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Characteristics of the spring/summer production in the Mecklenburg Bight (Baltic Sea) as revealed by long-term pCO2data

Schneider B., Buecker S., Kaitala S., Maunula P., Wasmund N.

Oceanologia

2015

No. 57 (4)

375--385

Automated CO2 partial pressure, pCO2, measurements were performed on a cargo ship that commutes between the Gulf of Finland and the Mecklenburg Bight in the southwest of the Baltic Sea. The data from 2004 to 2014 along a sub-transect in the Mecklenburg Bight are used to analyze the timing and intensity of the net community production (NCP). The start of the spring bloom, identified by the first continuous drop of the pCO2 below the atmospheric level, spanned from mid-February to mid-March. Converting the pCO2 decrease during spring to changes in the total CO2 concentration and taking into account air-sea gas exchange, the spring NCP was determined. The NCP increased by about 80% during 2004–2014, the mean amounted to 40 μmol L-1. In two years a distinct second pCO2 minimum in mid-summer succeeded the minimum in spring. This was attributed to production fuelled by nitrogen fixation since the nitrate concentrations were virtually zero and since the atmospheric deposition could not satisfy the NCP nitrogen demand. Furthermore, investigations of the plankton composition revealed a cyanobacteria biomass peak in the year with the highest mid-summer NCP. Based on the calculation of the mid-summer NCP in the two particular years and on the C/N ratio of particulate organic matter, the corresponding nitrogen fixation activity was calculated. These values and the analysis of the relationship between the integrated NCP and temperature indicated that the nitrogen fixation activity in the Mecklenburg Bight was by a factor 3–4 lower than in the central Baltic Sea.