Journal article
Spring phytoplankton bloom dynamics reveal thresholds of ocean alkalinity enhancement in the North Sea
Communications earth & environment, Vol.7(1), pp.1-19
21/09/2026
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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.
Ocean alkalinity enhancement displays critical thresholds, beyond which phytoplankton bloom development is delayed and organic matter concentrations are reduced, according to results of mesocosm experiments in the German North Sea.
Details
- Title
- Spring phytoplankton bloom dynamics reveal thresholds of ocean alkalinity enhancement in the North Sea
- Creators
- Juliane K. Tammen - GEOMAR Helmholtz Centre for Ocean Research KielLeila R. Kittu - GEOMAR Helmholtz Centre for Ocean Research KielMarkus Schartau - GEOMAR Helmholtz Centre for Ocean Research KielGiulia Faucher - GEOMAR Helmholtz Centre for Ocean Research KielKai G. Schulz - Southern Cross UniversityMaarten Boersma - Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und MeeresforschungUlf Riebesell - GEOMAR Helmholtz Centre for Ocean Research Kiel
- Publication Details
- Communications earth & environment, Vol.7(1), pp.1-19
- Publisher
- Nature Publishing Group UK
- Number of pages
- 19
- Grant note
- This study was funded by the German Federal Ministry of Education and Research (BMBF) through the RETAKE project (grant/award no. 03F0895A), within the framework of the DAM Mission “Marine carbon sinks in decarbonization pathways” (CDRmare). Additional support was provided through the Ocean Alk-Align project, funded by Carbon to Sea and the Thistledown Foundation. Open Access funding enabled and organized by Projekt DEAL.
- Identifiers
- 991013403072102368
- Copyright
- © The Author(s) 2026
- Academic Unit
- Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Journal article