06/18/2020 Congratulate Yuan Hui on her PhD defense

Yuan Hui successfully defended her PhD thesis today. Congratulations!

Title: Phosphorous Dynamics and Eutrophication in Nearshore Lake Ontario: Insights from Hydrodynamic and Ecological Modeling

Abstract

As the limiting nutrient for eutrophication, phosphorus cycling in large lakes is an important management consideration. Dynamics controlling the fate and transport of total phosphorus (TP) in lakes involve an integrated system including physical, chemical, and biological processes. TP studies are especially important for large lakes such as Lake Ontario, which provides significant ecosystem services, but also are facing challenging ecological issues such as the resurgence of the benthic algae Cladophora. Of particular concern and interest is that even with overall decreasing TP concentrations since implementation of the Great Lakes Water Quality Agreement (GLWQA) signed by the U.S. and Canada in 1972, nearshore concentrations are higher than target values, and have contributed to worsening eutrophication issues. This study focuses on generating a better understanding of TP dynamics and developing a tool for its management. The first part of this work focuses on lake hydrodynamics, as the basis of nutrient transport, and the second part considers lake ecology, water quality, and lower-food-web modeling, including dreissenid mussels and Cladophora. A state-of-the-art hydrodynamic (Environmental Fluid Dynamic Codes (EFDC)) and ecological (Advanced Aquatic Ecosystem Model (A2EM)) modeling framework is applied that considers all major processes in these areas. Application of the EFDC shows that runoff, which is often neglected in hydrodynamic modeling, could be an important contributor to water quantity. Both numerical and physical modeling are used to demonstrate the importance of nearshore circulation patterns for the distribution of TP and Cladophora, and to show the need for sufficiently fine scale modeling resolution in nearshore regions of the lake. Application of A2EM shows dreissenids decrease TP in the water column, and the decreased TP is stabilized in the nearshore sediment in the form of feces and pseudofeces, instead of being transported offshore. This finding represents a first quantification of the so-called “nearshore shunt hypothesis” for Lake Ontario. Apart from total annual loading of TP from tributaries, which is a common component of TP management, the timing of TP loads is also shown to be important for Cladophora abundance. These findings enhance the understanding of TP cycling in Lake Ontario, especially the roles played by invasive dreissenids and Cladophora, and demonstrate the usefulness of a well-developed modeling approach for TP management in large lakes in general.

Advisors: Dr. Joseph Atkinson and Dr. Zhenduo Zhu