Water quality monitoring in Basque coastal areas using local chlorophyll-<inline-formula><math display="inline" overflow="scroll"><mrow><mi>a</mi></mrow></math></inline-formula> algorithm and MERIS images

Abstract

Accurate estimation of chlorophyll-a (chl-a), a proxy of the eutrophication risk, is necessary in coastal areas for the assessment of water quality in accordance with European Direc- tives. Local parameterization of remote sensing algorithms is useful to cope with the variability and specificity of optically-active in-water constituents. Using the Bay of Biscay coastal waters, affected by Basque river runoffs, as a case study, the objectives of this investigation are to: 1. develop an empirical algorithm to estimate water surface chl-a for the optically-complex Basque coastal waters; 2. explore the influence of suspended matter, phytoplankton species, and pigment content on the algorithm developed for medium resolution imaging spectrometer instrument (MERIS) imagery; 3. compare the local algorithm to three ocean color algorithms (OC4v6, Gitelson's algorithm, and the OC5); and 4. apply the local algorithm to the MERIS images. For this purpose, two surveys were undertaken within the study area, the Batel-1 survey in 2007, and the Batel-2, in 2009. The empirical algorithm was developed with remote sensing reflectances (Rrs), undertaken with a TriOS field spectrometer, and chl-a measured in situ from the Batel-2 survey. The algorithm was not affected by different concentrations of suspended matter in surface waters, within the range from 0.0 to 6.6 g · m −3 . There was no significant effect of 23 accessory pigments found in the area on the algorithm. Eighty-four Rrs and chl-a measure- ments from the Batel-1 survey were used to validate the local algorithm and to compare it with output of the other algorithms. The local algorithm provided the lowest root-mean-square differ- ence (RMS ¼ 1.7 mg · m −3 ), the best correlation with the observed data (R ¼ 0.8), together with the best slope-intercept combination between predicted and observed chl-a (slope ¼ 0.5, intercept ¼ 0.6). The chl-a algorithm developed here for MERIS imagery can assist in the assess- ment of water ecological status in the southeastern part of the Bay of Biscay, in a cost-effective manner. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10.1117/ 1.JRS.6.063519)