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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. 

Abstract. Close coupling of Iberian hydroclimate and North Atlantic seasurface temperature (SST) during recent glacial periods has been identifiedthrough the analysis of marine sediment and pollen grains co-deposited on thePortuguese continental margin. While offering precisely correlatable records,these time series have lacked a directly dated, site-specific record ofcontinental Iberian climate spanning multiple glacial cycles as a point ofcomparison. Here we present a high-resolution, multi-proxy (growth dynamicsandδ¹³C, δ¹⁸O, and δ²³⁴Uvalues) composite stalagmite record of hydroclimate from two caves in westernPortugal across the majority of the last two glacial cycles (∼220ka).At orbital and millennial scales, stalagmite-based proxies for hydroclimateproxies covaried with SST, with elevated δ¹³C,δ¹⁸O, and δ²³⁴U values and/or growth hiatusesindicating reduced effective moisture coincident with periods of lowered SSTduring major ice-rafted debris events, in agreement with changes inpalynological reconstructions of continental climate. While in many cases thePortuguese stalagmite record can be scaled to SST, in some intervals themagnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appearto have been somewhat decoupled from SST.