Variability in fluvial suspended and streambed sediment phosphorus fractions among small agricultural streams
Sara Sandström, Martyn N. Futter, David O’Connell, Emma Lannergård, Jelena Raković, Katarina Kyllmar, Laurence Gill, Faruk Djodjic
Abstract
Abstract Agriculture is a major source of sediment and particulate phosphorus (P) inputs to freshwaters. Distinguishing between P fractions in sediment can aid in understanding its eutrophication risk. Although streams and rivers are important parts of the P cycle in agricultural catchments, streambed sediment and especially fluvial suspended sediment (FSS) and its P fractions are less studied. To address this knowledge gap, seasonal variations in FSS P fractions and their relation to water quality and streambed sediment were examined in three Swedish agricultural headwater catchments over 2 yr. Sequential fractionation was used to characterize P fractions in both streambed sediment and FSS. All catchments had similar annual P losses (0.4–0.8 kg ha –1 ), suspended solids (124–183 mg L –1 ), and FSS total P concentrations (1.15–1.19 mg g –1 ). However, distribution of P fractions and the dominant P fractions in FSS differed among catchments ( p < .05), which was most likely dependent on differences in catchment geology, clay content, external P sources, and flow conditions. The most prominent seasonal pattern in all catchments was found for iron‐bound P, with high concentrations during low summer flows and low concentrations during winter high flows. Streambed sediment P fractions were in the same concentration ranges as in FSS, and the distribution of the fractions differed between catchments. This study highlights the need to quantify P fractions, not just total P in FSS, to obtain a more complete understanding of the eutrophication risk posed by agricultural sediment losses.