This data is part of the BMBF project CUSCO (Coastal Upwelling Systems in a Changing Ocean). Here we report carbon and nitrogen uptake rates by phytoplankton collected 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). Samples were spiked with 13C labelled sodium bicarbonate and 15N labelled potassium nitrate. Samples were incubated for 24 hours at light intensities that resemble the light conditions in the mesocosms. After incubation a subsample (150-1000 ml) was filtered onto a 0.7 µm pre-combusted glass-fiber filters (GFF, Whatmann). The filters were acidified, then dried in the oven at 60 °C and later measured using an elemental analyzer (Flash IRMS, ThermoFisher) connected to a mass spectrometer (Delta V Advantage Isotope Ratio MS, ThermoFisher) with the ConFlo IV (ThermoFisher). Carbon and nitrogen uptake rates were calculated based on Slawyk et al. (1977, DOI:10.4319/lo.1977.22.5.0925) and Shiozaki et al. (2009, DOI:10.3354/meps07837) respectively.

Mesocosm 10 describes the control sample from the Pacificvalues marked with / are invalid