Urea decomposition activities in an ammonium enriched freshwater pond

Abstract

Urea decomposition activities, urea and ammonium concentrations, and the concentration of chlorophyll a (chl a) were measured in an ammonium enriched, hypertrophic freshwater pond at 5 to 7 h intervals during 3 diel cycles on June 27 to 28, September 6 to 7, 1992 and February 14 to 15, 1993. The study site was characterized by very high ammonium concentrations, ranging from 123.6 to 403.6 μM. During the study period, the urea concentration ranged from 1.3 to 9.1 μM and chl a concentration ranged from 81.5 to 211.8 μg 1-1. In 2 of 3 diel cycles the nighttime urea decomposition rate in whole waters was higher than the daytime decomposition rate; in 1 diel experiment the nighttime decomposition rate was 56.4 % of the daytime rate, indicating that the variation of urea decomposition rate was not associated with natural light/dark cycles. Despite the very high ammonium concentrations substantial urea decomposition rates (5.5 to 80.8 nM h-1) were observed, particularly in June and September. Such high decomposition rates were due to high chl a-specific activity rather than due to large phytoplankton biomass and low activity. When urea concentration was high (ca > 2 μM), the urea decomposition rate showed a linear increase with increasing urea concentration even in the presence of high ammonium concentrations (range of 123.6 to 403.6 μM), suggesting that elevated urea concentration mitigates the repressive effect of ammonium on urea decomposition. The urea decomposition rate in GF/C filtrates (mainly bacterial-size fractions) ranged from negligible to 39.3 nM h-1, representing an average of 47.1 % of the activity in whole pond waters. This study suggests that urea decomposition activity in an ammonium enriched freshwater pond is significantly elevated and controlled by the urea concentration, and that bacteria might contribute substantially to urea decomposition rate in a hypertrophic freshwater pond.