https://doi.pangaea.de/10.1594/PANGAEA.996209Petric, SvetlanaSvetlanaPetricGEOMAR - Helmholtz Centre for Ocean Research KielFaucher, GiuliaGiuliaFaucher0000-0001-8930-477XGEOMAR - Helmholtz Centre for Ocean Research KielRiebesell, UlfUlfRiebesell0000-0002-9442-452XGEOMAR - Helmholtz Centre for Ocean Research KielPANGAEA2026AbundanceBiomassciliatescommunity compositionCommunity composition and diversitydiatomsDinoflagellatesenhancementField experimentHelgolandMarine Carbon Dioxide Removal (mCDR)mesocosmmesocosm experimentmesocosm studymicroscopymicrozooplanktonNorth Seaocean alkalinity enhancementOcean Alkalinity Enhancement (OAE)PhytoplanktonPhytoplankton biomassPhytoplankton bloomspring bloomType of studyEvent labelSample code/labelDATE/TIMEDay of experimentTreatment: dilution typeTreatmentTreatment: alkalinity, totalAlkalinity, totalCarbon, inorganic, dissolvedpHFugacity of carbon dioxide in seawaterSalinityTemperature, waterSpeciesSpecies, unique identificationFunctional groupCellBiomass as carbon per volumeMesocosm experimentTitration device, Metrohm, 862 Compact Titrosampler [with built-in PT1000 temperature sensor]Automated Infra Red Inorganic Carbon Analyzer (AIRICA), MARIANDA; coupled with Differential infrared gas analyzer, LI-COR Biosciences, LI-7000Calculated using the CO2sys_v3.0 software (Pierrot et al. 2021)CTD 60M multiparameter probe (Sea & Sun Technology GmbH, Germany)Reference in WoRMSCalculatedMicroscopyKOSMOS_2023DAM CDRmare - RETAKE: CO2 removal by alkalinity enhancement: potential, benefits and risks (RETAKE)Ocean Alk-Align (OceanAlkAlign)Research Mission of the German Marine Research Alliance (DAM): Marine carbon sinks in decarbonisation pathways (CDRmare)2023-03-16T00:00:00/2023-04-21T00:00:00Dataset58140 data pointstext/tab-separated-valuesCreative Commons Attribution 4.0 InternationalData access is restricted (moratorium, sensitive data, license constraints)This dataset contains species-resolved phytoplankton and microzooplankton abundance and biomass data from the KOSMOS 2023 Helgoland mesocosm experiment conducted in the coastal waters off Helgoland Island in the North Sea, Germany. The experiment investigated the effects of Ocean Alkalinity Enhancement (OAE) on plankton communities during the development of a natural spring bloom. Six mesocosms (~6 m³) were subjected to an immediate dilution OAE scenario, establishing an alkalinity enhancement gradient ranging from ΔTA 0 to 1250 µmol kg⁻¹ in increments of 250 µmol kg⁻¹ through calcium-based alkalinity addition. The dataset includes taxonomically resolved phytoplankton and microzooplankton abundance (cells mL⁻¹), carbon biomass (µg C L⁻¹), functional group classifications, and World Register of Marine Species (WoRMS) taxonomic identifiers. Associated carbonate chemistry measurements, including total alkalinity (TA), dissolved inorganic carbon (DIC), pH, fugacity of carbon dioxide (fCO₂), salinity, temperature, and calculated ΔTA values are provided for each sampling event.7.899999999999999554.18330000000001HelgolandFederal Ministry of Education and Researchhttps://doi.org/10.13039/50110000234703F0895ADAM CDRmare - RETAKE, GEOMAR