Hydrological and Seasonal Controls of Phosphorus in Northern Great Plains Agricultural Streams

Abstract

Controls on nutrient transport in cold, low-relief agricultural regions vary dramatically among seasons. The spring snowmelt is often the dominant runoff and nutrient loading event of the year. However, climate change may increase the proportion of runoff occurring with rainfall, and there is an urgent need to understand seasonal controls on nutrient transport to understand how patterns may change in the future. In this study, we assess patterns and drivers of total P (TP) dynamics in eight streams draining agriculturally dominated watersheds, located in southern Manitoba, Canada. Data from three years of monitoring revealed highly coherent patterns of TP concentrations in streams, with pronounced peaks in the spring and midsummer across the region. This coherent pattern was in spite of considerable interannual variability in the magnitude and timing of discharge; in particular, a major storm event occurred in summer 2014, which resulted in more discharge than the preceding spring melt. Concentration-discharge model fits were generally poor or not significant, suggesting that runoff generation is not the primary driver of TP dynamics in the majority of streams. Seasonal patterns of conductivity and stream temperature suggest that mechanisms controlling TP vary by season; a spring TP concentration maximum may be related to surface runoff over frozen soils, whereas the summer TP maximum may be related to temperature-driven biogeochemical processes, which are not well represented in current conceptual or predictive models. These findings suggest that controls on stream TP concentrations are dynamic through the year, and responses to increases in dormant and nondormant season temperatures may depend on seasonally variable processes.

Nitrogen dynamics and nitrogen‐to‐phosphorus stoichiometry in cold region agricultural streams

“…Spring patterns of N concentrations in these streams were strongly driven by snowmelt runoff processes. These patterns are consistent with other agricultural streams in this region, (Corriveau et al, 2013;Casson et al, 2019). Hydrological connection to upland and cropland areas is highest in the early spring.…”

“…Effective drainage area is defined as the area of the catchment that is expected to contribute flow to the stream during an event with a return period of 2 yr (Godwin & Martin, 1975). The differences among catchments in wetland proportion, cropland proportion, size, and effective drainage area reflect the typical range in this region (Casson et al., 2019; Wilson et al., 2019). The catchments were similar in other characteristics such as riparian cropland proportion and riparian grassland proportion and somewhat similar in riparian forest proportion (Table 1).…”

“…Investigations into hydrological controls on P behavior in the northern Great Plains have demonstrated that concentration‐discharge (c‐Q) relationships are complex with low explanatory power (Ali et al., 2017; Casson et al., 2019). Work from other intensively managed agricultural catchments has suggested that large legacy sources of nutrients result in chemostatic conditions (Basu et al., 2010; Thompson et al., 2011).…”

Agricultural Water Quality in Cold Climates: Processes, Drivers, Management Options, and Research Needs

“…Another important pattern in cold climate regions is that nutrient concentrations in agricultural streams often show strong seasonality. In Manitoba, Canada, Casson et al (2019) examine the seasonality of P concentrations and their relationships to streamflow in eight, agriculture‐dominated streams. They find that although P dynamics in spring snowmelt runoff are significantly correlated with discharge and flowpath metrics, during the growing season, the P concentrations exhibit coherent seasonal peaks across years and catchments, independent of flow dynamics.…”

“…They find that although P dynamics in spring snowmelt runoff are significantly correlated with discharge and flowpath metrics, during the growing season, the P concentrations exhibit coherent seasonal peaks across years and catchments, independent of flow dynamics. Their results suggest that the drivers of P dynamics in streams vary by season, and the results point to a more important role of seasonality than landscape characteristics in influencing the dynamics of P concentrations in streams (Casson et al, 2019).…”