Particle transport of phosphorus in streams draining catchments with different land uses

Summary

Numerous studies on lake Vansjø, located in south-east in Norway, have identified eutrophication as one of the major causes for not achieving good water quality status in the western part of the lake. This has been shown to be the result of high nutrient concentrations in soils, sediments and water. Main causes for the high nutrient levels in water seem to be large flux of phosphorus leached from over-fertilized agricultural soils and release from sediments. Investigations have shown that the major P fraction in the streams and entering the lake is particulate-P. This study, particle transport of phosphorus in streams draining catchments with different land uses, has been carried out as part of the larger Eutropia project (RCN funded project number 190028) to evaluate phosphorus pools both in suspended and stream sediment samples thereby obtaining a better understanding of the catchment processes governing the mobilization, transport and variation in levels of particulate-P. Therefore, how P is bound to the particles i.e. the pools and forms of P in the suspended particles and sediments were assessed. Stream sediment samples were collected from nine inlet streams to lake Vansjø and analyzed for pH, loss on ignition, particle size distribution, and total major cations, Si and P using Inductively Coupled Plasma-Optical Emission Spectroscopy and X-ray Fluorescence. Furthermore, P-fractionation, total Phosphorus (Tot-P), total organic P, and inorganic P fractions, was conducted using the method Møberg and Pedersen and sequential extraction. In addition, the mineral contents of the stream sediments were identified by X-ray diffraction analysis. Scanning electron microscope analysis was done on selected samples in both stream and suspended sediments to verify presence of apatite, and see similarities between the sediments’ surface topography and micro elemental analysis. Water samples collected from the same streams were also analyzed for pH, total suspended particle matter (SPM) including organic, inorganic phosphorus and others parameters which are not included here. It is found that the main fraction of phosphorus in stream sediments is present in inorganic forms (soluble in 6 M H2SO4), which comprised a spatial mean of about 79 % of the total. More than 50 % of it is considered to be non-available apatite P associated with calcium (Ca-P). The average rank order of the different P-fractionations for these nine streams was Ca-P > Fe-P> Al-P > NH4Cl-P > occluded P. The P levels found in the sediments are in general high (average 3.2 g kg-1 of dry weight), although there is large spatial variability. Sperrebotn and Dalen streams sediments contained the highest and lowest total phosphorus contents, 5.6 and 2.2 g kg-1, respectively. This high P content in the sediments can be detrimental to the water quality even though most of it not readily available Ca-P. The highest tot-P values were found in streams which have combined effect of land uses (both agriculture and forest areas) while tot-P contents obtained in the sediments of streams draining mainly agricultural and mainly forest lands were not so high. The content of InO-P and SPM-I in water from the streams were high during periods of high flow. Particulate bound phosphorus can undergo transformations by (reductive) dissolution, desorption, organic mineralization or bacterial decomposition resulting in bioavailable forms. Transport of these particles to the lake may therefore potentially have significant effect in deteriorating quality of the lake water.