N:P stoichiometry in Canadian prairie streams: effects of land cover and hydrologic variability Kim J. Rattan 1 * , Patricia A. Chambers 1 1 Environment Canada and Climate Change, Canada Centre for Inland Waters, Burlington, Ontario, L7S 1A1, Canada * Corresponding author: kim.rattan@canada.ca; 1-905-336-4451
The Issue Satellite image of Lake Winnipeg algae bloom • Prairie streams are an important freshwater resource in North America Great Plains. • Runoff/seepage from agricultural land Lake Winnipeg introduces nutrients, resulting in eutrophication and loss of valuable ecosystem services. • Concentrations and loads of nitrogen (N) and phosphorus (P), in both dissolved and particulate fractions, are sensitive to land cover and hydrologic variability. • Temporal variation in N:P ratios affects lake food webs, particularly the productivity and composition of algal communities. • Yet little is known about riverine N:P ratios, particularly the effects of changing land use and hydroclimatology on the mobilization Blue-Green Algae on Grand Beach and delivery of N relative to P in tributaries.
Objectives To assess the response of N:P ratios (concentrations and loads) to changes in crop cover and hydrologic variability for streams draining the Canadian prairies.
Study Area Seven sub-watersheds: 66 - 677 km 2 Continental Climate: Long cold winters (avg -15 o C) followed by warm summers with precipitation falling predominately during summer months. Soils: clay and silt. Topography: low relief with natural land cover consisting of grasslands with permanent and temporary wetlands. Land use: row crops (small grains, canola) and livestock (cattle, poultry, swine).
Sampling and chemical measurements Chemistry • Grab samples for total and dissolved P and N collected daily during the rising limb and peak of snowmelt, weekly during the falling limb and biweekly thereafter until ice cover, for two years (2013 and 2014). Discharge • Pressure transducer in each stream measuring water level. • Discharge estimated from relationship between water level at site and discharge at long-term d/s government stations. Loads • Nutrient concentrations (measured or linearly interpolated between sampling dates) were multiplied by daily discharge and summed by season or year.
Hydrology and Climate 2014 2013 • In both 2013 and 2014, runoff volume and discharge peaked during snowmelt (orange bar) although snowmelt peaks were less in 2014 (note scale differences between years). • 2014 also experienced more rain events, characterized by additional peaks in the hydrograph (green square).
Hydrology & Climate 8 Rain Precipitation Snowmelt Mean Rain precipitation (mm/day) Spring Summer Fall 6 Annual 4 2 • In 2013, mean precipitation was 0 2013 2014 2014 2013 Year greatest during the spring. Year Runoff and discharge were 1600 Runoff greatest during snowmelt. 1400 1200 Runoff (dam3/day) Summer and fall were only 21% 1000 800 of annual runoff. 600 400 • In 2014, rain precipitation was 200 0 Year 2014 2013 greatest in summer. Runoff and Year Discharge discharge were still greatest 18 16 during snowmelt. Summer and 14 fall comprised 45% of annual 12 Q (m3/s) 10 runoff. 8 6 4 2 0 2014 2013 2014 2013 Year Year
4 a (a) TN Nitrogen Fractions Total N a 3 TN (mg/L) b b b 2 b b b 1 • For TN and DIN, concentrations 0 Year 2013 2014 2013 2014 2013 2014 2013 2014 were greatest during snowmelt Season SN SP SU F for both 2013 and 2014 (orange Dissolved N 4 bar). (b) TDN Dissolved N a 3 a DIN (mg/L) • For PN, concentrations were 2 b b b b b * greatest during snowmelt and b Ammonium 1 # spring in 2013 (orange bars). 0 Concentrations did not differ Year 2013 2014 2013 2014 2013 2014 2013 2014 Season SN SP SU F seasonally in 2014. a (d) PN Particulate N 0.8 a a a b 0.6 PN (mg/L) # a a • Letters (a,b,c,d) above bars identify seasonal means that b 0.4 differ (p<0.05) within a year; symbols (*,#) below bars * 0.2 identify seasonal means that differ (p<0.05) between years. 0.0 Year 2013 2014 2013 2014 2013 2014 2013 2014 • SN = snowmelt, SP = spring, SU = summer, and F= Fall Season SN SP SU F
