Event-scale riverine loading of nitrogen and phosphorus: Impacts of land use, seasonality, and antecedent conditions on N:P export ratios

Riverine nutrient loads, particularly during high flows, impact ecological processes in receiving waterbodies by altering nutrient concentrations and ratios. Given that nitrogen (N) to phosphorus (P) ratios influence a range of processes, and that anthropogenic activities are shifting the balance of these elements, it is important to understand N and P loading patterns across space and time. While general differences in N and P provenance and riverine loading dynamics are well-established, drivers of variability in intra- and inter-event loading patterns remains poorly understood. Yet these events tend to dominate annual nutrient loads in many climates. With the advent of in situ sensors and novel algorithms to predict N and P concentrations concurrently, opportunity exists to examine event-scale N and P loading patterns.

Here we use high-frequency N and P measurements from >350 events over 5 years to quantify riverine N and P loading and N:P export ratios in three low-order watersheds with different primary land uses and land covers (i.e., agricultural, forested, urban) in the Lake Champlain Basin in northern Vermont, USA. We also use discharge, precipitation, and high-frequency riparian soil sensor data to examine the influence of storm characteristics and antecedent conditions on N and P export.

Our results demonstrate: (1) the importance of event contributions to annual loads for both nutrients, (2) land use/land cover and season interact with storm size to determine event-scale N and P loading and N:P export ratios, and (3) it is important to consider storm characteristics and antecedent conditions to fully understand N & P export dynamics from watersheds.