Abstract
Diurnal variations in pH and dissolved oxygen (DO) concentrations are common seasonal phenomena in many eutrophic estuaries, yet few studies have investigated the concurrent effects of low pH and low DO on marine organisms inhabiting these coastal systems. Here, we assess the effects of diurnal variations in pH and DO on the early-life history of two bivalve species native to Northeast US estuaries, the bay scallop (Argopecten irradians) and hard clam (Mercenaria mercenaria). In one set of experiments, larval- and juvenile-life stage bivalves were exposed to ambient conditions (pHT ~ 7.9), two continuously-low pH levels (pHT ~ 7.3 and 7.6), and diurnal fluctuations between the ambient and low conditions yielding mean pH levels equal to the intermediate pH levels. In a second set of experiments, larval bivalves were exposed to ambient conditions (pHT ~7.9, DO ~ 7 mg L-1), two levels of low pH and DO (pHT ~ 7.2, DO ~1 mg L-1; pHT ~7.4, DO ~ 4 mg L-1) and diurnal fluctuations of both pH and DO between the ambient and low pH/DO levels that resulted in mean pH and DO levels equal to the intermediate pH and DO levels. Diurnal acidification treatments with ambient DO levels yielded survival rates for both species at both life stages that were consistent with the survival of individuals exposed to the same mean level of chronic pH with juveniles being more resistant to acidification than larvae. In contrast, when both pH and DO varied diurnally, the survival rates of larval bivalves were significantly lower than the survival of individuals chronically exposed to the same mean levels of pH and DO, an indication that bivalves were physiologically more vulnerable to concurrent fluctuations of both parameters compared to acidification alone. While both species displayed sensitivities to diurnal fluctuations in pH and DO, scallops were relatively more susceptible than hard clams. Since many shallow eutrophic estuaries presently experience diurnal cycles of both pH and DO when early-life stages of bivalves are present in estuaries, the populations of the bivalves studied are likely impacted by these conditions which are likely to intensify with climate change.
Highlights
Two consequences of climate change are acidification and deoxygenation of world oceans (Doney et al, 2012)
For the second set of experiments, pH and dissolved oxygen (DO) levels were altered in unison and four treatments were established: A control treatment, a chronic-intermediate pH-DO treatment (∼7.5, ∼4.0 mg L−1), a chronic-low pH-DO treatment (∼7.2, ∼2.0 mg L−1), and a diurnally-fluctuating pH-DO treatment that resulted in the control pH and DO levels by day and the low pH and DO levels at night
Survival of larval A. irradians was significantly reduced by low pH
Summary
Two consequences of climate change are acidification and deoxygenation of world oceans (Doney et al, 2012). Beyond anthropogenically-induced changes in climate, acidification, and low oxygen conditions are inextricably linked in the ocean via the processes of respiration and photosynthesis and often display highly similar patterns in marine ecosystems (Feely et al, 2010; Cai et al, 2011; Wallace et al, 2014). In urbanized regions receiving excessive nutrient loads (Melzner et al, 2012; Wallace et al, 2014) or within eutrophied river plumes (Cai et al, 2011) hypoxia and acidification can occur at extreme levels. Beyond the role of excessive nutrient loads in driving this trend, specific estuarine habitats such as salt marshes are naturally enriched in organic carbon that when respired can create hypoxic and acidified conditions, within warmer waters (Ringwood and Keppler, 2002; Baumann et al, 2014)
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