Highlights from the article:
• Blooms of N2 fixing cyanobacteria in the Baltic Sea do not depend on temperature;
• Production/N2 fixation rates are correlated with the rate of temperature increase;
• The ratio irradiation vs. mixing depth controls production and N2 fixation;
• The spring phosphate excess does not control the mid-summer N2 fixation;
• Phosphorus limitation of cyanobacteria production/N2 fixation was not detected;
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.
Calculating total CO2 calculationsfrom mean pCO2
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 (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.
Blooms of N2-fixing cyanobacteria independent of temperature
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 to use the ratio between irradiance and 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 favourable irradiation conditions.
Phosphorus limitation of cyanobacteria production/N2 fixation was not detected
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.
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