Assessing suspended sediment dynamics in relation to ecological thresholds and sampling strategies in two Irish headwater catchments.

Prediction of the impact of suspended sediment on aquatic ecosystems requires adequate knowledge of sediment dynamics in surface waters. Often, studies reporting the response of aquatic biota to suspended sediment are concerned with concentrations, while catchment erosion studies often report sediment delivery as annual loads and yields, making the comparison to documented ecological impacts difficult. Similarly, the European Union Freshwater Fish Directive (FFD) (78/659/EC) stipulates a guideline value of 25 mg l(-1) which should not be exceeded, with the exception of floods and droughts. In this respect, the significance of suspended sediment in two Irish rivers was assessed using turbidity sensors calibrated for suspended sediment. Sediment yields of 0.07 tonnes (t) ha(-1) year(-1) and 0.44 t ha(-1) year(-1) and annual FFD exceedance frequency of 8.3% and 17.8% were estimated for the two catchments. Contrasts in the frequency of exceedance events between both catchments was observed, yet duration was typically short (<5h). Additionally, this study evaluated different sampling resolutions to assess the impact on estimated loads and exceedance frequency. Increasing resolution improved accuracy and reduced uncertainty, with the 24-7 'Plynlimon' sampling method (sampling every 7h) providing the best solution to estimating both loads and exceedance. This study documents some of the first data on sediment dynamics in Ireland and indicates that periods of elevated suspended sediment concentration in the two study catchments may be significant.