Total, dissolved and particulate N: P stoichiometry in prairie streams in relation to land cover and hydrologic variability

Concentrations and loads of nitrogen (N) and phosphorus (P), in both dissolved and particulate fractions, are sensitive to land cover and hydrologic variability. Previous studies on relationships between watershed characteristics and stream chemistry have focused on the response of individual N and P fractions. However, marine and lakes studies have shown the value of using individual nutrient fractions as well as nutrient ratios (e.g., N:P) to assess ecosystem condition. This study examined the response of total, dissolved, and particulate N and P fractions as well as N:P ratios to changes in % crop cover and hydrologic variability in three agricultural catchments in the Red River Basin, southern Manitoba, Canada. While discharge peaks were greatest during snowmelt for both study years (2013 and 2014), flow ceased in early June 2013 due to lack of precipitation whereas discharge peaks were observed during summer and fall 2014 in response to persistent multi-day rain events. Despite hydrologic differences between the two years, total, dissolved, and particulate N:P concentration ratios did not differ (p>0.05) between years (expressed as either annual or seasonal means) or in response to % crop cover. In contrast, N: P load ratios were associated with watershed characteristics: total N: P load ratios differed (p<0.05) in relation to both % crop cover and season, dissolved N:P load ratios differed (p<0.05) with % crop cover, and particulate N:P load ratios differed (p<0.05) between seasons and years. Our finding that dissolved versus particulate N:P load ratios responded differently to land use and hydrologic changes suggests the dissolved load ratios are more closely linked to land use activities whereas particulate loads ratios are largely influenced by interannual climate and discharge variability. 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.