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1. Development of the boron isotope pH proxy in Arctica islandica shells

   Wanamaker, A.; Thatcher, D.; Stewart, J.; Williams, B.; Franklin, H.; Guay, K.; Jellison, B.; Whitney, N.; McMahon, T.; Jarosinski, L.; et al (May 2023, International Sclerochronology Conference)

   Coastal systems can exhibit large variability in pH compared to open marine conditions, thus the impacts of ocean acidification (OA) on their resident calcifying organisms are potentially magnified. Further, our understanding of the natural baseline and variability of pH is spatially and temporally limited in coastal settings. In the few coastal locations that have been monitoring seawater pH, records are generally limited to <10 years and are thus unable to provide the full range of centennial to decadal natural variability. This is the case for the Gulf of Maine (northwestern Atlantic), a highly productive region of strategic importance to U.S. fisheries, that is facing multiple environmental stressors including rapid warming and threats from OA. Paleoceanographic proxy records are therefore much needed in this region to reconstruct past pH conditions beyond instrumental records. A clear candidate for this is the boron isotope (d11B) pH proxy provided the d11B sensitivity to pH in long-lived shallow water marine carbonates can be established. To this end, we grew juvenile and adult Arctica islandica (ocean quahog) in flowing seawater tanks for 20.5 weeks in controlled pH (7.4, 7.6, 7.8 or 8.0 (ambient) ± 0.02) and temperature (6, 9 or 12 ± 0.56 °C) conditions at Bowdoin College’s Schiller Coastal Studies Center, Harpswell, Maine (USA). The clams were stained twice with calcein and supplemented with food (Shellfish Diet) throughout the experiment to ensure suitable growth. New shell growth (average 67% increase in maximum shell height and 522% increase in buoyant weight across all treatments), constrained by calcein markings, were sampled for boron isotope analysis (d11B) to determine if shell d11B varied as a function of pH similar to many other calcifying organisms. The results of the culture experiment will yield whether or not Arctica islandica preserves seawater pH information in their shells. If so, the transfer function relating shell d11B to pH will be used to hindcast pH in the central coastal region of the Gulf of Maine during recent centuries. Alternatively, if the shell d11B signal is independent of ambient seawater pH, this may reveal the capacity of Arctica islandica to regulate internal calcifying fluid chemistry and their resilience to OA. 

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2. Documenting Environmental and Oceanic Variability in the Down East Coastal Region of the Gulf of Maine using the long-lived bivalve Arctica islandica

   Walton, A.; Griffin, S.M.; Wanamaker, A.D.; Jarosinski, L. (April 2022, PAGES Open Science Meeting)

   The Gulf of Maine (GoM) is currently undergoing rapid environmental and ecological changes and this system is projected to become increasingly stressed in the coming decades. Thus understanding past spatial and temporal conditions of this region is key to understanding how future environmental changes and extreme events may impact fisheries and ecosystem dynamics in the GoM. Changes in the physical and chemical variations in the shells of mollusks can be used as a powerful proxy for studying past climates and environments. In this study, we used the growth and geochemistry signatures in the long-lived marine bivalve Arctica islandica collected from the Down East coastal region in the Gulf of Maine (Jonesport, ME) to evaluate past climatic and hydrographic variability in the northwestern North Atlantic Ocean. The recent collection of shells extends a previously developed master shell growth chronology by 11 years and now spans from 1954 to 2020 CE. Based on visual crossdating techniques, shell growth variability is highly coherent among the population indicating that environmental conditions are driving growth. Variability in annually resolved shell growth increments and stable oxygen isotope values are largely related to sea surface temperatures (SSTs) and water mass properties of the Eastern Maine Coastal Current. This master shell growth chronology and annually-resolved isotope series will fill data gaps prior to instrumental records and allow us to better understand the spatial oceanographic variability in the GoM. 

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3. Delayed onset of ocean acidification in the Gulf of Maine

   https://doi.org/10.1038/s41598-024-84537-3  

   Stewart, Joseph_A; Williams, Branwen; LaVigne, Michèle; Wanamaker, Alan_D; Strong, Aaron_L; Jellison, Brittany; Whitney, Nina_M; Thatcher, Diana_L; Robinson, Laura_F; Halfar, Jochen; et al (January 2025, Scientific Reports)

   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. 

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4. Temperature and pH Effects on Growth of Four Species of Commercially-Important Bivalves from Tank Experiments

   Thatcher, D.; McMahon, T.; Whitney, N.; Guay, K.; Franklin, H.; Stewart, J.; Williams, B.; Wanamaker, A.; LaVigne, M. (December 2022, American Geophysical Union Fall Meeting)

   The Gulf of Maine is a highly productive and economically important region in the northwestern Atlantic that has undergone rapid warming in recent decades and is susceptible to ocean acidification (OA). These stressors may have substantial impacts on local fisheries. Therefore, understanding the combined effects of warming and OA to commercially important shellfish is vital. To test responses to warming and OA, Mercenaria mercenaria (hard clam), Mya arenaria (soft-shell clam), Plactopectin magellanicus (sea scallop), and both juvenile and adult Arctica islandica (ocean quahog) were grown in flowing seawater tanks for 20.5 weeks in controlled pH (7.4, 7.6, 7.8 or 8.0 (ambient) ± 0.02) and temperature (6, 9 or 12 ± 0.56 °C) conditions at Bowdoin College’s Schiller Coastal Studies Center. The specimens’ diet was supplemented with high-quality food (Shellfish Diet) throughout the experiment. Temperature effects were a significant contributor in all shell growth metrics (maximum height, dry weight and buoyant weight) in all species except the height and dry weight of adult A. islandica. Additionally, pH effects were significant in the height of M. mercenaria and in the dry weight of juvenile A. islandica samples. Overall, mortality rates ranged from 1.5% in juvenile A. islandica to 24% in M. mercenaria, with results varying by species and treatment conditions. Additionally, differences in final shell condition were noted among the various treatments indicating that, although most of the organisms survived and grew, the elevated temperature and/or lower pH conditions might not have been ideal for thriving. Considering all results of growth and survival, the four species showed a differential response to the same warming and acidification conditions. As suggested by prior research, the availability of high-quality food may allow certain species to tolerate the future warming and/or OA conditions modeled in this experiment. Experimental results may reveal the species-specific resiliency of economically valuable shellfish to changing ocean conditions as well as guide future planning to safeguard regional ecosystems and fisheries. 

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5. Contrasting responses of commercially important Northwest Atlantic bivalve species to ocean acidification and temperature conditions

   https://doi.org/10.1371/journal.pclm.0000509  

   McMahon, Teagan; Thatcher, Diana; Williams, Branwen; Wanamaker, Alan; Jellison, Brittany; Franklin, Heidi; Guay, Katherine; Whitney, Nina M; Stewart, Joseph A; LaVigne, Michèle (November 2024, PLOS Climate)

   Cyr, Frédéric (Ed.)

   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. 

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