Control of the mid-summer net community production and nitrogen fixation in the central Baltic Sea: An approach based on pCO 2 measurements on a cargo ship

• Published in: Journal of marine systems 2014, Vol.136, p.1
• Main Authors: Schneider, Bernd, Gustafsson, Erik, Sadkowiak, Bernd
• Format: Article
• Language: English
• Subjects: Baltic Sea; CO2 partial pressure; Nitrogen fixation; Phosphorus; Solar radiation
• ISSN: 1879-1573; 0924-7963
• DOI: 10.1016/j.jmarsys.2014.03.007

Automated measurements of the surface CO 2 partial pressure, pCO 2 , were performed since 2003 on a cargo ship along a transect between Helsinki in the Gulf of Finland and Lübeck/Gdynia in the southwest of the Baltic Sea. The temporal and spatial resolution of the measurements amounted to 2–4 days and about 2 nautical miles, respectively. Based on temperature and salinity records and on the mean alkalinity, the total CO 2 concentrations, C T , were calculated from the mean pCO 2 in the northeastern Gotland Sea. The C T data were used to establish a CO 2 mass balance for the period from mid-June to the beginning of August in 2005, 2008, 2009 and 2011. Taking into account the air–sea CO 2 gas exchange, the mass balance yielded the net organic matter (C org ) production which is fuelled by nitrogen fixation at this time of the year. Several production events were detected with rates up to 8 μmol-C L − 1  d − 1 . The production rates were not related to temperature, but showed a distinct correlation with the rate of the temperature increase. This led to the conclusion that the exposure of nitrogen fixing cyanobacteria to irradiance is the dominating control for the C org production. Therefore, we suggest using the ratio of irradiance to the mixed layer depth as a variable for the parameterization of nitrogen fixation in biogeochemical models. The C org production and thus the nitrogen fixation rates remained almost constant as long as continuous rising temperatures indicated favorable irradiation conditions. A limitation of the rates by phosphate or any other factor could not be detected. Based on the C/N ratio of particulate organic matter during a cyanobacteria bloom, the C org production was used to estimate the mid-summer nitrogen fixation. The values varied from 102 mmol m − 2 to 214 mmol m − 2 (mean: 138 mmol m − 2 ) for the different years and did not show any correlation with the phosphate excess after the spring nitrate depletion.