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Control of the mid-summer net community production and nitrogen fixation in the central Baltic Sea: An approach based on pCO2 measurements on a cargo ship
Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.ORCID iD: 0000-0002-4215-9322
2014 (English)In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 136, 1-9 p.Article in journal (Refereed) Epub ahead of print
Abstract [en]

Automated measurements of the surface CO2 partial pressure, pCO2, 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 CO2 concentrations, CT, were calculated from the mean pCO2 in the northeastern Gotland Sea. The CT data were used to establish a CO2 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 CO2 gas exchange, the mass balance yielded the net organic matter (Corg) 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 Corg 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 Corg 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 Corg 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.

Place, publisher, year, edition, pages
2014. Vol. 136, 1-9 p.
Keyword [en]
Baltic Sea, CO2 partial pressure, Nitrogen fixation, Phosphorus, Solar radiation
National Category
Oceanography, Hydrology, Water Resources
URN: urn:nbn:se:su:diva-122363DOI: 10.1016/j.jmarsys.2014.03.007ISI: 000337865500001OAI: diva2:865793
Available from: 2015-10-29 Created: 2015-10-29 Last updated: 2015-11-11Bibliographically approved

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Gustafsson, Erik
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Baltic Nest Institute
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