This data was collected as a part of a mesocosm study to investigate the ecosystem impacts of ocean alkalinity enhancement, within the EU H2020 OceanNETs project. Nine mesocosms were deployed in Taliarte Harbour (Gran Canaria, Spain) and were regularly sampled using integrated water samplers between 10th September-25th October 2021. A gradient design was used in this experiment with a total of nine different alkalinity concentrations. Seawater alkalinity ranged between ambient (0 µeq kg-1 added alkalinity, OAE0) and 2400 µeq kg-1 additional alkalinity (OAE2400). The alkalinity levels increased in equal intervals of 300 µeq kg-1 across nine mesocosms (OAE0, OAE300, OAE600, OAE900, OAE1200, OAE1500, OAE1800, OAE2100, OAE2400). This data set contains image-based, biovolume-derived carbon mass (µgC per individual) of selected metazoan zooplankton taxa from these nine mesocosms. Here are included metazoan zooplankton image-based carbon mass measurements. C mass is calculated from biovolume, which was derived from length and width-based measurements (see Sanchez et al, in prep, for details on conversions). Metazoan zooplankton were sampled with apstein net (ø17cm, mesh size 55µm, 64.06285L) hauls taken every four days (except the first 11 days, when samples were only taken on days 1 and 11). Zooplankton were size fractioned and assessed in the correspondent size class (small: 55-200µm; medium: 200-500µm; large: 500µm-3mm). Only organisms in the medium and large size classes were photographed. The experiment, which lasted 33 days, was divided into four response phases (see Sánchez et al. (in prep)): i) pretreatment (days 1 to 4, treatment was implemented on day 4), ii) immediate (days 5-10), iii) shorter term (days 11-22), iv) longer term (days 23 to 33).

This data set is associated to the submission by Paul et al. (in review) (https://doi.pangaea.de/10.1594/PANGAEA.966941), so we refer to this data set for basic parameters like water temperature, salinity, pH and carbonate chemistry, to avoid repetition. It is also associated with the dataset "KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: metazoan zooplankton carbon biomass" (https://doi.pangaea.de/10.1594/PANGAEA.971764), which uses carbon mass conversion factors derived from it.