The ability of macroalgae to mitigate the negative effects of ocean acidification on four species of North Atlantic bivalve

Coastal ecosystems can experience acidification via upwelling, eutrophication, riverine discharge, and climate change. While the resulting increases in inline-formulapCO₂ 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 inline-formulapCO₂ levels (inline-formula∼1700 inline-formulaµatm) 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 inline-formulapCO₂ levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated inline-formulapCO₂ levels significantly improved their performance compared to the acidification-only treatment. Saturation states for calcium carbonate (inline-formulaΩ) were significantly higher in the presence of Ulva under both ambient and elevated inline-formulaCO₂ delivery rates, and growth rates of bivalves were significantly correlated with inline-formulaΩ 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 inline-formulapCO₂ levels and depressed inline-formulaΩ.