Epsilon Archive for Student Projects

Abstract

Agricultural streams play a key role in transporting sediment-associated phosphorus (P) from land to water. Yet the relative importance of different sediment sources remains poorly understood in Swedish catchments. This study integrates GIS-based erosion risk assessment, sediment colour analysis and phosphorus fractionation to improve process-level understanding of sediment and P mobilisation in the Lejstaån catchment, a small agricultural stream in Uppsala, Sweden. A GIS-based Erosion Index (EI) was adapted for use in a narrow, meandering headwater stream and identified several near-stream segments with elevated susceptibility to bank erosion. The EI-classes guided a sampling campaign where soil and sediment from fields, bank sediment, bed sediment, drainage pipes and floodplains from the catchment were represented (n=34). A Phillips sampler was deployed downstream during the period Jan-April representing a time-integrated sample of suspended sediment. Sediment colour analysis using scanned samples and RGB-based metrics showed that suspended sediment collected with a Phillips sampler most closely resembled a mixture of bank, bed and drainage material. However, substantial overlap between source types indicated that colour alone provides limited discriminatory power in this geologically homogeneous catchment. P fractionation revealed that bank material contained both stable (Ca-P, Al-P) and reactive P forms (Fe-P, part of Org-P), demonstrating that bank erosion can mobilise P with differing environmental relevance. Together, the results show that sediment-associated P transport in Lejstaån is strongly influenced by the interaction between geomorphological erosion risk, hydrological connectivity and the chemical reactivity of mobilised material. The findings highlight the importance of near-channel sources for P delivery and underscore the need to consider bank stability, vegetation cover in the near-stream zone and riparian processes in mitigation strategies for small agricultural streams.