Nitrogen fixation in a large shallow lake: rates and initiation conditions.

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 μgN l-1 d-1 and maximum rate (2.61 μgN 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 μgP l-1started to inhibit N2fix. [ABSTRACT FROM AUTHOR]