Characterising Organic Matter composition during extreme events in-stream and within terrestrial sources in a small, urban, headwater catchment in Birmingham, UK.

Abstract:

Urban catchments have strong terrestrial-riverine links leaving them vulnerable to extreme events. Organic Matter (OM) composition in urban streams is dominated by terrestrial inputs after storms. This acts as a strong control on the health of urban streams, and is a major factor in the “urban stream syndrome”. Long term characterisation of in-stream OM over short temporal resolutions is rare, and few studies have linked this to changes in OM composition in terrestrial sources.

We studied in-stream and terrestrial OM in the Bourn Brook, an urbanised, headwater catchment in Birmingham, UK. Continuous in-stream monitoring was carried out using an in situ fluorometer, measuring wavelengths corresponding with Tryptophan-like and humic-like fluorescence at hourly time steps from July 2016 to present. Samples from terrestrial sources (roads, roofs, grass, and a CSO) were collected manually in storms of varying magnitude. Terrestrial samples were measured for fluorescence spectra and absorbance. Initial analysis has calculated Coble peaks and numerous fluorescence indices and PARAFAC modelling is being undertaken to identify distinct fluorophores in terrestrial samples.

Higher intensity storms generally caused greater Tryptophan and humic-like peaks in stream, with a lag time in the humic-like peak. Following multiple consecutive rainfall events peaks in tryptophan and humic-like fluorescence were less intense regardless of event magnitude. Less intense peaks were also noted in terrestrial samples of OM in wet periods. Variation in peaks and indices for terrestrial samples of the same source type was almost as great as the total variation between all samples with differences in road samples particularly high.

Terrestrial inputs to the stream during storm events quickly dominate OM composition, but an exhaustion effect may weaken the terrestrial-riverine link in periods with multiple storms. Lower fluorescence peaks in the terrestrial samples in wet periods also suggest an exhaustion effect. Substantial variation in OM composition in terrestrial samples from the same source type show the importance of local anthropogenic activity in controlling OM composition. This study aids understanding of how terrestrial OM composition varies by storm in an urban catchment, and the concurrent in-stream response.