Estimation of Phytoplankton Chlorophyll-a Concentrations in the Western Basin of Lake Erie Using Sentinel-2 and Sentinel-3 Data

Abstract

Algae blooms have been a serious problem in coastal and inland water bodies across Canada. The temporal and spatial variability of algae blooms makes it difficult to use in situ monitoring of the lakes. This study aimed to evaluate the potential of Sentinel-3 Ocean and Land Color Instrument (OLCI) and Sentinel-2 Multispectral Instrument (MSI) data for monitoring algal blooms in Lake Erie. Chlorophyll-a (Chl-a)-related products of these sensors were tested by using the Great Lakes Chl-a NOAA’s monitoring data over summer 2016 and 2017, respectively. Our results show that while fluorescent light height (FLH) algorithm and models are limited to lakes with Chl-a < 8 mg/m3, maximum chlorophyll index (MCI) has the potential to be used effectively to monitor Chl-a concentration over Eutrophic lakes. Sentinel-3’s MCI is suggested for Chl-a > 20 mg/m3 and Sentinel-2 MCI for Chla > 8 mg/m3. Top of atmosphere (TOA) radiances showed a significantly better correlation with in situ data compared to TOA reflectance, which may be related to the poor pixel identification during the process of pixel flagging affected by the complexity of Case-2 water. Sentinel-2 MCI achieves better performance for Chl-a retrieval (R 2 = 0.92) than the existing methods. However, the FLH algorithms outperformed negative reflectance due to the shift of reflectance peak to longer wavelengths along with increasing Chl-a values.