Using optical sensors to assess the impact of infrequent sampling on the uncertainty of stream annual mean turbidity and total phosphorus concentrations, and how this can affect the water quality status

The annual mean concentration of nutrients is a commonly used parameter in implementing the Water Framework Directive, to assess current environmental status and distance from the environmental goal. However, the concentration of nutrients in streams may vary significantly over short time spans so finding the ‘true mean’ concentration can be difficult. We used hourly turbidity data from optical sensors in 10 streams in four Nordic countries, and we prepared mimic data series for weekly, fortnightly, and monthly sampling strategies. We calibrated the sensor turbidity data with the total phosphorus data from grab samples. We then assessed how the annual mean values of both turbidity and phosphorus can vary, depending not only on the number of samples collected per year but also on stream and catchment characteristics. We found that the uncertainty of the annual mean concentration of total phosphorus decreased with increasing sampling frequency and increasing catchment size, and with a decreasing proportion of agricultural land in the catchment. We also found that there was a higher risk of underestimating the mean TP than of overestimating it, meaning that managers will assume that water quality is better than it is. Our work has resulted in an initial model that calculates the number of samples needed to achieve a given uncertainty in annual mean TP concentration for streams of varying catchment size and land use.