygen in the water with deadly impacts on organisms. For example, after a decade of drought in southern Australia, floods in 2010-2011 led to a hypoxic blackwater event that spanned 2000 km of river channels in the southern Murray-Darling basin and lasted more than 6 months (Whitworth et al. 2012).
Hypoxic or oxygen-free zones are linked to mortalities of organisms, but responses of organisms are often based on presence-absence and short-term studies. Longer-term responses (e.g. >100 h) have rarely been investigated. Respirometry, a physiological approach used to measure oxygen consumption rates, presents a unique opportunity to predict an organism’s response to long-term exposure to environmental stressors such as reduced oxygen. Our research investigated the effects of long-term exposure (10 months) to low dissolved oxygen and temperature on the metabolic scope and tolerance of two freshwater fish species found in the Murray-Darling Basin (Gilmore et al. 2018).
We used juvenile golden perch (Percichthyidae: Macquaria ambigua ambigua) and silver perch (Terapontidae: Bidyanus bidyanus) in our experiments. The experimental design consisted of two oxygen treatments, normoxic (6–8 mgO2/L or 12–14 kPa) and hypoxic (3–4 mgO2/L or 7–9 kPa), combined with up to three temperature treatments (20, 24 and 28 degrees C). For the hypoxic treatments a simple and novel degassing system was developed, which used nitrogen gas to remove oxygen from the water. Respirometry was used to determine maximum metabolic rate, standard metabolic rate, absolute aerobic scope, tolerance limits to hypoxia and critical oxygen tension – resting respirometry was only conducted on golden perch due to high mortalities of silver perch in the initial experiment.
Golden perch had a much higher tolerance to hypoxia exposure than silver perch. Golden perch acclimated to hypoxia had reduced maximum metabolic rate (maximum rate that oxgyen can be transported to the organism for consumption) at 20 and 28 oC, but there was no change to standard metabolic rate (the minimum rate of oxygen consumption of a resting fish). Long-term exposure to hypoxia improved the tolerance of golden perch to hypoxia, compared to individuals held under normoxic conditions suggesting that golden perch can acclimate to levels around 3 mgO2/L (kPa ~ 7) and lower. The contrasting tolerance of two sympatric fish species to hypoxia highlights our lack of understanding of how hypoxia effects fish after long-term exposure. Some species may be able to develop natural resistance to poor oxygen conditions over time; however, this may be limited to only those with a naturally higher tolerance to hypoxia.
The video below was made by Kayla Gilmore, PhD student at the University of Adelaide, in 2016 for the Australian Society for Fish Biology Student Communication in Science competition. For 2018 information see ASFB Awards.
Breitburg et al. (2018) Declining oxygen in the global ocean and coastal waters. Science 359 (6371) DOI: 10.1126/science.aam7240
Gilmore et al. (2018) Testing hypoxia: physiological effects of long‑term exposure in two freshwater fishes. Oecologia 186: 37-47. https://doi.org/10.1007/s00442-017-3992-3
Whitworth et al. (2012) Drought, floods and water quality: Drivers of a severe hypoxic blackwater event in a major river system (the southern Murray–Darling Basin, Australia). Journal of Hydrology 450-451: 190-198.
Source of images
Suffocated spots - from Downtoearth; Source of data: World Resources Institute
Dead fish image - ABC news
Respirometry image - Kayla Gilmore