Sensitivity to ocean acidification parallels natural pCO ₂ gradients experienced by Arctic copepods under winter sea ice

Abstract

The Arctic Ocean already experiences areas of low pH and high CO 2 , and it is expected to be most rapidly affected by future ocean acidification (OA). Copepods comprise the dominant Arctic zooplankton; hence, their responses to OA have important implications for Arctic ecosystems, yet there is little data on their current under-ice winter ecology on which to base future monitoring or make predictions about climate-induced change. Here, we report results from Arctic under-ice investigations of copepod natural distributions associated with late-winter carbonate chemistry environmental data and their response to manipulated pCO 2 conditions (OA exposures). Our data reveal that species and life stage sensitivities to manipulated OA conditions were correlated with their vertical migration behavior and with their natural exposures to different pCO 2 ranges. Vertically migrating adult Calanus spp. crossed a pCO 2 range of >140 μatm daily and showed only minor responses to manipulated high CO 2 . Oithona similis, which remained in the surface waters and experienced a pCO 2 range of <75 μatm, showed significantly reduced adult and nauplii survival in high CO 2 experiments. These results support the relatively untested hypothesis that the natural range of pCO 2 experienced by an organism determines its sensitivity to future OA and highlight that the globally important copepod species, Oithona spp., may be more sensitive to future high pCO 2 conditions compared with the more widely studied larger copepods.climate change | diel vertical migration | ecophysiology | pH response O cean acidification (OA) has been highlighted as one of the most pervasive human impacts on the ocean (1). However, observational datasets that link oceanic carbonate chemistry with biotic responses on which to ground predictions of OA impacts remain limited, especially in the most susceptible and rapidly changing ocean, the Arctic (2). Recent observations indicate that several locations in the Arctic already experience seasonal undersaturation with respect to aragonite, concomitantly with elevated pCO 2 and lowered pH conditions (3), and such incidences are predicted to increase as OA progresses (4). However, knowledge of current seasonal and interannual variability in carbonate system parameters for the Arctic Ocean, particularly under winter sea ice, remains limited. Furthermore, information about the ecology of organisms that live in Arctic waters is primarily restricted to summer studies, with only a few investigations being conducted during the ice-covered winter period (5). Hence, predicting how organisms and ecosystems respond to OA is currently restricted to studies from subarctic and/or icefree Arctic systems because of the technical difficulties and costs involved in sampling remote ice-associated Arctic locations. Given that the Arctic is recognized as a "bellwether" for global OA processes (2) and that polar species are potentially more sensitive to these changes due to their reduced metabolic scope (6), this lack of data on Arctic under...

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