5. Surface Water Biogeochemistry as Derived from pCO₂ Observations

The surface-water pCO₂ in the central Baltic Sea shows a distinct seasonality, with minima in May and July. This pattern can be unambiguously attributed to the net community production (NCP) during the spring bloom and to the mid-summer NCP fueled by nitrogen fixation. Converting the pCO₂ data to concentrations units for the total CO₂ facilitated a detailed and quantitative analysis of the chronology of the NCP. The start of the spring bloom was triggered by the year’s first increase in the surface-water temperature and during the study period regularly occurred in the central Baltic Sea by the end of March. The first phase of NCP was based on the availability of nitrate and lasted, on average, until mid-April. However, NCP continued until the end of May despite the absence of dissolved inorganic nitrogen (nitrate + ammonia). This observation has led to questions regarding the occurrence of nitrogen fixation already during spring. A period of regenerated production that did not contribute to NCP followed the termination of the spring bloom. Mid-summer NCP fueled by nitrogen-fixing cyanobacteria was detected as discrete pulses and coincided with sudden increases in temperature. Distinct linear correlations between temperature and the accumulated NCP for the individual production events suggested that solar radiation controls and limits the efficiency of nitrogen fixation. The role of phosphate as limiting factor could not be confirmed.