Past and present productivity of the Baltic Sea as inferred from pCO2 data

The spring/summer decrease in the CO2 partial pressure, pCO2, in the surface waters of the Baltic Proper was used to estimate the net biomass production during recent years and at the beginning of the last century. Current pCO2 data were available from 17 cruises in the eastern Gotland Sea during 1995–2001, and provided the mean seasonal pCO2 cycle. Taking into account temperature and salinity, the pCO2 seasonality was used to calculate the changes in the total CO2 (NCT, normalized to total alkalinity) on the basis of the equilibrium relationships of the marine CO2 system. A carbon balance was established for the productive period taking into account the NCT depletion, the CO2 air/sea exchange and the formation of dissolved organic carbon. The closing term of the balance was attributed to the integral net production of particulate organic carbon, PPOC, that was considered to represent the biomass production. The maximum PPOC was 3.6 ± 0.3 mol m−2 and indicated the termination of net production by the end of July. Historic pCO2 data were available from measurements of alkalinity and pH performed during the spring and summer of 1927–1935 in the eastern Gotland Sea. The carbon balance yielded a PPOC of 1.4 mol m−2 with an error range of ± 0.4 mol m−2 that was mainly attributed to the uncertainty in the pH measurements. This indicates an increase in the net biomass productivity of the Baltic Proper by a factor of 2.5 (maximum uncertainty: 1.8–3.8) since the beginning of the last century.