Spring Phytoplankton Bloom Dynamics Reveal Thresholds of Ocean Alkalinity Enhancement in the North Sea

durch Juliane Katharina Tammen (FB2-BI)

Mittwoch 20.08.2025, 13:15 → 14:30 Europe/Berlin

8A-002 - Hörsaal Ostufer / Lecture Hall East (GEOMAR - Standort Ostufer / GEOMAR - East Shore)

Abstract:

Ocean alkalinity enhancement (OAE), a proposed carbon dioxide removal strategy, alters seawater chemistry in ways that could disrupt planktonic communities, yet its ecological consequences remain poorly understood. 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 both the intensity of alkalinity enhancement and short-term exposure to undiluted conditions (higher intensities) influence pelagic ecological and biogeochemical responses in a spring phytoplankton bloom. Using calcium chloride and sodium hydroxide to simulate a calcium-based, non-CO₂-equilibrated OAE scenario, we applied a gradient of 0 – 1250 µmol kg^-1 added alkalinity (∆TA) under immediate and delayed dilution scenarios. High alkalinity (>750 µmol kg⁻¹) led to short-term declines in chlorophyll a and delayed onset of the phytoplankton bloom. Organic matter accumulation declined at high ∆TA, especially under delayed dilution, with peak POC concentrations of the delayed dilution treatment at ∆TA 1250 µmol kg^-1 being half of what was found in the immediate dilution counterpart. Our findings reveal non-linear threshold responses in bloom timing and organic matter accumulation to OAE, highlighting the risk of suppressing key bloom phases under high or transiently elevated alkalinity. We further show that even brief exposure to high alkalinity can have lasting ecological effects influencing organic matter build-up. These insights provide critical constraints for defining ecologically safe OAE deployment in nutrient-rich temperate ecosystems, particularly regarding exposure intensity and dilution timescales.