ALBERT

Description: Estuaries are recognized as one of the habitats most vulnerable to coastal ocean acidification due to seasonal extremes and prolonged duration of acidified conditions. This is combined with co-occurring environmental stressors such as increased temperature and low dissolved oxygen. Despite this, evidence of biological impacts of ocean acidification in estuarine habitats is largely lacking. By combining physical, biogeochemical, and biological time-series observations over relevant seasonal-to-interannual time scales, this study is the first to describe both the spatial and temporal variation of biological response in the pteropod Limacina helicina to estuarine acidification in association with other stressors. Using clustering and principal component analyses, sampling sites were grouped according to their distribution of physical and biogeochemical variables over space and time. This identified the most exposed habitats and time intervals corresponding to the most severe negative biological impacts across three seasons and three years. We developed a cumulative stress index as a means of integrating spatial-temporal OA variation over the organismal life history. Our findings show that over the 2014–2016 study period, the severity of low aragonite saturation state combined with the duration of exposure contributed to overall cumulative stress and resulted in severe shell dissolution. Seasonally-variable estuaries such as the Salish Sea (Washington, U.S.A.) predispose sensitive organisms to more severe acidified conditions than those of coastal and open-ocean habitats, yet the sensitive organisms persist. We suggest potential environmental factors and compensatory mechanisms that allow pelagic calcifiers to inhabit less favorable habitats and partially offset associated stressors, for instance through food supply, increased temperature, and adaptation of their life history. The novel metric of cumulative stress developed here can be applied to other estuarine environments with similar physical and chemical dynamics, providing a new tool for monitoring biological response in estuaries under pressure from accelerating global change.

Description: Shelled pteropods are widely regarded as bioindicators for ocean acidification, because their fragile aragonite shells are susceptible to increasing ocean acidity. While short-term incubations have demonstrated that pteropod calcification is negatively impacted by ocean acidification, we know little about net calcification in response to varying ocean conditions in natural populations. Here, we examine in situ calcification of Limacina helicina pteropods collected from the California Current Ecosystem, a coastal upwelling system with strong spatial gradients in ocean carbonate chemistry, dissolved oxygen and temperature. Depth-averaged pH ranged from 8.03 in warmer offshore waters to 7.77 in cold CO2-rich waters nearshore. Based on high-resolution micro-CT technology, we showed that shell thickness declined by 37% along the upwelling gradient from offshore to nearshore water. Dissolution marks covered only 2% of the shell surface area and were not associated with the observed variation in shell thickness. We thus infer that pteropods make thinner shells where upwelling brings more acidified and colder waters to the surface. Probably the thinner shells do not result from enhanced dissolution, but are due to a decline in calcification. Reduced calcification of pteropods is likely to have major ecological and biogeochemical implications for the cycling of calcium carbonate in the oceans.