Defining CO₂ and O₂ syndromes of marine biomes in the Anthropocene

Abstract

Research efforts have intensified to foresee the prospects for marine biomes under climate change and anthropogenic drivers over varying temporal and spatial scales. Parallel with these efforts is the utilization of terminology, such as ‘ocean acidification’ (OA) and ‘ocean deoxygenation’ (OD), that can foster rapid comprehension of complex processes driving carbon dioxide (CO2) and oxygen (O2) concentrations in the global ocean and thus, are now widely used in discussions within and beyond academia. However, common usage of the terms ‘acidification’ and ‘deoxygenation’ alone are subjective and, without adequate contextualization, have the potential to mislead inferences over drivers that may ultimately shape the future state of marine ecosystems. Here we clarify the usage of the terms OA and OD as global, climate change‐driven processes and discuss the various attributes of elevated CO2 and reduced O2 syndromes common to coastal ecosystems. We support the use of the existing terms ‘coastal acidification’ and ‘coastal deoxygenation’ because they help differentiate the sometimes rapid and extreme nature of CO2 and O2 syndromes in coastal ecosystems from the global, climate change‐driven processes of OA and OD. Given the complexity and breadth of the processes involved in altering CO2 and O2 concentrations across marine ecosystems, we provide a workflow to enable contextualization and clarification of the usage of existing terms and highlight the close link between these two gases across spatial and temporal scales in the ocean. These distinctions are crucial to guide effective communication of research within the scientific community and guide policymakers responsible for intervening on the drivers to secure desirable future ocean states.

Additive impacts of deoxygenation and acidification threaten marine biota

“…Unlike other meta‐analyses of the impacts of acidification (Kroeker et al., 2013), we chose not to limit experimental levels of p O 2 and p CO 2 considered in our analysis. We did so because (a) absolute values of p O 2 and p CO 2 vary depending on the coastal ecosystem being tested, (b) p CO 2 values observed for deoxygenated coastal ecosystems often far exceed those for end‐of‐century OA projections for the open ocean (IPCC, 2014; Klein et al., 2020), and (c) a broad range of p O 2 and p CO 2 ratios of experimental conditions was required to test the reliability of the theoretical p O 2 / p CO 2 ‐ratio as a potential indicator of biological responses.…”

“…Within coastal marine ecosystems, deoxygenation and acidification commonly co‐occur when biological respiration outweighs primary productivity to consume O 2 and produce CO 2 (Baumann, Wallace, Tagliaferri, & Gobler, 2015; Cai et al., 2011; Feely et al., 2010). Although many processes have been attributed to the increasing instances of deoxygenation along coastlines (Klein et al., 2020), excessive nutrient input is a dominant cause, which stimulates the production of excess organic matter and ultimately drives deoxygenation and acidification via microbial respiration (Diaz & Rosenberg, 2008; Rabalais et al., 2010; Vaquer‐Sunyer & Duarte, 2008). The various processes that contribute to these coastal syndromes were recently discussed and definitions made available for coastal deoxygenation (CD) and coastal acidification (CA), respectively (see Klein et al., 2020).…”

“…Although many processes have been attributed to the increasing instances of deoxygenation along coastlines (Klein et al., 2020), excessive nutrient input is a dominant cause, which stimulates the production of excess organic matter and ultimately drives deoxygenation and acidification via microbial respiration (Diaz & Rosenberg, 2008; Rabalais et al., 2010; Vaquer‐Sunyer & Duarte, 2008). The various processes that contribute to these coastal syndromes were recently discussed and definitions made available for coastal deoxygenation (CD) and coastal acidification (CA), respectively (see Klein et al., 2020). These newly proposed definitions ultimately acknowledge that deoxygenation and acidification events in coastal zones are often far more extreme than those (Cai et al., 2011; Gobler, Depasquale, Griffith, & Baumann, 2014) expected in the global ocean under end‐of‐century projections for OD and OA (IPCC, 2014; Klein et al., 2020).…”

“…The various processes that contribute to these coastal syndromes were recently discussed and definitions made available for coastal deoxygenation (CD) and coastal acidification (CA), respectively (see Klein et al., 2020). These newly proposed definitions ultimately acknowledge that deoxygenation and acidification events in coastal zones are often far more extreme than those (Cai et al., 2011; Gobler, Depasquale, Griffith, & Baumann, 2014) expected in the global ocean under end‐of‐century projections for OD and OA (IPCC, 2014; Klein et al., 2020).…”

“…Carbon dioxide (CO 2 ) and oxygen (O 2 ) are stoichiometrically linked in the biosphere, owing to the tight coupling between these two gases in anthropogenic combustion and metabolic processes (Bender, Battle, & Keeling, 1998). In the ocean, this link is exemplified by the concurrent, long‐term processes of ocean acidification (OA) and ocean deoxygenation (OD; see Klein, Steckbauer, & Duarte, 2020). OD is primarily caused by reduced O 2 solubility and greater metabolic demand that occurs with warming (but see: Keeling, Körtzinger, & Gruber, 2010; Regaudie‐de‐Gioux & Duarte, 2012; Schmidtko, Stramma, & Visbeck, 2017), whereas OA is primarily driven by the uptake of rising CO 2 from the atmosphere.…”