Sources, composition and transport of fluvial suspended sediment and attached phosphorus in agricultural catchments : a cross-scale analysis

Sammanfattning: Soil loss from arable land is a major environmental issue, as well as freshwater and marine eutrophication. Both these issues are well connected, since particles often transport phosphorus (P) from agricultural areas to surface waters, causing eutrophication. The main objective of this thesis was to gain a deeper understanding of the relationship between P and suspended solids (SS) in agricultural streams, to study the quantity and quality of P on SS and the role of catchment properties. By using long-term monitoring data on small agricultural catchments it was established that SS is a vector for P transport, and that SS concentrations in small streams are well correlated to clay content and share agricultural land. These relations were also up-scaled and corroborated for the tributaries to Lake Mälaren in spite of their larger size and more heterogeneous land use. Testing stream nutrient concentration trends in these catchments over the last 20 years resulted in few significant decreasing trends, due to the low degree of implementation of mitigation measures and the historical legacy effects. Catchment properties such as elevation, land use and soil type were also used as input variables in high resolution distributed dynamic modelling of SS transport from six small catchments with good agreement to observed data, which makes possible reliable modelling of similar, unmonitored catchments. Field and laboratory studies of three small catchments sampling fluvial suspended sediment showed variation in iron-bound P with seasons, with increases during summer and decreases during winter, high flow periods. Each catchment had different dominating P fractions, in spite of similar total P concentrations. To achieve water quality goals we need to focus resources where they have the highest effect.

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