Detecting Chlorophyll-a in Lake Garda Using TOA MERIS Radiances

Abstract

The performance of MERIS as a tool for mapping chlorophyll-a concentrations in lake waters has been evaluated using simulated and measured top of atmosphere radiances for Lake Garda (Italy). MERIS observations were simulated using hyperspectral data collected by the MIVIS imaging spectrometer in July 2000. MIVIS data were radiometrically corrected at the sensor altitude using the 6S radiative transfer code. The MERIS simulation process was verified using ETM+ measurements acquired at the same time of the MIVIS flight and differences between simulated and actual radiance measurements in ETM+ bands 1, 2, and 3 were about 10 percent. In July 2003, a cloud-free MERIS image was available. MERIS radiances of both dates were used to describe the variation of chlorophyll-a content in the lake that was estimated synchronously to remote observations using continuous track fluorometer data. In 2000, when the mean value of chlorophyll-a was about 6 mg/m 3 the best performing algorithm (RMSE = 0.58 mg/m 3 ) was a ratio of band differences using VIS and NIR wavelengths. In 2003, when the chlorophyll-a concentration in the lake was very low (mean <1 mg/m 3 ), a single channel centered at 490 nm was the best index in describing spatial variations of chlorophyll-a (RMSE = 0.10 mg/m 3 ). The results suggest that MERIS observations are providing useful information for assessing and monitoring chlorophyll-a distribution in lacustrine ecosystems.