Remote sensing of biotic effects:: Zebra mussels (Dreissena polymorpha) influence on water clarity in Saginaw Bay, Lake Huron

In this study, Advanced Very High Resolution Radiometer (AVHRR) remote sensing reflectance (R(rs)), imagery from 1987-1993 is used to study changes in water clarity before and after zebra mussels (Dreissena polymorpha) were discovered in Saginaw Bay, Lake Huron. Spatial and temporal trends in the data indicate distinct and persistent increases in water clarity in the inner bay after the first large recruitment of zebra mussels in the fall of 1991. The pre-Dreissena imagery show that turbidity in the inner bay was influenced by the Saginaw River discharge in spring, biological production (plankton) in summer, and wind-driven resuspension in fall, with highest turbidity in spring and fall. Spatial patterns in the post-Dreissena images were more similar regardless of season, with low reflectances in the shallow regions of the inner bay where zebra mussel densities were highest. A regression model based on point data from 24 sampling stations over the 7-yr period indicates that reflectances varied significantly by site and zebra mussel densities, as well as seasonally. Trends in observed and predicted values of reflectances followed similar patterns at each station-highest values were found during 1991 and lowest during 1992 at all stations, with slightly higher R(rs) in 1993 compared to 1992. Whereas AVHRR R(rs) highlight the value of historical imagery for reconstructing seasonal and interannual turbidity patterns in near-shore waters, a new generation of operational ocean color satellites, such as SeaWiFS (Sea-viewing Wide Field-of-view Sensor) and the newly launched MODIS (moderate resolution imaging spectroradiometer), now provide for routine monitoring of important biological and physical processes from space.