Seasonal variations in the concentrations of methyl and ethyl nitrate in a shallow freshwater lake

Between February and October 2007, a time series of alkyl nitrate concentrations in the water column of a shallow freshwater lake, University of East Anglia Broad, located in southeastern England was carried out to determine whether methyl and ethyl nitrate are present in freshwater systems and to improve understanding of alkyl nitrate production mechanisms in aquatic environments. Concentrations ranged from 4.7 (+/- 0.5) to 53.7 (+/- 4.36) pmol L-1 methyl nitrate, and 2.5 (+/- 0.3) to 11.1 (+/- 0.4) pmol L-1 ethyl nitrate and were within the range of those measured previously in seawater. Peaks in the concentrations of methyl and ethyl nitrate at 4 m were observed in 9 and 6 of the 18 depth profiles measured, respectively. Gradients in concentrations within the hypolimnion suggest that the alkyl nitrates are produced in the bottom waters or sediments, are transported to the lake in groundwater, or both. Colored dissolved organic matter absorbance data suggests that the penetration of ultraviolet light was limited in the lake, so the deep maxima must be due to a non-photochemical alkyl nitrate source.