Effects of storm events on mobilisation and in-stream processing of dissolved organic matter (DOM) in a Welsh peatland catchment

Peatlands are important contributors of dissolved organic matter (DOM) to downstream aquatic systems. We investigated the effects of storm events on dissolved organic carbon (DOC) concentrations and DOM quality in a stream draining a Welsh peatland catchment. Intensive stream samples were collected and analysed for pH, DOC, dissolved organic nitrogen (DON), absorbance and fluorescence. Soil water samples and samples of sphagnum pore water were also collected, and a simple end-member mixing model was applied to account for changes occurring during the events. Fluorescence data were interpreted using parallel factor analysis (PARAFAC). DOC concentrations increased and pH decreased during the storm events. The soil water data and the mixing model indicated that this was due to a change of flow paths and draining of the DOC-rich acrotelm. Absorbance indices and the DOC/DON ratio suggested that the DOM released during events was less degraded. There was a striking, inversely related diurnal pattern in absorbance and fluorescence after the discharge peak. The diurnal pattern and a lack of fit with the mixing model suggested that fluorescing DOM was mainly produced in-stream. Fluorescence has been found to peak in the morning and decline during day-time due to photo-bleaching. We hypothesise that the input of additional DOM during events causes a change in the diurnal pattern, giving a peak at mid-day, when the processing of the additional DOM is highest.