Remote sensing of physical cycles in Lake Superior using a spatio-temporal analysis of optical water typologies

An optical class-based approach was adapted and applied to satellite imagery of aquatic color radiometry over Lake Superior. Lake Superior exhibits an optically complex environment whose physical and biogeochemical variability is unknown for much of the year. We characterized optical classes or optical water types (OWTs) from remotely sensed data and determined the temporal and spatial distribution of the OWTs. OWT distributions were interpreted through their inherent optical properties (IOPs) and physical drivers. Five identified OWTs based on spectrally normalized remote sensing reflectance imagery from the MODIS-Aqua mission revealed a gradient between class-specific IOPs for colored dissolved organic matter absorption (a(CDOM)) and particulate backscattering (b(bp)). The clearest OWT displayed widespread prominence over the lake and within and across years, indicating strongly stable physical dynamics including stratification and permanent circulation patterns. The most optically complex OWT (highest a(CDOM) and b(bp)) followed more stochastic dynamics coinciding with localized runoff and mixing events. OWTs with intermediate optical complexity delineated river outflow plumes, upwelling, and annual lake wide turnover events while revealing annual and biannual harmonic patterns. OWTs verified both previously observed and unobserved dynamics in Lake Superior, demonstrating a valuable utility to characterize spatio-temporal, optical, and biogeochemical variability. (C) 2015 Elsevier Inc. All rights reserved.