This dataset contains carbonate chemistry speciation data of the 2023 KOSMOS mesocosm study on Helgoland, Germany. This study tested the effects of ocean alkalinity enhancement simulating lime additions on pelagic ecosystem functioning during a spring bloom. Carbonate chemistry speciation (fCO2, pHT, calcium carbonate saturation state) was generally calculated from measurements of total alkalinity (TA) and dissolved inorganic carbon (DIC) in depth-integrated water samples. There were 12 mesocosms in total and in 6 of them an alkalinity gradient of up to +1250 umol/kg was established in steps of 250 umol/kg. In the remaining 6 the same amount of alkalinity was added only to the upper portion of the mesocosms, resulting in twice the alkalinity increase there, before being mixed in after 48 hours. The two treatments simulated the immediate dilution of TA after ship deployment as well as a delayed one from a point source.

DIC and TA derived pHT regressed against depth-integrated CTD pH allowed to identify outliersIf outliers detected in TA or DIC, corrected CTD pH (regression R2 of 0.99) was used to calculate carbonate chemistry as second parameterThe T5 files show data for the delayed treatments from day 3 to 7 where on day 3 and 7 the data shown is from depth-integrated samples while on day 5 and 6 it is from top layer samples.On day 5 and 6, the top layer samples have also been taken by a Niskin (0.3-0.8m).