Estimation of the width of the nearshore zone in Lake Michigan using eleven years of MODIS satellite imagery

The nearshore zone, that region of water directly influenced by its proximity to the coast, has received increasing attention in recent years. The extent of the nearshore zone has been defined by some constant descriptive feature: e.g., a specific depth or a particular distance offshore. This type of definition does not allow for the dynamic nature of the relationship between the land and water and how it may be influenced by local, seasonal, or transient effects. Here satellite observations examined evaluate how the width of the nearshore zone in Lake Michigan varies with position along the coastline and with time. Satellite-derived estimates of chlorophyll concentration along seventy-one shore-normal transects spaced approximately 10 km apart around the lake were used to determine the width of the nearshore zone, defined as the point at which the estimated chlorophyll concentration close to the shore approaches the more-uniform offshore concentration. Of a total of 23,807 transects extracted from MODIS observations made between 2003 and 2013, we successfully fit a bi-linear model relating chlorophyll concentration to distance offshore to 15,996. We found that the width of the nearshore zone is variable, both seasonally and spatially. Although the overall median width of 4.5 km (mean width 5.3 km) closely corresponds to the 5 km value used in a number of Great Lakes studies including Lake Michigan, ten percent of the estimates are greater than 8.9 km, likely representing times of enhanced mixing and transport of nearshore waters into the offshore.