Nitrogen fixation in a large shallow lake: rates and initiation conditions |
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Authors: | Ilmar Tõnno Tiina Nõges |
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Affiliation: | (1) Institute of Zoology and Hydrobiology, Estonian Agricultural University, Institute of Zoology and Botany, Võrtsjärv Limnological Station, Rannu, Tartu County, University of Tartu, 61101 Estonia |
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Abstract: | The fixation of molecular nitrogen (N2fix) by cyanobacteria in situ and in PO4-P enrichment experiments was investigated in large shallow Lake Võrtsjärv in 1998–2000. In this lake, N2fix started when TN/TP mass ratio was about 20, which is much higher than Redfield mass ratio 7. The rate of N2fix varied between 0.81 and 2.61 gN l–1 d–1 and maximum rate (2.61 gN l–1 d–1) was measured in 15.08.2000. In L. Võrtsjärv a lag period of a couple of weaks occurred between the set-up of favourable conditions for N2fix as the appearance of N2-fixing species and depletion of mineral nitrogen, and the real N2fix itself. However, if the favorable conditions for N2fix occurred in the lake, N2fix started after enrichment with PO4-P in mesocosms even then when no N2fix was detected in the lake. N2fix in mesocosms was also more intensive than in lake water. In our experiments PO4-P concentrations higher than 100 gP l–1started to inhibit N2fix. |
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Keywords: | cyanobacterial N2-fixation shallow lake TN/TP ratio PO4-P enrichment experiments |
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