Testing the response of natural plankton community to ocean alkalinity enhancement in the subtropical North Atlantic Ocean

Ocean alkalinity enhancement (OAE) is one approach under investigation to remove carbon dioxide (CO₂) from the atmosphere and durably sequester it in the ocean. Currently, little is known about possible biogeochemical or ecological changes that may result from this increase in seawater alkalinity from experimental data. To address this gap, we carried out a in situ mesocosm experiment to investigate how a plankton community in the subtropical eastern North Atlantic Ocean responds to enhancement of ocean alkalinity over a 33 day study period. A gradient of nine alkalinity treatments ranging from ambient (~2400 µmol kg^-1) to 2x ambient (~4800 µmol kg^-1) was implemented where the seawater CO₂ was already equilibrated with atmospheric CO₂. Here, we focus on changes in the biogeochemical element pools to determine if organic matter partitioning and nutrient cycling may be sensitive changes in seawater chemistry induced by deployment of OAE in an oligotrophic plankton community. Overall, only 3 out of 15 sampled biogeochemical pools displayed measurable changes. Nitrogen turnover processes in the surface ocean appear to be more susceptible than other elements to OAE as two of the impacts were on nitrogen-related pools and a significant phytoplankton bloom (3.5 – 5 µg L^-1) occurred in selected mesocosms where alkalinity was enhanced despite nitrate-limited growth in primary producers. However, overall this study suggests that as long as no additional nutrients are added (silicate, nitrogen, trace metals) in the process of enhancing seawater alkalinity, and the carbon is already sequestered (equilibrated with atmospheric pCO₂), the risk of negative impacts on primary producer biomass and biogeochemical functioning, appears to be low on time scales of up to 30 days, even with a doubling of seawater alkalinity.