Nutrients across Time: Relationships with Climate, Hydrology, and Land Use in Four Rivers of the Pacific Northwest
Authors: Forney William M. 
Published: February 2022

Understanding the temporal dynamics and drivers of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) provides a critical link to better management of nutrient‐related impacts such as eutrophication and harmful algal blooms (HABs). DIN and DIP are a primary control on persistent eutrophication and HABs in the Pacific Northwest (PNW). An understudied phenomenon, this paper examines multi‐decadal trends in DIN and DIP concentrations and loads, and their relationships to climatic and hydrologic factors (e.g., stream and air temperature, discharge, precipitation) in the PNW. Dissolved constituents act as a broad sentinel of linkages between watershed and in‐stream mechanisms such as nitrification, denitrification, nutrient use efficiency, evapotranspiration, hydrologic connectivity, groundwater extraction, irrigation, and land uses. As opposed to the total N and P often used in individual, autochthonous, lentic systems, DIN and DIP are used here as measures of multiscaled processes in allochthonous lotic systems with diverse flow paths. Time‐series data from public agencies were used for up to 20 years in river outlets from the Willamette, Salmon, Spokane, and Yakima watersheds. Seasonal Mann Kendall (SMK) tests suggest significant decreasing multi‐decadal trends in DIN and DIP loads for three out of four watersheds (for DIN, SMK = −0.104; for DIP, SMK = −0.081, −0.181, and −0.213), significant decreasing trends in DIN concentrations for one of the four watersheds (SMK = −0.144), and significant decreasing trends in DIP concentrations in three of the four watersheds (SMK = −0.120, −0.135, and −0.157). Multivariate regressions found significant relationships for concentrations, loads, and ratios when regressed against stream and air temperatures, precipitation, and discharge (16 significant regressions, with adjustedR2values between 0.016 and 0.65). Highlights of these regression results are as follows: (1) precipitation, discharge, and water and air temperatures help to explain DIN and DIP concentrations and loads, (2) changes in DIN concentrations are sensitive to more hydroclimatic variables than DIP concentrations, and (3) DIP concentrations are positively correlated with stream temperature while DIP loads are negatively correlated with stream temperature. Furthermore, seasonal changes in nutrients — and their potential to alter aquatic productivity during a year — has received little attention in the literature. Regressions established significant seasonality or monthly variation of DIN and DIP concentrations and loads in all four watersheds (20 significant regressions, with adjustedR2values between 0.038–0.65). Nutrient thresholds of DIN (0.3–0.5 mg/L) and DIP (0.05–0.005 mg/L) concentrations were used to analyze N‐ and P‐limitation. P‐limitation is known to occur in lakes, and N‐limitation is known to occur in rivers. Surprisingly, except for one watershed (Salmon), nutrient concentrations for both DIN and DIP in all watersheds were shown to be above the limitation thresholds across multiple seasons. In certain situations, such as where significant decreasing trends continue, the DIN:DIP ratio suggests seasonal switching between N‐ and P‐limited could create ideal conditions for HABs. The findings of this study have important implications for water resource management issues such as agriculture, land use development, fish populations, timber harvests, water quality, and public health.