JSR Special Section Oa Primer and Introduction

Having watched the flood and ebb of research on shellfish responses to ocean acidification over the past 13 y now, I am struck by the many advances we have made as a community on this topic, as well as the many still outstanding questions we cannot yet conclude. It has been a privilege serving as a guest editor for this compilation of articles. I first need to thank the seven excellent lead authors for their contributions and patience with the process. These articles help bring attention to advances in ocean acidification research among shellfish biologists and ecologists as well as the many important industry partners who support research and the society that produces this journal. Second, the Ocean Acidification Program of the National Oceanic and Atmospheric Administration helped financially support the publication of these articles. Finally, the shellfish industry partners who have been adapting to our changing ocean and engaging policy makers inWashington DC about the threat and responses to ocean acidification. Their work has helped ensure this topic remains as a high priority in legislative and management realms. Despite these research and policy efforts, the collective community is still trying to determine how significant this section may in fact be for both commercially and ecologically important shellfish. Ocean acidification came to the forefront of the U.S. West Coast s oyster industry concerns in the late 2000s (Mabardy et al. 2015) when it became clear that despite decades of successful production of Pacific oyster seed in the region s hatcheries, the ocean s chemistry had begun to change in ways previously not experienced (Barton et al. 2015). Although it is well known that the U.S. Pacific Northwest coast experiences seasonal upwelling, bringing with it nutrientand CO2-rich waters to the nearshore and estuaries, these hatcheries were only able to continue producing oyster seed by chemically buffering the incoming ocean water (Barton et al. 2015). The addition of anthropogenic carbon to those metabolically active waters pushes the conditions more rapidly across what appear to be proximate thresholds for Pacific oyster larvae both in hatcheries (Barton et al. 2015), and in coastal bays and estuaries of the region (Hales et al. 2017, Pacella et al. 2018). Feely et al. (2016) estimated the increase of anthropogenic carbon in the region s nearshore coastal ocean waters to be greater than 2% of the total dissolved inorganic carbon. This seemingly small increase has lowered the median saturation state of the coastal upwelled waters by;0.5 units (Harris et al. 2013) and reduces the ability to buffer natural fluctuations on short time scales (Pacella et al. 2018). It is the amplification of the natural conditions from a changing baseline CO2 that upended the region s oyster hatcheries and because improving the carbonate chemistry has restored a large majority of the previously lost production capacity (Barton et al. 2015). In a longer geologic perspective, our current understanding of the rates of change in marine chemistry due to increasing anthropogenic CO2 are unparalleled in at least the past 1 million years from direct measures of CO2 in ice cores (Higgins et al. 2015) and from the past 66 million years using various proxies (Zeebe et al. 2016). Importantly, it is not the absolute amount of CO2 in the atmosphere that is the section regarding ocean acidification; the increase is faster now than any of the earth s natural systems can counter or buffer that change (Hönisch et al. 2012). In fact, very recent research on the impact of the Chicxulub meteor that is attributed with causing the mass extinction at the Cretaceous–Paleogene boundary (66 million years ago) has been found to have caused similarly ‘‘rapid’’ acidification of the world s oceans (Henehan et al. 2019). The acidification event that occurred was estimated to have been 0.25 pH units decrease over less than 1,000 y, with the result of an equally rapid decline in calcified foraminifera in the fossil record (Henehan et al. 2019). Although the cause of the shift in ocean acid–base chemistry was not believed to be chemically identical to the current anthropogenic source of acidification, the rate of change is within the bounds of what is currently happening. Estimates put the change in oceanic pH at about 0.1 unit in the past 200 y; much of this change occurring in the last 50–70 y, with more rapid changes predicted with our current projections of CO2 emissions. Acknowledging this geologic perspective on ocean acidification should elicit a pause to recognize since the fall of the dinosaurs, the animals we have in our oceans have never experienced a baseline shift in chemistry such as is currently happening. The following is intended to provide a simple primer on carbonate chemistry, and, in particular, how it related to ocean acidification. Others have provided a far more comprehensive treatment of this topic (Zeebe &Wolf-Gladrow 2001); here, the intent is simply to remind the readers of some of the basics. The late Lars Gunar Sillen noted that the ocean s chemistry on geologic timescales results from a balance between acids leaking from the earth s interior and the basic weathering products from rocks on land, maintaining the pH of the ocean at a slightly basic condition. Thus, the reason we focus so much on carbonate chemistry is that it is the acid–base system in greatest concentration in the ocean and with active changes in speciation in the pH range of typical marine waters. In other words, other acid–base systems do not really matter much in most marine waters. Therefore, it is critical to note pH is technically only an indicator of the acid–base status of water; a function of the relative concentrations of linked acid and base systems, which are driven by natural (and anthropogenic) processes. So while one frequently sees plots of pH on an x-axis and the concentrations of inorganic carbon species on a y-axis (a Berjjum plot), it is incorrect to infer that pH is driving the distribution of the dissolved inorganic carbon species (dissolved CO2, bicarbonate, and carbonate ions). The only cases where a system other than the dissolved inorganic carbon system is altering pH in significant ways is where other acids or bases are added to the system in large quantities, such as what appears to have happened from the meteor impact 66 million years ago (Henehan et al. 2019). It would be incorrect, however, to not recognize other systems will alter the acid–base status, but in most cases, only in small degrees relative to the carbonic acid system. So, returning to Dr. Sillen s note, then the pH of the ocean varies roughly as the ratio of CO2 and carbonate ion (CO3 ) varies, on daily time scales from photosynthesis/respiration and calcification/dissolution *Corresponding author. E-mail: george.waldbusser@oregonstate.edu DOI: 10.2983/035.038. 0322 Journal of Shellfish Research, Vol. 38, No. 3, 707–710, 2019.