Variable metabolic responses of Skagerrak invertebrates to low O<sub>2</sub> and high CO<sub>2</sub> scenarios

Abstract

Coastal hypoxia is a problem that is predicted to increase rapidly in the future. At the same time we are facing rising 15 atmospheric CO 2 concentrations, which are increasing the pCO 2 and acidity of coastal waters. These two drivers are well studied in isolation however; the coupling of low O 2 and pH is likely to provide a more significant respiratory challenge for slow moving and sessile invertebrates than is currently predicted. The Gullmar Fjord in Sweden is home to a range of habitats such as sand and mud flats, seagrass beds, exposed and protected shorelines, and rocky bottoms. Moreover, it has a history of both natural and anthropogenically enhanced hypoxia as well as North Sea upwelling, where salty water reaches 20 the surface towards the end of summer and early autumn. A total of 11 species (Crustacean, Chordate, Echinoderm and Mollusc) of these ecosystems were exposed to four different treatments (high/low oxygen and low/high CO 2 ; varying pCO 2 of 450 and 1300 ppm and O 2 concentrations of 2-3.5 and 9-10 mg L -1 ) and respiration measured after 3 and 6 days, respectively. This allows us to evaluate respiration responses of species of contrasting habitats and life-history strategies to single and multiple stressors. Results show that the responses of the respiration were highly species specific as we observed 25 both synergetic as well as antagonistic responses, and neither phylum nor habitat explained trends in respiratory responses.Management plans should avoid the generalized assumption that combined stressors will results in multiplicative effects and focus attention on alleviating hypoxia in the region. KEYWORDS:Hypoxia, ocean acidification, low O 2 , high CO 2 , low pH, invertebrates, Gullmar Fjord. 30Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-321 Manuscript under review for journal Biogeosciences Discussion started: 18 September 2017 c Author(s) 2017. CC BY 4.0 License. 1 IntroductionResolving the responses of marine organisms to the multiple pressures associated with global change is a major challenge for marine scientists (Duarte et al., 2014). This challenge is particularly pressing for coastal ecosystems, where human populations and impacts often concentrate. Among these pressures, decreasing O 2 concentrations (hypoxia) and ocean acidification (OA) are receiving particular attention (Diaz and Rosenberg, 2008; Doney et al., 2009; Vaquer-Sunyer 5 and Duarte, 2008; Kroeker et al., 2013). Whereas uptake of anthropogenic CO 2 , is leading to more acid waters in the open ocean (Doney et al., 2009;Caldeira and Wickett, 2003), the control of pH in coastal waters is more complex, often involving metabolic processes (Duarte et al., 2013). This metabolic control is particularly evident when eutrophication stimulates algal blooms, leading to increased sedimentation of organic matter, subsequently degraded through microbial respiration, therebyconsuming O 2 and releasing CO 2 (Conley et al., 2009). Hence, elevated CO 2 through excess respiration is associated with 10 reduc...

Additive impacts of deoxygenation and acidification threaten marine biota

“…If we consider our findings in the context of the previously proposed “RI” thresholds, the relationship between Ln RR and theoretical p O 2 / p CO 2 ‐ratio obtained supports criticisms of Seibel and Childress (2012) that the thresholds are inadequate and do not fully capture sources of variation in Ln RR observed in this analysis. Although its use has been challenged (Seibel & Childress, 2012), the only experimental tests thus far available involved marine invertebrates from the Chilean coast, Baltic Sea, and Eastern Australia (Fontanini et al., 2018; Steckbauer et al., 2015; Treible et al, 2018). The responses of marine organisms to RI values within the range of 0.7–1.0, which was proposed to delineate the level at which aerobic respiration must be severely compromised (Brewer & Peltzer, 2009), were modest (Ln RR ≈ −0.4, i.e., 33% reduction in performance compared to controls with RI values of 1.5–1.8, Figure 3a).…”

“…A simple linear constraint describing p O 2 and p CO 2 conditions in marine systems may provide a useful metric to understand how the combined extremity of deoxygenation and acidification drives impacts on marine biota. For instance, invertebrates of the Chilean coast and the Baltic Sea experimentally exposed to various p CO 2 and p O 2 combinations exhibited reduced metabolic rates with declining O 2 and rising p CO 2 values (Fontanini, Steckbauer, Dupont, & Duarte, 2018; Steckbauer et al., 2015). However, the reliability of the p O 2 / p CO 2 ‐ratio as a potential indicator of marine taxa responses to CA and CD remains to be tested.…”