Interannual variations in nitrification in a hypereutrophic urban lake: occurrences and implications

The irregular occurrence of high rates of nitrification (nitrification events) in the upper waters of a nitrogen polluted urban lake, Onondaga Lake, NY, during the fall mixing period is documented. The analysis is supported by eight years (1988–1995) of measurements of total ammonia (T-NH3) and oxidized forms of N (NOx) in the lake and its inflows, and dissolved oxygen (DO) and temperature profiles. A tested system-specific mass balance model for N is used to estimate nitrification rates for these years, and a system-specific DO model is applied to demonstrate the impact of a nitrification event on the lake’s oxygen resources for a single year. Rates of nitrification for different years ranged from near zero throughout the fall mixing interval (1994) to an average of about 0.18 d−1 for one month (1995). The nitrification events cause major interannual variations observed in the pools of T-NH3 and NOx in the lake’s upper waters in fall, and are probably responsible for the particularly severe lake-wide depletion of DO observed in certain years during this interval. Nitrification was the dominant loss process for T-NH3 and source for NOx in years that the events occurred. The exacerbating effect of the events on the oxygen resources of the lake needs to be accommodated in related remediation efforts.