Assessing the impacts of simulated Ocean Alkalinity Enhancement on viability and growth of near-shore species of phytoplankton

Abstract. Over the past 250 years, atmospheric CO2 concentrations have risen steadily from 277 ppm to 405 ppm, driving global climate change. In response, new technologies are being developed to reduce emissions and to remove carbon from the atmosphere using negative emission technologies (NETs). One proposed NET is Ocean Alkalinity Enhancement (OAE), which would mimic the ocean’s natural weathering processes, raising alkalinity and pH and sequestering carbon dioxide from the atmosphere. The potential impacts of OAE were assessed through an analysis of prior studies investigating the effects of elevated pH on phytoplankton growth rates and by experimental assessment of the pH-dependence of viability and growth rates in two near-shore isolates of phytoplankton. Viability was assessed with a modified Serial Dilution Culture – Most Probable Number assay. Chlorophyll a fluorescence was used to test for changes in photosynthetic competence and apparent growth rates. There were no significant impacts on the viability or growth rates of the diatom Thalassiosira pseudonana and the prymnesiophyte Diacronema lutheri (formerly Pavlova lutheri) with short-term (10-minute) exposure to elevated pH. However, there was a significant decrease in growth rates with long-term (days) exposure to elevated pH. Short-term exposure is anticipated to more closely mirror the natural systems in which OAE will be implemented because of system flushing and dilution. These preliminary findings suggest that there will be little to no impact on a variety of taxonomic groups of phytoplankton when OAE occurs in naturally flushed systems.