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

Abstract

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-NH 3) and oxidized forms of N (NO x ) 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-NH 3 and NO x 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-NH 3 and source for NO x 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.