Nitrogenase activity and estimates of nitrogen fixation by freshwater benthic blue-green algae

The sandy substrate of Lake Taharoa (west coast, North Island, New Zealand 35°50'S, 173°41'E) is covered by communities of filamentous algae that extend from the exposed beach down to 21 m depth. The algae bind the sand to form crusts and mats which may break off as discrete plates. The dominant species are the blue-greens Microcoleus, Nostoc, Phormidium, Lyngbya, Oscillatoria, Scytonema, Stigonema, Shizothrix, Calothrix, Dichothrix, Tolypothrix, and Anabaena, with occasional high concentrations of the desmid Cylindrocystis. Nitrogenase activity, measured by acetylene reduction, showed a wide range of rates (4-150 μmol C2H4 m h ). Estimates of annual rates of nitrogen fixation by the Taharoa communities are comparable with those for periphytic blue-green algae-dominated systems reported elsewhere.