TOWARD A NEW MODELING APPROACH FOR MANAGEMENT OF NUISANCE CLADOPHORA GROWTH IN THE GREAT LAKES

Cladophora glomerata, a filamentous green alga that grows on hard substrate, first started to attract significant attention in the mid-1970s, when the species spread in Lakes Huron, Erie, and Ontario. High concentrations of soluble reactive phosphorus in discharges led to increased Cladophora growth, which impaired beaches, ecosystem services, and water intakes. The Great Lakes Water Quality Agreement of 1972 (amended in 1978) presumably helped curb the algal proliferation by setting phosphorus limits for any discharges to the Great Lakes. The scientific literature indicates success in this management strategy as measured maximum biomass values decreased. Recent studies, however, show that – while phosphorus limitations are still in effect – Cladophora has returned, and at greater depths. The introduction and establishment of invasive zebra and quagga mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) to the Great Lakes in the late 1980s to early 1990s cause perturbations in the light environment that lead to favorable conditions for Cladophora. This dissertation first investigates the potential drivers of the Cladophora resurgence by comparing historical data and conducting a modeling exercise that – for the first time – quantifies the effect of each driving force on the Cladophora resurgence (Chapter 2). In the spirit of monitoring and analyzing existing data, this section establishes that the Cladophora resurgence is not only perceived but real and that changes in light conditions following the invasion of dreissenids allow for renewed algal proliferation. Chapter 3 of this work describes the development, calibration, and confirmation of a hydrodynamic model, which is applied to describe mass transport in the nearshore of northern Lake Ontario. This work is in response the Great Lakes Water Quality Protocol of 2012, which recognizes that a whole-lake (offshore) approach to controlling nuisance algal growth will not be effective where effluent and tributary discharges are received immediately and locally in the nearshore. Chapter 4 focuses on the development, calibration, and confirmation of the Great Lakes Cladophora Model version 3 (GLCM v3), which includes several improvements of the previous version of the GLCM: redefined light/temperature rate of photosynthesis and respiration response curves, inclusion of a growth-inhibiting self-shading term, a newly defined relationship between the phosphorus uptake rate and stored phosphorus content, replacement of the Droop relationship between the rate of photosynthesis and stored phosphorus content with a similar relationship based on experimental results, and treatment of self-shading and sloughing for an improved understanding and algorithm describing those processes. The

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