Nitrification in the water column and sediment of a hypereutrophic lake and adjoining river system

Nitrification in lakes and rivers is usually modeled using first-order water column kinetics. Over the years, a number of scientists have, however, recognized the importance of sediment nitrification, especially in shallow systems. The aim of this study is to determine the extent of nitrification in the water column and sediments of a eutrophic system and to propose simple kinetic equations to describe the process. An intensive experimental study was performed (summer and fall 1992, fall 1993) on Onondaga Lake and the adjoining Seneca River, a hypereutrophic system with high ammonia concentrations (2-10 mgN l(-1)). Water samples and sediment cores were collected and laboratory nitrification studies performed. The results clearly demonstrate that there is rapid nitrification in the sediments of both Onondaga Lake (0.37 gN m(-2) d(-1)) and the Seneca River (0.32 gN m(-2) d(-1)). In contrast, no apparent nitrification was observed in the water column of the lake or river, despite high ammonia concentrations. The findings of the Most Probable Number (MPN) test provide a likely reason for this phenomenon, showing that the lack of nitrification in the water column is probably due to low nitrifier densities (similar to 10(1) cells ml(-1)), whereas the rate is rapid in the sediments due to high nitrifier populations (similar to 10(5) cells ml(-1)). It is therefore believed that nitrification in this relatively deep system is localized to the sediments and simple zero order kinetics is proposed as a first approach to model the process. It also stimulates the question of whether nitrification in most freshwater systems is principally a sediment process and should be modeled accordingly. (C) 2000 Elsevier Science Ltd. All rights reserved.