The Laurentian Great Lakes contain approximately 2% of the world's freshwater and provide drinking water for more than 30 million people. Lake Erie is the shallowest and smallest of the Great Lakes. The western basin is the shallowest portion of Lake Erie and receives a majority of its water from the Maumee River and Detroit River. The Maumee River Watershed is the largest watershed in the Great Lakes and land use in the watershed dominated by urban and agricultural areas. Farm field run-off and combined sewer overflows from urban and agricultural areas contribute high concentrations of nutrients to the Maumee River. The high nutrient water flowing out of the Maumee River into the Western Basin of Lake Erie creates a good environment for Harmful Algal Blooms (HABs), which are primarily composed of Microcystis, to rapidly grow. HABs are a serious issue for Lake Erie because Microcystis produces a toxin that is harmful to humans. Research focused on decreasing HABs has identified phosphorus as the limiting nutrient. Phosphorus enters the lake through either external (phosphorus entering from outside the lake i.e. farm field run-off or inputs from tributaries) or internal loading (release of phosphorus from lake sediments). For Western Lake Erie, there are numerous studies on external loading of phosphorus, however there is considerably less research on internal phosphorus loading. Estimating internal loading of phosphorus is important because phosphorus concentrations in lake sediments can be orders of magnitude higher than phosphorus concentrations in lake water and can be a significant source of phosphorus. Phosphorus release rates from lake sediments can be affected by a variety of environmental factors such as pH, temperature, and dissolved oxygen concentration. If lake sediments become warmer and more hypoxic in response to climate change, the contribution of phosphorus from lake sediments may increase. The research conducted in this thesis investigates the effect of temperature on the release of phosphorus from anoxic lake sediments. This study aims to estimate phosphorus release rates for a range of temperatures under anoxic conditions for Western Lake Erie and to improve estimates of internal phosphorus loading. Sediment cores collected during the summer of 2014 and 2015 were incubated at 10°C, 20°C, 22°C, 27°C, and 30°C under anoxic conditions. Results from the sediment core incubations indicate that temperature, location, and duration of anoxia are significant influences on phosphorus release. Extrapolating the phosphorus release rates for the entire western basin, four days of anoxia at 30°C for the entire basin could result in the release of ~240 metric tons of phosphorus.

Chapter 2: Stratification and hypoxia in the western basin of Lake Erie (WBLE) has been shown to result in phosphorus flux from the underlying sediment, which could provide necessary nutrients for harmful algal bloom (HAB) growth. Studying the duration and frequency of hypoxic events would provide pivotal information for estimations of phosphorus flux from underlying sediments. However, due to the ephemeral nature of hypoxic events in the WBLE, planned weekly vessel-based sampling trips are inadequate for alerting researchers of the onset of hypoxia, making sampling such events difficult. Instead, water quality instruments can be deployed to collect and relay live data to researchers in a much more frequent timeline. In this study, a buoy equipped with a thermistor string and an EXO3 sonde (Yellow Springs Institute) was deployed to monitor for potential stratification and depleting lake bottom oxygen concentrations. This system measured water quality parameters and posted the data online every 20 minutes. Using these data, immediate vessel-based sampling trips to 7 sites were made according to observed hypoxia. Data captured show a hypoxic event occurred in the WBLE during early July 2020 that persisted for several days before being mixed by a storm on July 11, 2020. This hypoxic event coincided with 8 days of stratification. In addition, hypolimnion water warmed to over 23 °C while remaining stratified from the overlying waters, which could facilitate higher phosphorus flux from sediments. On average, phosphorus concentrations in the hypolimnion were 1.06 μ/L (~43%) higher than in the epilimnion by the end of the event, suggesting that sediments were releasing phosphorus into the overlying waters. Chapter 3: The western basin of Lake Erie (WBLE) has been experiencing Harmful Algal Blooms (HABs) for over a decade. These blooms have been detrimental to the health of Lake Erie and the safety of drinking water for surrounding communities. Nutrient inputs (namely phosphorus) have been exacerbating these problems, providing necessary nutrients for HAB growth. While external loading of phosphorus is being addressed at a large scale, more needs to be discovered about the effects and likelihood of internal loading from Lake Erie sediments. Studies have suggested that anoxic sediments release phosphorus into overlying waters at increasing rates that correlate with increasing temperature, particularly between 20 °C and 30 °C where there was a 7 times increase in phosphorus flux (Matisoff et al., 2016; Gibbons and Bridgeman, 2020). However, the trajectory of the increase in phosphorus is to be determined. In this study, sediment cores were collected from two sites in the WBLE (sites 4P and 7M) and incubated in anoxic conditions at varying temperature treatments within the range of 20 °C and 30 °C. Temperature treatments consisted of 20 °C, 23 °C, 26 °C, and 29 °C. Results indicate the largest increase in phosphorus flux for site 4P came between the temperatures of 26 °C and 29 °C, with an average phosphorus concentration increase of 100.9 μg/L. However, site 7M showed the largest increase in phosphorus flux between 23 °C and 26 °C, with an average phosphorus concentration increase of 252.4 μg/L. Such a large flux between the temperatures of 23 °C and 26 °C is potentially alarming as lake bottom temperatures already exceed 23 °C and are likely to increase in the future due to climate change.