The value of high resolution nutrient monitoring: a case study of the River Frome, Dorset, UK

The River Frome was sampled at sub-daily sampling interval, with additional storm sampling, through an annual cycle. Samples were analysed for total phosphorus (TP), soluble reactive phosphorus (SRP), total oxidisable nitrogen (TON) and dissolved reactive silicon (Si). The resulting data set was artificially decimated to mimic sampling frequencies from 12 hourly to monthly time interval. Monthly sampling interval resulted in significant errors in the estimated annual TP and SRP load of up to 35 and 28 % respectively, and the resulting data sets were insufficient to observe peaks in P concentration in response to storm events. Weekly sampling reduced the errors in annual load estimate to ca. ± 7 %. TON and silicon concentrations were less variable with changing river flow, and monthly sampling was sufficient to predict annual load estimates to within 10 %. However, to investigate within-river nutrient dynamics and behaviour, it is suggested that a weekly sampling interval would be the minimum frequency required for TON and Si studies, and daily sampling would be a minimum requirement to adequately investigate phosphorus dynamics. Hysteresis in the nutrient concentration / flow relationships for all 32 storm events during the study period were modelled and seasonal patterns discussed, to infer nutrient sources and behaviour. The high resolution monitoring in this study identified, for the first time,major peaks in phosphorus concentration in autumn and winter that coincide with sudden falls in air temperature, and was associated with biofilm breakdown. This study has shown that to understand complex within-channel nutrient processes,accurately quantify nutrient exports from catchments, and observe changes in water quality as a result of nutrient mitigation efforts over time, it is vital that the newly emerging automated sampling and chemical analysis technologies begin to be deployed, to allow for high-resolution monitoring of our rivers in the future.