Spring phytoplankton bloom dynamics reveal thresholds of ocean alkalinity enhancement in the North Sea

Abstract Spring phytoplankton blooms drive primary production and food web dynamics in marine ecosystems. Ocean alkalinity enhancement, a proposed carbon dioxide removal strategy, alters seawater chemistry and could disrupt planktonic food webs, yet its ecological consequences remain poorly understood. We conducted a 39-day mesocosm experiment in spring 2023 in the German North Sea to assess the effects of calcium-based alkalinity additions on phytoplankton dynamics. Twelve 6-m³ mesocosms contrasted immediate dilution (well-mixed conditions), and delayed dilution (poorly mixed conditions) across an alkalinity addition gradient of ~0-1150 µmol kg⁻¹. We show that phytoplankton bloom and particulate organic matter dynamics are largely unaffected at low to moderate alkalinity additions of ~0-700 µmol kg −1 regardless of dilution scenario. At alkalinity additions ≥~900 µmol kg −1 , bloom development is delayed by ~6-9 days and particulate organic phosphorus is significantly reduced in both dilution treatments, while particulate organic carbon and nitrogen are lower only under delayed dilution. These findings identify critical alkalinity thresholds for phytoplankton bloom development in temperate, diatom-dominated spring blooms, informing boundaries for ecologically safe deployment.