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Continuous measurements of the CO2 and O2 partial pressure on a cargo ship in the Baltic Sea



Bernd Schneider, Bernd Sadkowiak, Robert Schmidt

Surface water CO2 partial pressure (pCO2) measurements are performed since 2003 on a cargo ship that commutes regularly at two day intervals between Lübeck and Helsinki. In summer 2005 the measurement system was complemented by a module for the determination of the O2 partial pressure (pO2). After the cargo ship line was carried over by a new ship (FINNMAID), the fully automated system was technically improved and will be in operation for at least three more years.The pCO2 measurements are performed by equilibration of surface water with air and detection of the equilibrium CO2 concentration in air by IR spectroscopy. The determination of the pO2 was based on the same principle using an optode for determination of O2 at equilibrium. With these investigations we are aiming at:

  1. Establishing a CO2 mass balance in order to quantify the net biomass production in particular during the mid-summer nitrogen fixation period and to characterize the interannual and regional variability.
  2. Identification of the Baltic Sea as a sink/source for atmospheric CO2 by a CO2 gas exchange balance
  3. Determination of the gas exchange transfer velocity by a coupled CO2/O2 mass balance (contribution to „Surface Ocean Processes in the Anthropocene“, SOPRAN/SOLAS).
Fig. 1: Route of cargo ship FINNPARTNER (since 2007 FINNMAID)
Fig. 2: Seasonality of the CO2 partial pressure between Lübeck and Helsinki in 2005

The distribution is characterized by two minima in May and July which are most pronounced in the northeast and which are caused by the spring bloom and by the production during the nitrogen fixation period, respectively. Isolated pCO2 peaks with values larger than 700 µatm occur during early fall close to the coast of the Island of Gotland and are a consequence of upwelling events.