The Azolla-Anabaena symbiosis in the lower Truckee River, Nevada

Azolla mexicana growth and distribution along the lower Truckee River, NV were used to define how the river's environmental conditions affect the growth of nitrogen-fixing organisms. Suitable conditions for germination of Azolla's spores (temperatures >15°C and irradiances >1500 μmol m −2 s−1) were dependent on low flows (∼5.7 m3 s−1). The occurrence of these conditions at sites downstream of a flow diversion (>131 km downstream from river source) prior to sites upstream resulted in abundant Azolla biomass at downstream habitats in late summer. Azolla was abundant at the upstream habitats by mid-fall. Photosynthetic active radiation (PAR) best explained the Azolla growth rate variability. Nitrogen fixation rates of Azolla and its symbiont, Anabaena azollae, (peak of 0.22 μmoles N₂ g wet wt −1 hr−1) were comparable to those found in other studies. Potential nitrogen loads by Azolla to the Truckee River were not substantial (<1% of the river's nitrogen load).