Abstract
Global environmental change is increasing hypoxia in aquatic ecosystems. During hypoxic events, bacterial respiration causes an increase in carbon dioxide (CO2) while oxygen (O2) declines. This is rarely accounted for when assessing hypoxia tolerances of aquatic organisms. We investigated the impact of environmentally realistic increases in CO2 on responses to hypoxia in European sea bass (Dicentrarchus labrax). We conducted a critical oxygen (O2crit) test, a common measure of hypoxia tolerance, using two treatments in which O2 levels were reduced with constant ambient CO2 levels (~530 µatm), or with reciprocal increases in CO2 (rising to ~2,500 µatm). We also assessed blood acid-base chemistry and haemoglobin-O2 binding affinity of sea bass in hypoxic conditions with ambient (~650 μatm) or raised CO2 (~1770 μatm) levels. Sea bass exhibited greater hypoxia tolerance (~20% reduced O2crit), associated with increased haemoglobin-O2 affinity (~32% fall in P50) of red blood cells, when exposed to reciprocal changes in O2 and CO2. This indicates that rising CO2 which accompanies environmental hypoxia facilitates increased O2 uptake by the blood in low O2 conditions, enhancing hypoxia tolerance. We recommend that when impacts of hypoxia on aquatic organisms are assessed, due consideration is given to associated environmental increases in CO2.
Highlights
Global environmental change is increasing hypoxia in aquatic ecosystems
This was indicated by measurements of O2crit in European sea bass being significantly different between fish exposed to either constant, or rising CO2 levels during O2crit tests when accounting for variation in standard metabolic rate (SMR) (Fig. 1; ANCOVA, F1,12 = 7.525, p = 0.0178)
A CO2 increase during O2crit tests resulted in a 20% reduction of O2crit (3.88 ± 0.19 kPa O2, 18.7 ± 0.9% air saturation, mean ± S.E.) when compared to tests in which CO2 levels were maintained at ambient levels (4.87 ± 0.22 kPa O2, 23.4 ± 1.1% air saturation, mean ± S.E.)
Summary
Global environmental change is increasing hypoxia in aquatic ecosystems. During hypoxic events, bacterial respiration causes an increase in carbon dioxide (CO2) while oxygen (O2) declines. Non-linear interactive effects between higher atmospheric CO2 and CO2 accumulation during hypoxia will lead to increased CO2 levels during hypoxia in future oceans[14] This means that effects of rising CO2 during hypoxia in marine systems will be amplified by climate change. Experiments which test responses to hypoxia or impacts from hypoxia on aquatic organisms create hypoxic conditions by off-gassing oxygen from water by gassing with pure nitrogen or a mix of nitrogen (N2) and O2 (for examples see[15,16,17,18]) This creates low O2 conditions without the concurrent CO2 increase that would be expected in the environment. Several studies on marine fish and invertebrates have demonstrated interactive effects of low oxygen and increased CO29,19,20, with some species exhibiting loss of equilibrium (LoE) and death at higher O2 concentrations when
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