WRS Masters Defense

Temporal Changes in Carbonate System Variables in Lake Superior Tributaries

Northeastern Minnesota, like other regions, has exhibited recent climate change effects; predicted changes over the next century include longer dry periods, less snow accumulation, and more large rain and snow events. Carbonate system variables including pH, total inorganic carbon (TIC) concentrations, and total alkalinity (TA) are all highly dependent on and have feedbacks with these climate factors. These variables can also influence the quality of life for various species, such as trout, which rely on many streams along the north shore of Lake Superior for spawning habitats. Therefore, studying the current conditions of waterbodies in northeastern Minnesota is crucial for setting a baseline to estimate and predict the potential ramifications of regional climate change. Amity Creek, Tischer Creek, and the St. Louis River are three tributaries of Lake Superior varying in watershed size and overall length. This study investigated how pH, TA, and TIC of these tributaries are affected by seasonality, stormflow, and diurnal conditions, as well as their inter-annual variability based on available data. Seasonal sampling showed that carbonate system variables were relatively stable throughout fall 2021 before decreasing to minimum levels in spring 2022 when large freshets and rainfall events occurred. Carbonate system variables reached maximum levels in the summer of 2022 when water and air temperatures were greatest in the sampling period and photosynthetic activity levels were also likely the highest. Autumn 2022 levels were similar to summer 2022 levels. During storm events, pH, TIC, and TA decreased compared to baseflow.  Specific trends during storms were dependent on storm characteristics and prior weather conditions. Over a diurnal period in Amity Creek, pH increased during the morning hours to a midday maximum and then decreased. TIC and TA increased throughout the day, likely due to the increased influence of groundwater as the stream recovered from a rain event that occurred 4 days prior. Historically, pH has been increasing in the St. Louis River and Amity Creek over the past several years while Tischer Creek pH shows no long-term inter-annual trend but appears highly susceptible to inter-annual variability. Alkalinity trends were more difficult to assess due to data availability. However, where available, alkalinity trends closely follow the pH trends in the case of Amity Creek, while in the St. Louis River, alkalinity remained stagnant. In Tischer Creek, hardness data was used as a proxy variable. Hardness levels appeared to be increasing but had notably similar inter-annual variability as pH. This study demonstrates how carbonate system variables in streams and rivers can shift over long time frames, but more importantly how much variability can occur over the course of storm events and hours.