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 metazoan zooplankton trophic level, calculated from zooplankton and particulate matter δ15N stable isotopes (‰ air). 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 not assessed for trophic level; medium: 200-500µm; large: 500µm-3mm). Organisms were picked into functional groups within each size class, and their delta 15N isotope signal was measured using a mass spectrometer coupled to an elemental analyser: i) bulk zooplankton (MZP), which consisted of a random mix of all zooplankton, ii) and copepod zooplankton (COP). Delta 15N stable isotopes were also measured in acidified and non-acidified particulate matter filters (no consistent differences observed between them), assumed as the potential food for zooplankton. Here, we present the delta 15N measurements, and the calculated trophic level of the different metazoan zooplankton groups assessed (see Sanchez et al. in prep for further details on calculations). 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. This data set also contains in greater detail the particulate matter δ15N values used to calculate zooplankton trophic level. It is also associated with the dataset "KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: metazoan zooplankton carbon and nitrogen mass, and 13C and 15N stable isotopes" (https://doi.pangaea.de/10.1594/PANGAEA.971765), which contains in greater detail the zooplankton δ15N values used to calculate their trophic level.