Assessing the impacts of simulated ocean alkalinity enhancement on viability and growth of nearshore species of phytoplankton

Over the past 250 years, atmospheric CO2 concentrations have risen steadily from 277 to 405 ppm, driving global climate change. In response, new tools are being developed to remove carbon from the atmosphere using negative emission technologies (NETs), in addition to reducing anthropogenic emissions. One proposed NET is ocean alkalinity enhancement (OAE), in which artificially raising the alkalinity favours formation of bicarbonate from CO2, leading to a decrease in the partial pressure of CO2 in the water. Subsequent invasion of atmospheric CO2 results in net sequestration of atmospheric carbon. The aim of this study was to investigate the impact of simulated OAE, through the alteration of pH, on phytoplankton representative of the spring and fall blooms in nearshore, temperate waters. The potential impacts of OAE were assessed through (1) an analysis of prior studies investigating the effects of elevated pH on phytoplankton growth rates and (2) an experimental assessment of the potential impact of short-term (10 min) and long-term (8 d) elevation of pH on the viability and subsequent growth rates of two representative nearshore species 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 min) exposure to elevated pH. However, there was a significant decrease in growth rates with long-term (8 d) exposure to elevated pH. Short-term exposure is anticipated to more closely mirror the natural systems in which land-based OAE will be implemented because of system flushing and dilution. The analysis of prior studies indicates wide variability in the growth response to elevated pH within and between taxonomic groups, with about 50 % of species expected to not be impacted by the pH increase anticipated from unequilibrated mineral-based OAE. To the extent that the growth responses reflect (largely unreported) parallel reductions in dissolved inorganic carbon (DIC) availability, the susceptibility may be reduced for OAE in which CO2 ingassing is not prevented.