Effects of copper contamination on nitrogen cycling in a freshwater sediment : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

The effects of trace elements on microbial populations and function can be wide-ranging and difficult to disentangle, hence only a few studies report specific effects on processes in elemental biogeochemical cycles. This study used a laboratory mesocosm experiment to test the effect of two levels of copper (Cu) contamination (65 mg/kg and 270 mg/kg) on nitrogen cycling in a freshwater sediment. The mobilisation of trace elements (copper, iron and manganese) and nitrogen (as nitrate and ammonium) in the sediments was analysed using diffusive gradients in thin films. A sequential extraction protocol was used to measure the partitioning of the introduced Cu into various geochemical fractions in the sediment. The release of nitrous oxide from the treated sediments was determined over an 82-h period. The results showed that, at these levels, Cu stimulated nitrate removal and increased the production of nitrous oxide (48 – 70 µg/m2/h) and accumulation of NH4-N in the sediments. Most of the total extractable-Cu in the sediment (69 %) was bound to the organic matter fraction, while the rest was associated with the relatively inert residual fraction (>20 %). Only a small amount (~2%) was found exchangeable, carbonate, and Fe/Mn oxide fractions. This research concludes that Cu contamination can affect the nitrogen cycle in NZ freshwater sediments, which may have further implications on water quality and greenhouse gas inventories from freshwater environments.