Evaluation of atmospheric correction and high-resolution processing on SeaDAS-derived chlorophyll-a: an example from mid-latitude mesotrophic waters

Over the last 15 years, great effort has gone into the development of chlorophyll-<i>a</i> (chl-<i>a</i>) retrieval algorithms for case 2 waters, where variations in the water leaving radiance signal are not well correlated with concentrations of chl-<i>a</i>. In this study, we investigate the effectiveness of Moderate Resolution Imaging Spectroradiometer (MODIS)-derived chl-<i>a</i> retrieval algorithms in the less productive coastal waters around Tasmania, Australia. Algorithms were evaluated using matches between satellite imagery and <i>in-situ</i> water samples (number of samples, <i>n</i> = 16–65) derived from a 604 sample data set collected over a 9-year period. Three aerosol correction models and three chl-<i>a</i> retrieval algorithms were evaluated using both standard and high-resolution processing procedures using the National Aeronatics and Space Adminstration’s SeaDAS software package. chl-<i>a</i> retrievals were evaluated in Bass Strait, where <i>in-situ</i> chl-<i>a</i> was less than 1 mg m⁻³ and retrievals were less affected by coloured dissolved organic matter. chlor_a, the default SeaDAS chl-<i>a</i> product, with the Management unit of the North Sea Mathematical models aerosol correction algorithm performed best (root mean square error (RMSE) = 0.09 mg m⁻³; mean absolute percentage error (MAPE) = 34%; coefficient of determination, <i>R</i>² = 0.75). The fluorescence line height algorithm using Rayleigh corrected top of atmosphere reflectances (RMSE = 0.11 mg m⁻³, MAPE = 41%, <i>R</i>² = 0.61) may provide an alternative in waters where full atmospheric correction is problematic and the two-band red/near-infrared algorithm failed to provide a meaningful estimate of chl-<i>a</i>. High-resolution processing of MODIS imagery improved spatial resolution but reduced chl-<i>a</i> retrieval accuracy, reducing the agreement between measured and predicted levels by between 12% and 25% depending on the retrieval algorithm. The SeaDAS default chlor_a product proved superior to the alternatives in mid-latitude mesotrophic coastal waters with low chl-<i>a</i> concentrations. In addition, there appears little benefit in using MODIS high-resolution processing mode for chl-<i>a</i> retrievals.