Water clarity of the Upper Great Lakes: Tracking changes between 1998-2012

Water clarity trends in three upper Great Lakes, Lakes Superior, Michigan, and Huron, were assessed via satellite imagery from 1998 to 2012. Light attenuation coefficients (Kd490) from SeaWiFS and Aqua MODIS satellites compared favorably with in situ measurements. Significant temporal and spatial trends and differences in Kd490 were noted within all three of the lakes. Lake-wide average Kd490 for Lake Superior did not exhibit any changes between 1998 and 2012. Annual Kd490 values for Lake Huron, however, showed a significant negative trend during the study period using both SeaWiFS and MODIS datasets. Similarly, annual Kd490 values of Lake Michigan declined between 1998 and 2010. Only in the offshore waters (> 90 m depth) of northern Lake Michigan did Kd490 increase but just after 2007. Photic depth increased significantly in both Lake Michigan (≃ 5 m), and Lake Huron (≃ 10 m) when comparing annual Kd490 for pre- (1998-2001) and post-dreissenid mussel (2006-2010). At seasonal level, significant decreases in Kd490 in lakes Michigan and Huron were mainly noted for the spring/fall/winter mixing periods. After these recent changes in water clarity, lake-wide photic depths in lakes Michigan and Huron superseded Lake Superior; thus, making Lake Superior no longer the clearest Great Lake. Several factors (e.g. filtering activities of quagga mussels, phosphorus abatement, climate change, etc.) are likely responsible for these large changes.