Standard and Regional Bio-Optical Algorithms for Chlorophyll $a$ Estimates in the Atlantic off the Southwestern Iberian Peninsula

This study investigates standard and regional algal pigment index 1 (API1) estimates in the Atlantic off the southwestern Iberian Peninsula. Standard API1 data are those delivered by the Medium Resolution Imaging Spectrometer (MERIS) orbiting sensor. Equivalent quantities are computed by applying a regional inversion scheme using as input both MERIS and in situ remote sensing reflectances $(R_{\mathrm{rs}})$ . Reference data for the development of the regional algorithm and for the analysis of tested products include field measurements of total concentration of chlorophyll $a$ (TChl a ) and coincident $R_{\mathrm{rs}}$ values collected at different distances from the coast. Validation results, based on matchup analysis, identifies a systematic overestimation of standard API1 versus the reference TChl a values. The additional comparison of product maps in selected regions of interest confirms this tendency and demonstrates the feasibility and relevance of using regional algorithms for investigating spaceborne products. Analogous applications are hence devised for the early-stage evaluation of the forthcoming Sentinel-3/OLCI data products.