1.4 (a) TP Phosphorus Fractions Total P 1.1 a TP (mg/L) a 0.8 b b • Like N, total and dissolved P b b 0.5 b b concentrations were highest during 0.2 snowmelt (orange bar). Year 2013 2014 2013 2014 2013 2014 2013 2014 Season SN SP SU F • Like PN, PP was greatest during a 0.5 snowmelt and spring in 2013 (orange (c) SRP Reactive P a b bars). Concentrations showed less 0.4 SRP (mg/L) seasonal change in 2014. 0.3 b b b b 0.2 b • In 2013 (a typical snowmelt-driven year), nutrients were largely exported 0.1 2013 2014 2013 2014 2013 2014 2013 2014 in particulate forms. SN SP SU F • Under “wetter” (i.e., rainier) 0.4 (d) PP a Particulate P conditions of 2014, more nutrients 0.3 were exported in the dissolved form. a b PP (mg/L) 0.2 b a a # a # * # 0.1 • Letters (a,b,c,d) above bars identify seasonal means that * b differ (p<0.05) within a year; symbols (*,#) below bars * 0.0 identify seasonal means that differ (p<0.05) between Year 2013 2014 2013 2014 2013 2014 2013 2014 Season SN SP SU F years. • SN = snowmelt, SP = spring, SU = summer, and F= Fall
Particulate N:P Load Ratios 5 Annual • Results from a two-way ANCOVA PN: PP (Tonne: Tonne) 4 (season and year as factors, crop cover as co-variate) showed that 3 PN:PP ratios differed on an annual and seasonal basis. # 2 • Annually, PN:PP ratios were greater 1 * in 2014. Within years, values were 2013 2014 Year greater during seasons when the soil was wet but not frozen (spring 2013; summer 2014) These findings a 16 suggest that under wetter soil PN:PP (Tonne: Tonne) Seasonal b conditions, greater quantities of PN 12 c were exported relative to PP. b a 8 b d. • Export of PN:PP was not associated b 4 .. with the extent of crop cover in the c 0 watershed. * # # * Season SN SP SU F SN SP SU F Year 2013 2014
Total and Dissolved N:P Load Ratios N: P Load Ratios • Total and dissolved N:P load ratios 16 b TN:TP showed associations (p<0.05) with b TN:TP (Tonne: Tonne) season and crop cover for both 2013 12 c and 2014. a 8 a • a Total N:P ratios were higher (p<0.05) a a 4 during summer and fall. DIN:SRP load ratios were lowest during fall. 0 Season SN SP SU F SN SP SU F • The seasonality in total and dissolved Year 2013 2014 N:P load ratios likely relates to the fact that delivery of P to streams is a a 12 a DIN:SRP DIN: SRP (Tonne: Tonne) influenced by hydrological activity whereas N moves through the 9 a b landscape in dissolved forms. b 6 b • The positive correlation between N:P 3 load ratios and crop cover indicates c 0 greater N loss, relative to P, under Season SN SP SU F SN SP SU F intensive crop cultivation. Year 2013 2014
Conclusions • Dissolved versus particulate N:P load ratios responded differently to land use and hydrologic variability: – land use was the major driver of dissolved N:P load ratios – hydrology was the main driver in particulate N:P load ratios. • Predicting stoichiometry is important because of its strong effects on ecological processes such as primary production. • Improved knowledge of the dominant nutrient forms and their transport pathways will assist in determining appropriate mitigation practices to reduce nutrient loads under a changing climate.
Acknowledgements Collaborators Funding – EC Lake J. Corriveau Winnipeg Basin A. Yates Initiative B. Brua – Canadian Rivers Institute (UNB) J. Culp Technical Support Partners Z. Duggan –Manitoba Gov’t R. MacKay – Land Owners Thank You! Kim.Rattan@canada.ca
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