The Export And Algal Availability Of Sediment-Associated Phosphorus In Agricultural Catchments In Eastern Canada

Phosphorus (P) inputs from agricultural sources have been identified as the primary contributor to eutrophication of fresh waters. Sediment associated P is the dominant form of P lost from agricultural lands. This thesis involved three projects being undertaking in three typical agricultural catchments in Eastern Canada and investigated the sediment and P losses during the snowmelt period, and the sediment P storage, algal availability, release potential and possible controlling factors during snow/ice free periods in streams and constructed wet ponds. This knowledge would help us assess the magnitude of soil erosion and P loss at catchment scale and develop beneficial management practices (BMPs) to conserve soil fertility and to reduce water contamination risk during snowmelt period. A further look into the sediment P characteristics in receiving waters during snow free period will help us better understand the fate of sediment P and their role in contributing to the aquatic ecology. The main conclusions are as following:1. Using two agricultural catchments in Eastern Canada, the suspended solids (SS) and P losses during the snowmelt period was quantified to investigate how snowmelt contributes SS and P loss. Water samples were collected from the outlets of the Bras d'Henri Catchment (BHW,2007-2009) and Black Brook Catchment (BBW,2008-2009) and measured for SS and P concentrations. Hydrological parameters (precipitation, snow water equivalent, and runoff discharge), soil frozen status and soil temperature were also measured. Results revealed inter-annual variation of snowmelt conditions and SS and P losses in each catchment. The 2008 snowmelt in BHW and BBW mainly occurred on unfrozen soils, while the 2007 and 2009 snowmelts in BHW and 2009 snowmelt in BBW mainly on frozen soils. In BHW,2008 snowmelt caused much higher median concentrations of SS, total P (TP), dissolved P (DP) and particulate P (PP) in steam water than 2007 and 2009; ratios of PP fractions in TP were variable with events but the median values were similar, suggesting both DP and PP important contrubutor to TP loss. In BBW, the median concentration of dissolved reactive phosphorus (DRP) in stream water was greater in 2008 snowmelt than in 2009 snowmelt; PP dominated TP loss. This study also suggests that soil state (i.e. frozen status) and rainfall were the most important factors influencing SS and P losses during snowmelt. Furthermore, snowmelt P export represented more than 20% of the total annual P export in BHW, and more than 12% of the annual DRP export in BBW. Thus, we strongly recommend adopting Best Management Practices (BMPs) that specifically target sediment and P loss during snowmelt.2. Suspended and streambed sediments were collected from four sites (the outlets of the intervention and control catchments in Bras d'Henri River Catchment, and two branches within the intervention catchment) and from June 2008 to December 2009 and November 2007 to November 2009 respectively. The objective of this study is to assess the role of stream sediments in controlling stream water P by investigating the sediment P storage, algal availability and possible affecting factors. Streambed sediment analysis showed that: (1) the average concentration of TP and AAP (0.1M NaOH extractable P) in Streambed sediments were comparable in intervention and control outlets, but were significantly higher in Brl5 than in Brl4; AAP accounted for approximately 10% of the sediment TP. (2) In all the four sites, the immediately available P (P1) represented less than 1% of the TP in streambed sediments, while more than 60% of the TP presented in the form bound to carbonate and apatite-P (P4). Intervention and control outlets exhibited distinctive concentration differences in P forms—P1, P2i (redox-sensitive P), P3o (P in microorganisms) and P5 (organic and refractory P), with P1, P2i and P5 greater in the control outlet than in the intervention outlet, but in the opposite for P3o. Br15 had significantly greater amounts of P1, P2i, P2o, P3o and P4 in streambed sediments than Br14 did. (3) P saturation index (PSI) were more than 15% and significantly greater in control outlet than in the sites within the intervention catchment (the outlet, Brl4 and Br15), indicating that streambed sediments in control outlet likely acted as a source of stream water P while those in intervention catchment mainly as a sink. P analysis in suspended sediments showed that:(1) TP and AAP in suspended sediments were also comparable between intervention and control outlets, but was significantly higher in Br15 than in Brl4. AAP accounted for 10-30% of the sediment TP. (2) P1, P2i and P2o in intervention suspended sediments were significantly lower than in the control outlet. Br15 had significantly greater amounts of P1, P2i, P2o and P3o than Branch 14 did. (3) PSI values suggested in most of the occasions suspended sediments from all the sites likely acted as a sink of stream water P. Despite of mainly acting as a P sink, the streambed and suspended sediments in Br15 had even greater amounts of TP and AAP than those in the control catchment. This highlighted the high risk of P loss in Br15 draining area and thus BMPs targeting this loss should be implemented in this area in the future.Sediment properties (OM, M3A1 and M3Ca) and the pedology of source soils mainly or partly explained the spatial variations in sediment P characteristics. Redox potential in the sediment-water interface was presumed to be an important factor influencing sediment P characteristics because of groundwater recharge, but pipe and seepage meter experiments in Br14 and Br15 during the base flow period indicated a minor contribution of sediment reducible P release to stream water P in Br14 where anoxic ground water recharge occurred. This was due partly to the low reduction of P-containing Fe/Mn compounds under anaerobic conditions and partly to the poor hydrological connectivity at the sediment-water interface.3. Bottom sediments were regularly taken from three constructed wet ponds to investigate P storage, algal availability and release potential in order to provide information for evaluating the wet ponds'function as a nutrients remover. The sediment TP concentrations hadn't varied significantly among the ponds, while the algal available fractions (AAP and M3P) were significantly greater in ponds B1 and B2 than in pond B3. The relative contribution of AAP to TP varied from 9% to 20% indicating that a considerable part of the TP could be algal available. The majority of sediment P was closely associated with carbonate and apatite (P4) which was relatively stable, while P1 and P2o accounted for the least parts. The maximum PSI values being less than 5% further implied that the three ponds sediments potentially act as a P sink rather than a source in the short-term. In the long term, as long as there is the incoming agricultural runoff, the bottom sediments eventually may play a limited role in removing P from water column due to P sorption sites saturation. This study also indicated the importance of organic matter in controlling the algal availability of sediment P in studied wet ponds.