. . . . . . "Open-source Sentinel-2 chlorophyll-a retrieval validated against in situ data in the Westerschelde estuary" . . . . . . "Five deliberate deviations, each recorded rather than hidden:\n\n1. Atmospheric-correction processor. The paper selected C2RCC for its chlorophyll-a\n time series (the \"cGS\" chain, C2RCC + Gons). C2RCC exists only inside ESA SNAP\n and crashes natively in the container, so it is excluded; this replication runs\n Acolite instead, making the chain \"aGS\" (Acolite + Gons), an open-source\n analogue. Note the original study itself reports Acolite as its worst-performing\n processor.\n\n2. Site. Validation is in the Westerschelde estuary, not the Sado, because the Sado\n in situ match-up data are available only on request (paper Data Availability\n Statement) and no open, contemporaneous, in-estuary substitute exists. The\n Westerschelde is a comparable mesotidal, well-mixed, turbid estuary.\n\n3. Period. The record spans January 2016 to July 2026 (the full Sentinel-2 era),\n not the original's March 2018 to March 2020 field campaign, to gain the \"more\n complete set of match-ups\" the paper called for.\n\n4. In situ reference. Coincident chlorophyll-a comes from Rijkswaterstaat routine\n monitoring stations, not the original AQUASado sampling campaign.\n\n5. Fully open-source chain. Every step (Sentinel-2 L1C, Acolite, the Gons algorithm)\n is open and reproducible, whereas the original's selected chain depends on the\n proprietary SNAP/C2RCC processor." . . "The satellite half of the paper's pipeline is reproduced with a fully open-source\nchain (the \"aGS\" chain) and validated against an independent in situ reference.\nSentinel-2 A/B/C Level-1C granules over the Westerschelde estuary are\natmospherically corrected with Acolite to water-leaving reflectance, clipped to an\nestuary bounding box covering the along-axis stations. Chlorophyll-a is retrieved\nwith the Gons et al. (2005) red-edge three-band algorithm, using the 665, 705 and\n783 nm bands, transcribed from the original paper's Table 2; the bands are matched\nby nearest wavelength to absorb the Sentinel-2 A/B/C band-centre drift. At each in\nsitu station a 3x3 pixel window is cut on the native 10 m UTM grid, with per-pixel\nquality control (water only, no cloud/shadow/glint, atmospheric-correction success),\nand the valid pixels are averaged. Satellite retrievals are paired with coincident\nin situ chlorophyll-a from Rijkswaterstaat monitoring stations on station and time\nwithin a plus-or-minus two-hour window. Agreement is scored with the paper's metric\nsuite (coefficient of determination, slope, intercept, RMSE, bias, unbiased RMS,\nabsolute and relative percentage difference), with the regression-family metrics\ncomputed in log10 space for chlorophyll-a and the percentage-difference metrics on\nuntransformed values, exactly as the original specifies. Statistics are reported\nboth over the full Sentinel-2 record and over the original study's field-campaign\nwindow, to separate the effect of changing the period from the effect of changing\nthe site." . "This study tests only the chlorophyll-a limb of the original claim: the assertion\nthat Sentinel-2 MSI retrieval of chlorophyll-a is the weak side of a parameter-\ndependent accuracy — that chlorophyll-a is the parameter for which further research\nand a more complete set of match-ups are still needed. In scope is whether an\nindependent, satellite-derived chlorophyll-a product agrees with coincident in situ\nchlorophyll-a measurements in a mesotidal, well-mixed, turbid estuary, and whether\nthe original study's weak-chlorophyll-a caution still holds when the retrieval chain\nis fully open source and transferred to a different estuary and a longer period.\nOut of scope, and left to separate atomic chains, are the turbidity, suspended-\nparticulate-matter and coloured-dissolved-organic-matter limbs of the paper's\nasymmetry, the seasonal time-series analysis, and any grid-sensitivity (HEALPix /\nDGGS) representation of the corrected fields." . . . "Anne Fouilloux" . "2026-07-25T16:23:23.490Z"^^ . . . . . "Open-source Sentinel-2 chlorophyll-a retrieval validated against in situ data in the Westerschelde estuary" . . "RSA" . 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