Denitrification within riverine systems of north-east England

Abstract

This study was undertaken to investigate denitrification and nitrous oxide production in sediments and the key environmental factors influencing these within selected river systems of the LOIS (Land-Ocean Interaction Study) area in North-East England and southern Scotland. Seasonal and spatial trends were evident in both environmental and denitrification data measured monthly for 1.5 years along the Swale-Ouse system from source to tidal limits. Denitrification, measured in sediment cores using acetylene inhibition and expressed by unit area of sediment, increased with distance from source down to freshwater tidal limits. Results from a supplementary survey of the freshwater tidal reaches of the Yorkshire Ouse showed a decrease from the tidal limits. Denitrification activity showed a spring (March to May) peak, particularly in the lowland sites. The highest rate (883±134 µmol N m(^-2) h(^-1)) was measured on the River Wiske, a highly eutrophic lowland tributary to the Swale. A high degree of colinearity was evident between environmental variables, although a significant relationship between denitrification, nitrate and temperature was found through multiple regression. For comparison, measurements were made in the less populated Tweed river system. The seasonal and spatial trends evident in both the environmental and denitrification data from the River Tweed, under a more limited sampling programme, were generally consistent with those observed in the Swale-Ouse system. An intensive field investigation of 50 river sites showed that both potential denitrification rate and N(_2)O production in sediment slurries were positively correlated with nitrate water concentration, sediment water content and percentage of fine (<100 µm) sediment particles. An experimental study investigating the kinetic parameters for denitrification, found that sediment cores taken along the Swale-Ouse exhibited a saturation type curve with added nitrate. Apparent affinity and estimates of apparent maximum velocity for mixed populations of denitrifying bacteria showed an increase on moving downstream and were highest on the Wiske.