Drivers of excess phosphorus and stream sediments in a nested agricultural catchment during base and stormflow conditions

Abstract

A variety of landscape and hydrological characteristics influence nutrient concentrations and suspended sediments in freshwater systems. Yet, the combined influence of these characteristics within nested agricultural catchments is still poorly understood, particularly across varying flow states. To tease apart potential drivers at within catchment scales, it is necessary to sample at a spatio‐temporal resolution that captures how landscape drivers change over time. The overall objective of this study was to evaluate the relative influence of landscape and hydrological characteristics at subcatchment scales in relation to total phosphorus (TP), soluble reactive phosphorus (SRP), the ratio of SRP and TP (SRP/TP), and total suspended solids (TSS) across varying flow conditions. Synoptic surveys were conducted at 13 longitudinal sampling sites at a variety of flow conditions (n = 14) between 2016–2017 in the Innisfil Creek watershed, Southern Ontario. The surveys were grouped into baseflow and stormflow conditions, and partial least squares regression (PLSR) was used to characterize the relationships between catchment characteristics, median concentrations of phosphorus, and TSS. Soil texture (i.e., clay dominated), winter wheat, and constructed drain density had the largest influence on stormflow SRP, and SRP/TP ratios; but, measures of soil erosion, like bank erosion hazard index and sinuosity had the largest influence on stormflow TSS. During baseflow periods, these landscape characteristics were not informative; and they were difficult to tie to in‐stream conditions. Overall, our PLSR models indicated that buried tile drainage was a major source of SRP in Innisfil Creek, whereas bank erosion was a dominant source of TSS. This article is protected by copyright. All rights reserved