Long term water clarity changes in North America's Great Lakes from multi-sensor satellite observations

Water clarity in North America's Laurentian Great Lakes has undergone considerable change over the last several decades as a consequence of invasive species, eutrophication, and implemented nutrient management practices. Satellite observations from the CZCS, SeaWiFS, and MODIS-Aqua sensors have been used in tandem with long term records of Secchi disk depth (Z(SD)) to provide a retrospective analysis of spatial and temporal variations in water clarity over the Great Lakes. A simple empirical algorithm is presented, relating Z(SD) to remote-sensing reflectance at approximate to 550 nm (R-rs approximate to 550). Results suggest remarkable and complex changes in water clarity over the Great Lakes. Lakes Ontario, Huron, and Michigan have seen increases in average Z(SD) over the three sensor periods of 58%, 49%, and 62%, respectively. Lake Erie shows highly variable Z(SD) with no consistent long term trends, while Lake Superior has remained fairly consistent in its lake-wide water clarity conditions. Temporal trends document the decrease in whiting events on Lake Michigan while capturing the ongoing occurrence of these seasonal bright-water events on Lake Ontario. Results indicate a divergence in Z(SD) trends between nearshore and offshore environments; with larger increases in offshore than nearshore Z(SD) and some nearshore areas suggesting a decrease in Z(SD). Offshore regions of Lakes Huron, Michigan, and Superior show diminished Z(SD) seasonality in contrast to increasing seasonal variance in the nearshore. Spatial and temporal variations in Z(SD) are in agreement with documented reductions in Great Lakes bioproductivity, degrading nearshore water quality, and changing biogeochemical processes influencing whiting events and sediment resuspension.