Assessing the Influence of OAE on Particulate Matter Stoichiometry in the North Sea – Insights from a Mesocosm Study

The natural dissolution of calcium- or silicate-based rock minerals in the ocean increases the alkalinity and enhances the uptake of atmospheric CO₂. Deliberate large-scale addition of such minerals to the surface ocean has been proposed as a promising method to drive negative CO₂ emissions through ocean alkalinity enhancement (OAE), mitigating climate change. However, the environmental safe and sustainable implementation of OAE requires a comprehensive understanding of the potential ecological implications of this marine-based Carbon Dioxide Removal technology. In contributing to this understanding, a 39-day mesocosm experiment was conducted in the temperate-eutrophic waters of the German North Sea off Helgoland, during the spring of 2023. The primary objective was to examine how the intensity of alkalinity and the duration of alkalinity exposure before dilution in a calcium-based non-equilibrated OAE application (elevated pH) affects the pelagic ecology and biogeochemistry during a phytoplankton spring bloom. We simulated alkalinisation via calcium hydroxide through the addition of calcium chloride and sodium hydroxide in total alkalinity (∆TA) increments of 250 µmol kg^-1 (∆TA = 0, 250, 500, 750, 1000, 1250 µmol kg^-1) in one set of six mesocosms (each with a volume of 6 m³). This treatment intended to represent the successful dilution of OAE application through ship-deployment. A second set of six mesocosms was used to simulate a delayed dilution of alkalised waters from a point source. For this, the top layer of these mesocosms was manipulated with twice the amount of TA and mixed with the untreated bottom layer after 48 hours, ultimately leading to the same ∆TA gradient as the immediate dilution treatment. Here, we report on the influence of OAE on phytoplankton bloom dynamics and particulate matter stoichiometry, which are key characteristics of marine ecosystems and carbon cycling. The first results indicate a delay in phytoplankton bloom timing with increasing alkalinity and pH, with no discernible impact of dilution type. Surprisingly, significant differences in Chlorophyll a dynamics at the lowest ∆TA level of 250 were observed in both dilution types. Furthermore, peak concentrations of particulate organic carbon (POC) exhibit a significant decrease with increasing ∆TA and pH in the delayed dilution treatment, particularly evident in the two highest ∆TA treatments. Conversely, the immediate dilution treatment displays a positive trend in POC with increasing ∆TA and pH, indicating the influence of alkalinity intensity and duration of alkalinity exposure before dilution on bulk POC build up by phytoplankton. Given that changes in phytoplankton bloom dynamics and particulate organic matter can alter the ocean’s CO₂ uptake and sequestration potential, our results address significant knowledge gaps to determine an ecologically safe operating space for OAE implementation under nutrient rich conditions.