Long-term trends in aquatic pollutants: Chloride and phosphorus dynamics in lakes embedded in urban and agricultural watersheds

Aquatic ecosystems are increasingly affected by human activities such as land use change in the watershed. Changes in land use affect the delivery of non-point source loads of nutrients and pollutants. I explore long-term trends and drivers of two aquatic pollutants, chloride and phosphorus, associated with changes in land use and land cover. Comparison of these two pollutants provided an opportunity to explore simple and complex models for solute transport and processing in lakes embedded in changing landscapes.;Excessive chloride loading has become pervasive due to increases in road miles and rates of road salt application associated with urbanizing watersheds in northern climates. In the first chapter, I examined the relationship between road salt application and chloride dynamics in an urban watershed, Lake Wingra. Using a model calibrated with long-term data, I explored scenarios for changes in road salt management. I found that, under current conditions, the lake should quickly respond to changes in road salt application and projected mean concentrations are unlikely to exceed guidelines for aquatic organisms. However, trends in chloride concentrations of the groundwater underlying the Wingra watershed suggested a need to mitigate road salt application.;In the second portion of my dissertation, I explored phosphorus dynamics in Lake Mendota to assess the relative importance of external loading and recycling of this limiting nutrient. Using a combination of field sampling, long-term data analysis and biological and physical modeling, I assessed the contribution of three different mechanisms of phosphorus recycling: biotic recycling, entrainment and sediment release. Both biotic recycling and entrainment provided important sources of P to primary producers during the stratified season. Entrainment was spatially and temporally variable, but could be reasonably represented using both a 3-D hydrodynamic model and sampling based on a single central location. Finally, although long-term P dynamics were primarily driven by external loading, P in the hypolimnion increased over time. This change was driven by changes in the stability of the water column and increased length of stratification. Changes in the physical condition of the lake held implications for phosphorus recycling and water clarity during the summer.