Experimental measurement of sediment nitrification and denitrification in Hamilton Harbour, Canada

Abstract

This research examines the role of sediment nitrification and denitrification in the nitrogen cycle of Hamilton Harbour. The Harbour is subject to large ammonia and carbon loadings from a waste-water treatment plant and from steel industries. Spring ammonia concentrations rapidly decrease from 4.5 to 0.5 mg 1^-1, while spring nitrate concentrations increase from 1 to 2 mg 1^-1, by mid-summer. A three-layer sediment model was developed. The first layer is aerobic; in it, oxidation of organics and nitrification occurs. The second layer is for denitrification, and the third layer is for anaerobic processes. Ammonia sources for nitrification include diffusion from the water column, sources associated with the oxidation of organics, sources from denitrification and from anaerobic processes. Diffusion of oxygen, ammonia and nitrate across the sediment-water interface occurs. Temperature effects are modelled using the Arrhenius concept. A combination of zero-order kinetics for nitrate or ammonia consumption with diffusion results in a half-order reaction, with respect to the water column loss rate to sediments.

From experimental measurement, the rate of nitrification is 200 mg N 1^-1 sediment per day, while that of denitrification is 85 mg N 1^-1 sediment per day at 20 °C. The Arrhenius activation energy is estimated as 15 000 cal/mole-K and 17 000 cal/mole-K for nitrification and denitrification, respectively, between 10 °C and 20 °C. Calculations of the flux of ammonia with the sediments, using the biofilm model, compare favourably with experimental observations. The ammonia flux from the water column is estimated to account for 20% of the observed decrease in water column stocks of ammonia, while the nitrate flux from the water column is estimated to account for 25% of the total nitrogen produced by the sediments.