Prairie stream nutrient stoichiometry across space and time: Influence of discharge, season, and crop type

Abstract Stream water nutrient ratios are often influenced by flow variation and landscape characteristics. However, the influence of these drivers on total and dissolved nutrient ratios remains understudied, especially in prairie ecosystems where hydrologic connectivity between soils and streams exhibits substantial spatial and seasonal variability. Here, we ask how hydrology and land cover drive patterns of nitrogen (N), phosphorus (P), and N : P ratios across streams and rivers draining northern prairie ecosystems. To answer this, we compiled nutrient concentration data for tributaries of the Red River, Manitoba, Canada, to assess seasonal and annual variation in nutrient ratios, as well as the relationship between crop cover, discharge, and ratios, over 1‐yr, 10‐yr, and 30‐yr time spans. Total nitrogen : total phosphorus ratios were near the Redfield mass ratio (N/P = 7.23) across 24 streams in the Red River Valley. By comparison, dissolved inorganic nitrogen : total dissolved phosphorus ratios in these streams were N depleted and generally declined from spring through to autumn. The types of crops grown did not appear to be a consistent influence on nutrient ratios in streams throughout the region. In contrast, stream flows strongly influenced spring and summer nutrient ratios in four tributaries over a 30‐yr period. Specifically, increasing stream flow tended to decrease TN : TP and DIN : TDP in the two eastern tributaries but increase DIN : TDP in the western tributaries. Our findings that nutrient ratios in prairie streams are impacted by seasonality and fluctuating hydrologic conditions suggest that nutrient ratios in Red River tributaries may be impacted by future climate change.