The pteropod Limacina helicina frequently experiences seasonal exposure to corrosive conditions (Ωar  < 1) along the US West Coast and is recognized as one of the species most susceptible to ocean acidification (OA). Yet, little is known about their capacity to acclimatize to such conditions. We collected pteropods in the California Current Ecosystem (CCE) that differed in the severity of exposure to Ωar conditions in the natural environment. Combining field observations, high-CO2 perturbation experiment results, and retrospective ocean transport simulations, we investigated biological responses based on histories of magnitude and duration of exposure to Ωar < 1. Our results suggest that both exposure magnitude and duration affect pteropod responses in the natural environment. However, observed declines in calcification performance and survival probability under high CO2 experimental conditions do not show acclimatization capacity or physiological tolerance related to history of exposure to corrosive conditions. Pteropods from the coastal CCE appear to be at or near the limit of their physiological capacity, and consequently, are already at extinction risk under projected acceleration of OA over the next 30 years. Our results demonstrate that Ωar exposure history largely determines pteropod response to experimental conditions and is essential to the interpretation of biological observations and experimental results.

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Declines in mean ocean pH and aragonite saturation state (ΩA) driven by anthropogenic CO2 emissions have raised concerns regarding the trends of pH and ΩA in estuaries. Low pH and ΩA can be harmful to a variety of marine organisms, especially those with calcium carbonate shells, and so may threaten the productive ecosystems and commercial fisheries found in many estuarine environments. The Strait of Georgia is a large, temperate, productive estuarine system with numerous wild and aquaculture shellfish and finfish populations. We determine the seasonality and variability of near‐surface pH and ΩA in the Strait using a one‐dimensional, biophysical, mixing layer model. We further evaluate the sensitivity of these quantities to local wind, freshwater, and cloud forcing by running the model over a wide range of scenarios using 12 years of observations. Near‐surface pH and ΩA demonstrate strong seasonal cycles characterized by low pH, aragonite‐undersaturated waters in winter and high pH, aragonite‐supersaturated waters in summer. The aragonite saturation horizon generally lies at ∼20 m depth except in winter and during strong Fraser River freshets when it shoals to the surface. Periods of strong interannual variability in pH and aragonite saturation horizon depth arise in spring and summer. We determine that at different times of year, each of wind speed, freshwater flux, and cloud fraction are the dominant drivers of this variability. These results establish the mechanisms behind the emerging observations of highly variable near‐surface carbonate chemistry in the Strait.

Abstract. Wind-driven upwelling is an important control on surface nutrients and water properties in stratified lakes and seas. In this study, a high-resolution biophysical coupled model is used to investigate upwelling in the Strait of Georgia on the Canadian Pacific coast. The model is forced with surface winds from a high-resolution atmospheric forecast and has been tuned in previous studies to reproduce extensive observations of water level, temperature, salinity, nutrients and chlorophyll with competitive skill relative to similar models of the study region. A total of 5 years of hourly surface nitrate and temperature fields are analyzed in order to characterize the dominant upwelling patterns of the basin. A prevailing along-axis wind pattern steered by mountainous topography produces episodic upwelling along the western shore during the spring and fall southeasterlies and along the eastern shore during the summer northwesterlies, as indicated by positive nitrate anomalies. Principal component analysis reveals that these cross-axis upwelling patterns account for nearly one-third of the surface nitrate variance during the summer productive season. By contrast, nearly half of the surface temperature variance over the same period is dominated by a single, combined mixing and diurnal heating–cooling pattern. The principal components associated with these patterns correlate with along-axis wind stress in a manner consistent with these physical interpretations. The cross-axis upwelling response to wind is similar to other dynamically wide basins where the baroclinic Rossby deformation radius is smaller than the basin width. However, the nitrate anomaly during upwelling along the eastern shore is stronger in the northern basin, which may be indicative of an along-axis pycnocline tilt or an effect of the background along-axis stratification gradient due to the Fraser River. Our findings highlight an important spatiotemporal consideration for future ecosystem monitoring.