Modeling ocean acidification in the California Current System

Ocean acidification is a global phenomenon, triggered by the oceanic uptake of anthropogenic CO2. Regions with a naturally low pH and aragonite saturation state (Ωarag), such as the high latitudes and Eastern Boundary Upwelling Systems (EBUS), are prone to become undersaturated with respect to Ωarag within a few decades. Low resolution global models fail to simulate the seasonal upwelling of dissolved inorganic carbon (DIC) enriched waters close to the coast and do not resolve physical mesoscale activities that dictate the carbon chemical patterns. These models therefore miss the acuteness of ocean acidification in EBUS. To fill this gap, I use a 5 km resolution U.S. West Coast set-up of the Regional Oceanic Modeling System coupled to a simple ecosystem model to study the progression of ocean acidification in the California Current System (CCS), one of the four major EBUS. A preindustrial simulation was conducted to study the natural carbon chemistry dynamics of this region. Results show that even before anthropogenic CO2 perturbed the carbon chemistry of the CCS, ∼84% of the benthic ecosystems on the continental shelf off northern and central California experienced about one short (∼6 days) aragonite undersaturation event per year (Ωarag>0.96). To compare these results with present day conditions and future projections a transient simulation from 1995 to 2050 was conducted. While atmospheric pCO2 and DIC followed the IPCC SRES ”business as usual” scenario, aspects of climate change, such as increased temperature and winds were not taken into account. Under presentday conditions, the aragonite saturation horizon (Ωarag=1) is located at around 200 m, ∼100 m higher than during the preindustrial era. During strong upwelling events, the aragonite saturation horizon shoals up to the surface in the central CCS, which is supported by recent observations. Despite the high temporal variability of pH and Ωarag, the central and northern subregions of the CCS have already moved out of their preindustrial natural variability envelope of pH and Ωarag. Moreover, aragonite undersaturation events on the continental shelf affect the whole studied region, quadrupled in number and have become longer than during the preindustrial era. They last anywhere between a few days up to a year. Future projections show that ocean acidification progresses at high speed in the CCS. By 2030 and 2035 respectively, nearshore aragonite of the northern and central CCS subregions are projected to move out of their present-day variability envelope. The aragonite saturation horizon is projected to shoal into the upper 75 m of the water column beginning in 2025 in the central and 2035 in the northern subregion. Due to the greater exposure to aragonite undersaturation, benthic organisms are likely already negatively influenced today. In view of the fast approaching continuous and 10 times higher intensity of aragonite undersaturation, the chemical threat to economically important aragonite calcifiers such as oysters, clams and mussels will be substantially amplified. Changes in the structure of the ecosystems will lead to unknown feedbacks into fisheries, global biogeochemical cycles and climate.