| Continuous measurements of the surface water CO2 partial pressure were performed since
2003 on a cargo ship that commutes at 2 – 3 day intervals between the Mecklenburg
Bight in the southwest and the Gulf of Finland in the northeast of the Baltic Sea. In
2006 the fully automated measurement system was complemented by a module
for recording the oxygen concentrations. The data were exploited under different
aspects:
1.) Determination of the net biomass production: The decrease of the pCO2 after the onset of
the spring bloom was used to calculate changes in total CO2. To determine the loss of total
CO2 due to the formation of organic carbon, a mass balance was established that included
the CO2 gas exchange. Taking into account the formation of dissolved organic
carbon yielded the net production of particulate organic that represents the total
biomass. It was shown that con-ventional production estimates based on the nutrient
availability may underestimate the bio-mass production by a factor of more than
two.
2.) Determination of the nitrogen fixation: Nitrogen fixation rates were calculated by
combin-ing the organic carbon production with a DIN (dissolved inorganic nitrogen
compounds) budget. The obtained N2 fixation rates exceeded previous estimates and
indicated the impor-tant role of this process for the eutrophication of the Baltic
Sea.
3.) The Baltic Sea – sink/source for atmospheric CO2: The pCO2 data were used to calculate
the CO2 gas exchange. On an average the flux balances indicated that the Baltic Sea is a weak
sink (< 1 mol/m2 yr) for atmospheric CO2. However, the regional and interannual variability
was large and mainly caused by the variability of the convective mixing during
winter.
4.) Ocean acidification: The pH is directly related to the pCO2, hence, pCO2 time series are a
potential tool to identify long-term changes in the surface water acid/base system. However,
due to the large interannual and seasonal variability a trend in pCO2 was not yet
detectable.
Our studies of the marine CO2 system in connection with the use of an automated
measure-ment systems deployed on a cargo ship have proven to be an efficient (and
economical) tool for biogeochemical studies in the Baltic Sea. Considerable progress has
been achieved with regard to the characterization of important processes whereas at the same
time new interesting questions emerged. |