DISSOLVED NITROGEN UPTAKE BY A CYANOBACTERIAL BLOOM (ANABAENA-FLOS-AQUAE) IN A SUB-ARCTIC LAKE

Uptake of dissolved nitrogen (NH4+ + NO3- + urea + N2) by a cyanobacterial [Anabaena flos-aquae (Lyngb.)] De Breb 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-1 h-1, with a mean of 5.75 mumol liter-1 h-1; the euphotic zone accounted for 91% of the uptake. The mean turnover time for dissolved combined nitrogen (NH4+ + NO3- + Urea) in the euphotic zone was less than 14 h, and that for NH4+ was only 3.6 h. The mean relative preference indices for NH4+ (2.4), NO3- (0.4), and urea (0.5) established NH4+ 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 NH4+ for the cyanobacterial growth. The high half-saturation constant for NH4+ with low ambient NH4+ concentration nevertheless resulted in the simultaneous utilization of several forms of nitrogen.