This data is part of the BMBF project CUSCO (Coastal Upwelling Systems in a Changing Ocean). Here we report the dissolved inorganic carbon concentration and total alkalinity during a 35-day experiment, where we enclosed natural plankton communities in in-situ mesocosms off Peru. The experiment investigated the interactive effects of light and upwelling on the Humboldt upwelling ecosystem by mimicking a gradient of upwelling intensities (0%, 15%, 30%, 45% and 60%) under summer-time high light and winter-time low light. Integrated seawater samples from a depth between 0 and 10m were collected using a 5L Integrating Water sampler (IWS; Hydro-Bios, Kiel). Dissolved inorganic carbon (DIC) and total alkalinity (TA) samples were obtained by 0.2µm gentle pressure filtration, poisoned with saturated 7.5 % mercury chloride (HgCl2) solution and frozen at -20°C until measurement. Samples for Total Alkalinity (TA) were measured by means of potentiometric titration with 0.05 M HCl using an automated titration device (862 Metrohm Compact Titrosampler). All DIC samples taken until day 17 were measured using an Automated Infra-Red Inorganic Carbon Analyzer (AIRICA) with a LICOR detector (LI-7000 CO2/H20 Analyzer, MARIANDA, Kiel). Certified reference material (Dickson standard for oceanic CO2 Measurements - CRM Batch 142 with salinity = 33.389 and DIC = 2038,07 µmol/kg) was measured and used to correct measured sample values. Additional DIC samples were measured using gas chromatography-mass spectrometry (GC-MS) to determine the 13C signal. The data of the GC-MS was adjusted to the AIRICA data using a linear transformation. Missing days were filled using an average of the day before and after.

Mesocosm 10 describes the control sample from the PacificAs AIRICA data is more precise than GC-MS data, AIRICA data was used where availablelinear regression showed that GC-MS and AIRICA data linearly correlatefor days after T17 a linear conversion factor was applied to adjust GC-MS values: AIRICA=-1180+1.475*GCMS