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 p CO 2 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 6 of 7 experiments, exposure to elevated p CO 2 levels (~ 1,700 µatm) resulted in depressed shell- and/or tissue-based growth rates of bivalves compared to control conditions ( p Ulva in all experiments ( p p CO 2 levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated p CO 2 levels significantly improved their performance compared to the acidification only treatment ( p Ulva under both ambient and elevated CO 2 delivery rates ( p 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 p CO 2 levels and depressed Ω.