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 production rates, calculated by measuring δ13C uptake following an enrichment of the dissolved inorganic carbon pool using 13-C bicarbonate. Secondary production is measured by tracing uptake of 13C from particulate matter (potential food for zooplankton) to metazoan zooplankton, following an enrichment of the dissolved inorganic carbon through 13C-bicarbonate addition on day 11 after sampling (see Sanchez et al (in prep)). We used the following formula, adapted from De Kluivjer et al. (2013) Biogeosciences, and further details can be found on Sánchez et al. (in prep): ((consumer AT%13C t - consumer AT%13C t-n)/(food AT%13C t - consumer AT%13C t-n))*(consumer biomass/time). AT13C of food, or particulate matter (PM), is the average of all measurements between t-n and t. Biomass of consumer (zooplankton) is the average of biomass measurement between t-1 and t+1. Metazoan zooplankton biomass used to calculate production rates is the biomass calculated from abundances and C mass conversion factors (see correspondent data set for more detail). Metazoan zooplankton were sampled for stable isotopes 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 13C 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) (copepod zooplankton production could only be calculated in the medium size class due to biomass limitations in the large size class - not enough organisms). Delta 13C 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 atomic percent 13C measurements in dissolved inorganic carbon, particulate matter, and different metazoan zooplankton groups, along with their carbon biomass, and the calculated daily production rates of said metazoan zooplankton groups (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 δ13C values used to calculate secondary production. It is also associated with the datasets "KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: metazoan zooplankton carbon biomass" (https://doi.pangaea.de/10.1594/PANGAEA.971764) and "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 contain in greater detail the zooplankton biomass and δ13C values used to calculate production.