Abstract
Aquatic ecological responses to climatic warming are complicated by interactions between thermal effects and other environmental stressors such as organic pollution and hypoxia. Laboratory experiments have demonstrated how oxygen limitation can set heat tolerance for some aquatic ectotherms, but only at unrealistic lethal temperatures and without field data to assess whether oxygen shortages might also underlie sublethal warming effects. Here, we test whether oxygen availability affects both lethal and nonlethal impacts of warming on two widespread Eurasian mayflies, Ephemera danica, Müller 1764 and Serratella ignita (Poda 1761). Mayfly nymphs are often a dominant component of the invertebrate assemblage in streams, and play a vital role in aquatic and riparian food webs. In the laboratory, lethal impacts of warming were assessed under three oxygen conditions. In the field, effects of oxygen availability on nonlethal impacts of warming were assessed from mayfly occurrence in 42 293 UK stream samples where water temperature and biochemical oxygen demand were measured. Oxygen limitation affected both lethal and sublethal impacts of warming in each species. Hypoxia lowered lethal limits by 5.5 °C (±2.13) and 8.2 °C (±0.62) for E. danica and S. ignita respectively. Field data confirmed the importance of oxygen limitation in warmer waters; poor oxygenation drastically reduced site occupancy, and reductions were especially pronounced under warm water conditions. Consequently, poor oxygenation lowered optimal stream temperatures for both species. The broad concordance shown here between laboratory results and extensive field data suggests that oxygen limitation not only impairs survival at thermal extremes but also restricts species abundance in the field at temperatures well below upper lethal limits. Stream oxygenation could thus control the vulnerability of aquatic ectotherms to global warming. Improving water oxygenation and reducing pollution can provide key facets of climate change adaptation for running waters.
Highlights
Human dependency on ecosystem services provided by rivers and streams makes them one among the world’s most important resources (Postel & Carpenter, 1997; Vo€ro€smarty et al, 2010; Durance et al, 2016)
Experimental manipulation demonstrated that water oxygenation significantly affected the heat tolerance in mayfly nymphs of E. danica (Fig. 1a) and especially S. ignita (Fig. 1b) (Kruskal–Wallis test: v2 = 11.78; P = 0.0028 for E. danica and v2 = 20.77; P < 0.0001 for S. ignita)
For E. danica, mean values for critical thermal maxima (CTmax) (ÆSD) were 5.5 °C (Æ2.13) higher in normoxia relative to hypoxia
Summary
Human dependency on ecosystem services provided by rivers and streams makes them one among the world’s most important resources (Postel & Carpenter, 1997; Vo€ro€smarty et al, 2010; Durance et al, 2016). While many current studies attempt to address the consequences of such vulnerabilities by appraising the effects of changes in discharge or thermal regimes alone (Domisch et al, 2011; Isaak & Rieman, 2013), in reality climate risks to freshwaters arise through complex interactions with a wide range of other stressors (Ormerod et al, 2010). Better oxygenation of the medium should improve heat tolerance, whereas lower temperatures improve hypoxia tolerance (Whitney, 1939; Knight & Gaufin, 1964; Po€rtner et al, 2006; Woods et al, 2009; Verberk et al, 2013) These interactions are understood mostly through experiments where temperatures exceed those experienced by species in the field and mortality was used as an endpoint. It remains unknown whether good oxygenation can offset adverse warming effects under ecologically relevant temperatures and oxygen conditions, for example by enhancing survival or through effects on sublethal processes such as growth and reproduction
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