Epilithic Algae From An Urban River Preferentially Use Ammonium Over Nitrate

Epilithon is a taxonomically diverse assemblage of aquatic organisms which grow on rocks; this biological compartment is involved in several reactions that contribute to the dynamics of dissolved organic nitrogen in water. Using ceramic tiles colonized in an urban river, this experimental study assessed the relative importance of ammonium uptake by inhibiting nitrification blocking ammonium oxidation with acetylene, as well it tested the hypothesis that epilithon preferentially assimilated ammonium over nitrate. In our experiments, ammonium uptake by epilithon accounted for 46% - 100 % of the ammonium decrease in the water column, whereas nitrate uptake accounted for 0% - 11% of the nitrate decrease. Ammonium uptake rates ranged from 197 to 519 mu mol N-NH4+ m(-2) h(-1), while nitrate uptake rates were from 47 to 85 mu mol N-NO3- m(-2) h(-1). The rate of preferential assimilation (RPI) was between 1.15 and 1.26, indicating preference for ammonium over nitrate. The results of this research provide valuable information regarding the relative contribution of algal uptake relative to nitrification in epilithon from an urban river.