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Abstract The Gulf of Maine holds significant ecological and economic value for fisheries and communities in north-eastern North America. However, there is apprehension regarding its vulnerability to the effects of increasing atmospheric CO₂. Substantial recent warming and the inflow of low alkalinity waters into the Gulf of Maine have raised concerns about the impact of ocean acidification on resident marine calcifiers (e.g. oysters, clams, mussels). With limited seawater pH records, the natural variability and drivers of pH in this region remain unclear. To address this, we present coastal water pH proxy records using boron isotope (δ¹¹B) measurements in long-lived, annually banded, crustose coralline algae (1920–2018 CE). These records indicate seawater pH was low (~ 7.9) for much of the last century. Contrary to expectation, we also find that pH has increased (+ 0.2 pH units) over the past 40 years, despite concurrent rising atmospheric CO₂. This increase is attributed to an increased input of high alkalinity waters derived from the Gulf Stream. This delayed onset of ocean acidification is cause for concern. Once ocean circulation-driven buffering effects reach their limit, seawater pH decline may occur swiftly. This would profoundly harm shellfisheries and the broader Gulf of Maine ecosystem. 

Modern calcifying marine organisms face numerous environmental stressors, including overfishing, deoxygenation, increasing ocean temperatures, and ocean acidification (OA). Coastal marine settings are predicted to become warmer and more acidic in coming decades, heightening the risks of extreme events such as marine heat waves. Given these threats, it is important to understand the vulnerabilities of marine organisms that construct their shells from calcium carbonate, which are particularly susceptible to warming and decreasing pH levels. To investigate the response of four commercially relevant bivalve species to OA and differing temperatures, juvenileMercenaria mercenaria(hard shell clams), juvenileMya arenaria(soft shell clams), adult and juvenileArctica islandica(ocean quahog), and juvenilePlacopecten magellanicus(Atlantic sea scallops) were grown in varying pH and temperature conditions. Species were exposed to four controlled pH conditions (7.4, 7.6, 7.8, and ambient/8.0) and three controlled temperature conditions (6, 9, and 12°C) for 20.5 weeks and then shell growth and coloration were analyzed. This research marks the first direct comparison of these species’ biological responses to both temperature and OA conditions within the same experiment. The four species exhibited varying responses to temperature and OA conditions. Mortality rates were not significantly associated with pH or temperature conditions for any of the species studied. Growth (measured as change in maximum shell height) was observed to be higher in warmer tanks for all species and was not significantly impacted by pH. Two groups (juvenileM.arenariaand juvenileM.mercenaria) exhibited lightening in the color of their shells at lower pH levels at all temperatures, attributed to a loss of shell periostracum. The variable responses of the studied bivalve species, despite belonging to the same phylogenetic class and geographic region, highlights the need for further study into implications for health and management of bivalves in the face of variable stressors. 

The distribution of dissolved iodine in seawater is sensitive to multiple biogeochemical cycles, including those of nitrogen and oxygen. The iodine-to-calcium ratio (I/Ca) of marine carbonates, such as bulk carbonate or foraminifera, has emerged as a potential proxy for changes in past seawater oxygenation. However, the utility of the I/Ca proxy in deep-sea corals, natural archives of seawater chemistry with wide spatial coverage and radiometric dating potential, remains unexplored. Here, we present the first I/Ca data obtained from modern deep-sea corals, specifically scleractinian and bamboo corals, collected from the Atlantic, Eastern Pacific, and Southern Oceans, encompassing a wide range of seawater oxygen concentrations (10–280 μmol/kg). In contrast to thermodynamic predictions, we observe higher I/Ca ratios in aragonitic corals (scleractinian) compared to calcitic corals (bamboo). This observation suggests a strong biological control during iodate incorporation into deep-sea coral skeletons. For the majority of scleractinian corals, I/Ca exhibits a covariation with local seawater iodate concentrations, which is closely related to seawater oxygen content. Scleractinian corals also exhibit notably lower I/Ca below a seawater oxygen threshold of approximately 160 μmol/kg. In contrast, no significant differences in I/Ca are found among bamboo corals across the range of oxygen concentrations encountered (15–240 μmol/kg). In the North Atlantic, several hydrographic factors, such as temperature and/or salinity, may additionally affect coral I/Ca. Our results highlight the potential of I/Ca ratios in deep-sea scleractinian corals to serve as an indicator of past seawater iodate concentrations, providing valuable insights into historical seawater oxygen levels.