Assessing the Physical Drivers of Water Quality along Lake Superior's South Shore

This study aims to identify mechanisms that influence the retention of various chemical species near Chequamegon Bay and the Apostle Islands National Lakeshore (AINL), which are key ecosystems along Lake Superior’s southern shore. During the summer months of 2008, 2012, and 2014, these waters experienced highly variable conditions. In 2008, the Environmental Protection Agency measured phosphorus levels that were nearly 10 times above the national standard. In the latter part of June of 2012, the AINL experienced its first ever algal bloom. Yet in 2014, these waters along the southern shoreline experienced relatively pristine conditions. Using satellite surface temperatures and reanalysis, it was found that the ten meter winds were stronger in those years, and ten-meter air temperatures were warmer near ice-out during poor water quality years. High early season temperatures also increased the lake temperature gradient between the near- and far-shore. This suggests a strengthened coastally trapped current during 2008 and 2012. The influence of atmospheric temperatures, winds, and the coastal current on Lake Superior water quality were examined using ROMS (Regional Ocean Modelling System). It was found that the strength of the coastal current was the dominant driver of water quality in these years.