Hypoxia and acidification in ocean ecosystems: coupled dynamics and effects on marine life

Abstract

There is increasing recognition that low dissolved oxygen (DO) and low pH conditions co-occur in many coastal and open ocean environments. Within temperate ecosystems, these conditions not only develop seasonally as temperatures rise and metabolic rates accelerate, but can also display strong diurnal variability, especially in shallow systems where photosynthetic rates ameliorate hypoxia and acidification by day. Despite the widespread, global co-occurrence of low pH and low DO and the likelihood that these conditions may negatively impact marine life, very few studies have actually assessed the extent to which the combination of both stressors elicits additive, synergistic or antagonistic effects in marine organisms. We review the evidence from published factorial experiments that used static and/or fluctuating pH and DO levels to examine different traits (e.g. survival, growth, metabolism), life stages and species across a broad taxonomic spectrum. Additive negative effects of combined low pH and low DO appear to be most common; however, synergistic negative effects have also been observed. Neither the occurrence nor the strength of these synergistic impacts is currently predictable, and therefore, the true threat of concurrent acidification and hypoxia to marine food webs and fisheries is still not fully understood. Addressing this knowledge gap will require an expansion of multi-stressor approaches in experimental and field studies, and the development of a predictive framework. In consideration of marine policy, we note that DO criteria in coastal waters have been developed without consideration of concurrent pH levels. Given the persistence of concurrent low pH-low DO conditions in estuaries and the increased mortality experienced by fish and bivalves under concurrent acidification and hypoxia compared with hypoxia alone, we conclude that such DO criteria may leave coastal fisheries more vulnerable to population reductions than previously anticipated.

Exposure to elevated pCO2 does not exacerbate reproductive suppression of Aurelia aurita jellyfish polyps in low oxygen environments

“…In laboratory settings, hypoxia is usually created by sparging seawater with N 2 gas to displace oxygen. An artifact of this method is that pH is elevated (up to 8.6) due to simultaneous displacement of CO 2 , creating different conditions to those that occur in the field , Gobler & Baumann 2016. Therefore, studies of hypoxia that have sparged seawater with N 2 (Condon et al 2001, Decker et al 2004, Grove & Breitburg 2005, Miller & Graham 2012 may have produced results that are inconsistent with the response of biota in the natural environment.…”

“…This pattern also manifests as metabolic responses (Steckbauer et al 2015). According to the conceptual model presented by Gobler & Baumann (2016) (originally proposed by Pörtner et al 2005), aerobic performance (within a thermal range) will be reduced in hypoxic environments, and further reduced in environments with both low oxygen and elevated pCO 2 . Indeed, combining reduced pH with hypoxia caused both additive and synergistic reduction in respiration compared to hypoxia alone across a suite of invertebrate taxa (Steckbauer et al 2015).…”

“…(Brewer & Peltzer 2009, Cai et al 2011, Wallace et al 2014, Gobler & Baumann 2016. Reduced pH conditions (i.e.…”

You Better Repeat It: Complex CO2 × Temperature Effects in Atlantic Silverside Offspring Revealed by Serial Experimentation

“…Given their co-occurrence in nature, physiological responses to elevated CO 2 and low DO are likely connected. Intermediate CO 2 exposure can elicit important adaptive responses which may mediate sublethal effects of low DO [84], yet more extreme exposures may act synergistically to elevate stressor sensitivity [85]. Thus, factorial CO 2 × DO × temperature experiments would be insightful for more robust characterizations of coastal climate effects on fish early life stages.…”

Diurnal Fluctuations in Acidification and Hypoxia Reduce Growth and Survival of Larval and Juvenile Bay Scallops (Argopecten irradians) and Hard Clams (Mercenaria mercenaria)

“…In shallow, well-mixed estuaries with high rates of respiration, hypoxia and acidification can occur diurnally, as photosynthesis during the day results in high DO and pH levels, while respiration during night decrease DO and pH levels. The intensity of this process can be related to the depth of the water column, given that sediments are known to be strong sources of CO 2 and sinks of O 2 (Green and Aller, 1998) and their influence is inversely proportional to the depth of the water column (Gobler and Baumann, 2016). Within deeper, stratified ecosystems including oxygen minimum zones (OMZs), hypoxic and acidified water can persist for weeks, months, or longer, as benthic and/or deep water respiration continually consumes oxygen and produces CO 2 faster than it is replenished via diffusion and mixing from surface waters (Paulmier et al, 2011;Wallace et al, 2014).…”