Phytoplankton and Microzooplankton responses to Calcium-based Ocean Alkalinity Enhancement during a Spring bloom in the North Sea
durch
8A-002 - Hörsaal Ostufer / Lecture Hall East
GEOMAR - Standort Ostufer / GEOMAR - East Shore
Abstract:
Ocean Alkalinity Enhancement (OAE) is an emerging carbon dioxide removal strategy that leverages the natural dissolution of calcium- or silicate-based rock minerals in the ocean to increase seawater alkalinity, thereby enhancing CO₂ ingassing. However, its ecological implications remain uncertain, necessitating a thorough assessment for safe and sustainable deployment. To address this, we conducted a 39-day mesocosm experiment during the spring phytoplankton bloom of 2023 in the temperate, eutrophic waters off Helgoland in the North Sea, using hydrated lime additions across a ΔTA gradient (0–1250 μmol kg⁻¹ in increments of 250 μmol kg⁻¹). The non-equilibrated approach lowered pCO₂ (from ~400 to <50 μatm at ΔTA ≥750) and raised pH (pH >9 in ΔTA ≥1000), allowing us to test biological responses to large carbonate-chemistry shifts. Phytoplankton community blooms were delayed at the highest ΔTA (ΔTA ≤1000 μmol kg⁻¹), with community composition changes captured by microscopy and HPLC-CHEMTAX. Microzooplankton shifted from early ciliate dominance to later dinoflagellate prevalence across treatments, with a change in temporal development above ΔTA ≤750 μmol kg⁻¹. These results highlight the sensitivity of spring bloom dynamics to OAE, underscoring the need to consider the benefits of CO₂ removal against ecosystem responses before scaling up deployment.
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