A retrospective analysis of the impacts of land management and climate drivers on nutrient contributions to freshwater sources: a Maumee River case study

The return of harmful algal blooms to western Lake Erie has increased focus on managing nutrient contributions from its tributaries. Increased dissolved reactive phosphorus (DRP) loads over the last twenty years is suspected to be a primary cause of the recurrence and severity of these blooms. Understanding the drivers of past loading trends are relevant for assessing the impacts of future conservation practices and their impacts on water quality. This work looks back in time to 1) explore how changes in land management and climate have impacted water quality trends in the region and 2) determine how well our models, developed using considerable data sources from the near present-day, are able to predict water quality in previous time periods. Utilizing the Soil and Water Assessment Tool (SWAT), we investigated changes in climate and land management between 1980 and 2014 to identify key factors driving changes in nutrient trends and our model's prediction power as we moved further back in time. We found that the dominant drivers of discharge and nutrients varied in the historical period: climate trends drove discharge, DRP load, and nitrogen concentrations, while historical management drove phosphorus concentrations and total phosphorus loads. Among the land management changes examined, the rising adoption of minimal- and no-tillage strategies had the greatest impact on nutrient trends. While we saw that our model was able to capture the general shape of trends, predictive power decreased significantly as we moved further from the data rich period the model was originally developed in. This work shows that changes that we implement from a land management perspective can make a difference in achieving water quality goals and standards, even under a changing climate.