Hypoxia and Acidification Have Additive and Synergistic Negative Effects on the Growth, Survival, and Metamorphosis of Early Life Stage Bivalves

Abstract

Low oxygen zones in coastal and open ocean ecosystems have expanded in recent decades, a trend that will accelerate with climatic warming. There is growing recognition that low oxygen regions of the ocean are also acidified, a condition that will intensify with rising levels of atmospheric CO2. Presently, however, the concurrent effects of low oxygen and acidification on marine organisms are largely unknown, as most prior studies of marine hypoxia have not considered pH levels. We experimentally assessed the consequences of hypoxic and acidified water for early life stage bivalves (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria), marine organisms of significant economic and ecological value and sensitive to climate change. In larval scallops, experimental and naturally-occurring acidification (pH, total scale = 7.4–7.6) reduced survivorship (by >50%), low oxygen (30–50 µM) inhibited growth and metamorphosis (by >50%), and the two stressors combined produced additively negative outcomes. In early life stage clams, however, hypoxic waters led to 30% higher mortality, while acidified waters significantly reduced growth (by 60%). Later stage clams were resistant to hypoxia or acidification separately but experienced significantly (40%) reduced growth rates when exposed to both conditions simultaneously. Collectively, these findings demonstrate that the consequences of low oxygen and acidification for early life stage bivalves, and likely other marine organisms, are more severe than would be predicted by either individual stressor and thus must be considered together when assessing how ocean animals respond to these conditions both today and under future climate change scenarios.

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

“…Measured traits included survival, metabolism, growth, activity and metamorphosis in either larvae/juveniles or adults. In larval bay scallops (Argopecten irradians) and juvenile hard clams (Mercenaria mercenaria) survival and growth were sensitive to low DO, low pH and low DO Â low pH [23]. While low DO conditions (1.0-1.6 mg l 21 ) reduced larval scallop growth but not survival, low pH conditions (7.65-7.70) reduced survival but had no effect on growth.…”

“…Five independent studies so far have assessed a total of six commercially and ecologically important mollusc species, including four bivalves [23,29,30] and two gastropods [27,28]. Measured traits included survival, metabolism, growth, activity and metamorphosis in either larvae/juveniles or adults.…”

“…Synergistic negative effects of low DO and low pH (high CO 2 ) on different traits and marine taxa. (a) Synergistic decrease in respiration rate in small and big sea urchins [27]; (b) growth rate of juvenile quahog was unaffected by low DO or low pH individually, but decreased under combined stressor conditions [23]; (c) survival of Atlantic silverside larvae to 10 dph. Survival was robust against low pH and sensitive to low DO, but decreased synergistically under combined stressors (green arrow, [22]); (d) representation of Pörtners [25] 'Oxygen-and capacity-limited thermal tolerance' framework, adapted to the multiple stressor scenario of acidification and hypoxia.…”

“…In contrast, coastal zone management plans rarely, if ever, set target pH or pCO 2 levels or base DO criteria on the concurrent effects of acidification, despite the fact that many estuaries experience acidification at levels and durations known to be harmful to marine life. Given that concurrent exposure to acidification and hypoxia reduces the survival of fish and shellfish below the survival observed from hypoxia alone [22,23], it is likely that water quality standards based on DO alone have made marine fisheries more vulnerable to population reductions than appreciated. While there is currently not enough information to make quantitative recommendations regarding the precise changes that should be made to existing DO criteria to better protect ocean animals against these two stressors, raising DO standards to higher minimums to accommodate concurrent low pH would be one managerial approach to address this issue.…”

Community-Level Actions that Can Address Ocean Acidification

“…This category includes restoration activities and plans that account for multiple stressors, such as hypoxia, nutrient runoff, and temperature change, which chemically enhance ocean acidification (Cai et al, 2011) or synergistically challenge marine organisms (Poertner, 2010;Gobler et al, 2014). In some locations, actions may even already be under way to address other issues (e.g., hypoxia, nutrient runoff) that coincidentally enhance the resilience of marine resources at risk from ocean acidification.…”