Dissolved Nitrogen Uptake by a Cyanobacterial Bloom (Anabaena flos-aquae) in a Subarctic Lake

Uptake of dissolved nitrogen (NH(4) + NO(3) + urea + N(2)) by a cyanobacterial [Anabaena flos-aquae (Lyngb.)] De Brèb population in Smith Lake, Alaska, was measured every 2 to 4 days during the spring of 1990. Total dissolved nitrogen uptake ranged from 0.34 to 24.75 mumol liter h, with a mean of 5.75 mumol liter h; the euphotic zone accounted for 91% of the uptake. The mean turnover time for dissolved combined nitrogen (NH(4) + NO(3) + urea) in the euphotic zone was less than 14 h, and that for NH(4) was only 3.6 h. The mean relative preference indices for NH(4) (2.4), NO(3) (0.4), and urea (0.5) established NH(4) as the preferred nitrogenous nutrient. The uptake rates were apparently dependent on biomass, temperature, and light. Regeneration, probably due to zooplankton excretion and bacterial remineralization of dissolved organic nitrogen, was the main source of NH(4) for the cyanobacterial growth. The high half-saturation constant for NH(4) with low ambient NH(4) concentration nevertheless resulted in the simultaneous utilization of several forms of nitrogen.