Northern Gulf of Mexico estuarine coloured dissolved organic matter derived from MODIS data

Coloured dissolved organic matter CDOM is relevant for water quality management and may become an important measure to complement future water quality assessment programmes. An approach to derive CDOM using the Moderate Resolution Imaging Spectroradiometer MODIS was developed that may be more accessible to water quality managers by selecting an off-the-shelf software and algorithm with standard atmospheric correction. This study focused on demonstrating the transferability of a remote-sensing reflectance Rrs band ratio algorithm, Rrs667/Rrs488, previously developed to derive CDOM absorption agλ at multiple MODIS wavebands in open ocean and coastal waters to the United States northern Gulf of Mexico estuaries. In situ calibration regressions at 412, 443, 469, and 488 nm had coefficients of determination R2 of 0.76, 0.71, 0.69, and 0.57, respectively. Waveband calibrations at 531, 547, and 555 nm were below R2 = 0.50, and therefore were not considered further. MODIS Rrs, from the standard atmospheric correction, followed nearly identical spectral shapes to the in situ HyperSAS Rrs, but were on average 0.002 ± 0.0004 sr−1 less. A satellite to in situ validation match-up window of ≤1 hour was selected with an R2 = 0.82 and root mean square error RMSE = 1.79 at 412 nm. An in situ water quality mooring demonstrated that the overall response and range of MODIS ag412 were similar, with relative mean error from –32% to 42%. The advantage to managers was synoptic coverage across multiple estuaries and the ability to provide estimates of derived water quality parameters between the water quality assessment programme sample collection periods, which could offer more holistic assessment.