Novel in situ CO₂ enrichment system reveals seagrass meadows are a refugium against coastal acidification for North Atlantic bivalves

Abstract. While the accumulation of anthropogenic CO₂ in the atmosphere is causing a decline in global ocean pH, many eutrophic estuaries are already experiencing acidification due to accelerated respiration driving the consumption of dissolved oxygen (DO) and production of CO₂, decreasing available carbonate ions (CO₃^2-) and threatening marine calcifiers. Here, a novel in situ CO₂ enrichment system was constructed to examine the effects of coastal acidification on the growth and survival of two species of North Atlantic bivalves (Argopecten irradians and Crassostrea virginica) in two distinct estuarine habitats: a seagrass meadow and an unvegetated sandy bottom in an open water estuary. The in-situ system captured natural diel dynamics as ambient chambers displayed chemistry nearly identical to the surrounding water, while CO₂-enriched, acidified chambers maintained a consistent ~Δ 0.3–0.5 pH offset. At the unvegetated sandy bottom site, A. irradians and C. virginica displayed significant reductions in growth and survival in the acidified chambers (pH_T = 7.3–7.5; saturation state of aragonite, Ω_Ar = 0.6–0.9) relative to ambient conditions (pH_T = 7.6–7.9; Ω_Ar = 1.6–2). At the seagrass site, while growth of A. irradians and C. virginica in the acidified treatments (pH_T = 7.3–7.7; Ω_Ar = 0.7) receiving the same delivery of CO₂ was, again, significantly slowed compared to the control (pH_T = 7.5–8.1; Ω_Ar = 2 – 2.8), the growth reduction, mortality rates, and levels of acidification were attenuated compared to the sandy bottom experiment, evidencing the ability of seagrass to buffer seawater and serve as a potential acidification refuge for bivalves. Collectively, the novel experimental CO₂ enrichment system constructed for this project demonstrates that coastal acidification can have deleterious effects on marine bivalve populations, and that future conditions as well as the habitat refuge offered by seagrasses must be considered when developing management and restoration plans for temperate estuaries.