Abstract
Abstract. Coastal ecosystems can experience acidification via upwelling, eutrophication, riverine discharge, and climate change. While the resulting increases in pCO2 can have deleterious effects on calcifying animals, this change in carbonate chemistry may benefit some marine autotrophs. Here, we report on experiments performed with North Atlantic populations of hard clams (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), bay scallops (Argopecten irradians), and blue mussels (Mytilus edulis) grown with and without North Atlantic populations of the green macroalgae, Ulva. In six of seven experiments, exposure to elevated pCO2 levels (∼1700 µatm) resulted in depressed shell- and/or tissue-based growth rates of bivalves compared to control conditions, whereas rates were significantly higher in the presence of Ulva in all experiments. In many cases, the co-exposure to elevated pCO2 levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated pCO2 levels significantly improved their performance compared to the acidification-only treatment. Saturation states for calcium carbonate (Ω) were significantly higher in the presence of Ulva under both ambient and elevated CO2 delivery rates, and growth rates of bivalves were significantly correlated with Ω in six of seven experiments. Collectively, the results suggest that photosynthesis and/or nitrate assimilation by Ulva increased alkalinity, fostering a carbonate chemistry regime more suitable for optimal growth of calcifying bivalves. This suggests that large natural and/or aquacultured collections of macroalgae in acidified environments could serve as a refuge for calcifying animals that may otherwise be negatively impacted by elevated pCO2 levels and depressed Ω.
Highlights
The continued delivery of CO2 into surface oceans is expected to cause significant shifts in pools of inorganic carbon by the end of this century, with projected increases in CO2 and HCO−3 and decreases in CO23− and the saturation states of calcite and aragonite (Feely et al, 2009; Meehl et al, 2007)
Calcifying organisms are highly vulnerable to the projected shifts in the various pools of total dissolved inorganic carbon (DIC), with the deleterious effects of ocean acidification being well documented for corals (Hoegh-Guldberg et al, 2007; Kleypas et al, 1999), coralline algae (Gao and Zheng, 2010; Martin and Gattuso, 2009), and bivalves (Barton et al, 2012; Gazeau et al, 2007; Talmage and Gobler, 2011)
Gobler: Ability of macroalgae to mitigate the negative effects of ocean acidification performed using SigmaPlot 11.0 to assess significant differences in growth rates based on shell length, tissue weight, shell weight, and survival during experiments, for which the main treatment effects were pCO2 and the presence of Ulva
Summary
The continued delivery of CO2 into surface oceans is expected to cause significant shifts in pools of inorganic carbon by the end of this century, with projected increases in CO2 and HCO−3 and decreases in CO23− and the saturation states of calcite ( calcite) and aragonite ( aragonite) (Feely et al, 2009; Meehl et al, 2007). Eutrophication-enhanced respiration in coastal zones can lead to the accumulation of respiratory CO2 that can exceed concentrations projected for the end of the century (> 2000 μatm), as well as result in the undersaturation of aragonite ( aragonite < 1; Cai et al, 2017; Wallace et al, 2014). Since bivalves provide numerous ecosystem and economic services (Newell, 2004), and elevated pCO2 is a common occurrence in many coastal ecosystems (Feely et al, 2008; Salisbury et al, 2008; Wallace et al, 2014), it is important to understand how other co-occurring
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