https://w3id.org/sciencelive/np/RAoh82dxkJvR73OU_ttKf2_mtwgXY5_8qgcp3S7tmhcVM
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sub:assertion {
sub:chla-weak-limb-open-source-westerschelde a <https://w3id.org/sciencelive/o/terms/FORRT-Replication-Outcome>;
<http://schema.org/endDate> "2026-07-25"^^xsd:date;
<http://www.w3.org/2000/01/rdf-schema#label> "Open-source Sentinel-2 chlorophyll-a retrieval in the Westerschelde qualifies the Sado weak-Chl-a finding";
<https://w3id.org/sciencelive/o/terms/hasConclusionDescription> """This replication qualifies the original claim. Sent et al. (2021) found that
Sentinel-2 MSI retrieves water-quality parameters with parameter-dependent
accuracy — strong for turbidity but weak for chlorophyll-a, concluding that \"for
the key parameter Chl-a further research is needed, with a more complete set of
match-ups.\" Testing that chlorophyll-a limb in an independent, more turbid estuary
(the Westerschelde, 2016–2026) with a fully open-source atmospheric-correction
chain (Acolite + Gons et al. 2005, the aGS chain), we obtained no useful agreement
with in situ chlorophyll-a: R² = 0.10 over the full record (N = 33) and R² = 0.13
for the 2018–2020 subset (N = 14), with a negative regression slope and a positive
bias. This does not confirm that an operationally reproducible, open-source chain
can retrieve estuarine chlorophyll-a at the accuracy of the paper's selected cGS
chain (R² = 0.63, C2RCC + Gons). Rather than contradicting the paper, the result
supports and extends its central caution: chlorophyll-a is the parameter for which
MSI retrieval is not yet reliable, and that weakness persists — indeed deepens —
when the proprietary C2RCC step is replaced by an open-source processor and the
method is transferred to a different optical regime.""";
<https://w3id.org/sciencelive/o/terms/hasConfidenceLevel> <https://w3id.org/sciencelive/o/terms/Moderate>;
<https://w3id.org/sciencelive/o/terms/hasEvidenceDescription> """From results/chla_satellite_acolite.parquet, agreement computed in log10 space
(scripts/matchup_stats.py), matching the paper's log-scale treatment of Chl-a:
Full record (2016–2026): N = 33, R² = 0.10, slope = −0.20,
RMSE = 0.63 (log10), BIAS = +0.40
Original-window subset (2018–2020): N = 14, R² = 0.13, slope = −0.14,
RMSE = 0.59 (log10), BIAS = +0.30
Satellite chlorophyll-a (median 8.7 µg/L, range 1.9–27.7) systematically exceeds
the coincident Rijkswaterstaat in situ chlorophyll-a (median 3.1 µg/L, range
0.9–26.0) across six along-axis stations. The negative slope is the decisive
feature: the retrieval does not track in situ chlorophyll-a at all, consistent with
the Gons red-edge algorithm responding to backscatter/turbidity rather than pigment
in this high-SPM estuary. For comparison, the original study's selected cGS chain
reported R² = 0.63. Match-up figure: figures/main_result.png.""";
<https://w3id.org/sciencelive/o/terms/hasLimitationsDescription> """Three factors bound this conclusion and must be read together.
1. Processor change. The paper's headline Chl-a number (R² = 0.63) is the cGS
chain (C2RCC + Gons). We ran the aGS chain (Acolite + Gons) because C2RCC exists
only inside ESA SNAP and crashes natively in the container
(docs/atmospheric-correction-choice.md). We therefore did not test cGS and
cannot dispute its number. The paper itself reports Acolite as its worst-
performing processor (mean BIAS 2.78, mean APD 254 percent), so a weaker aGS
result is partly anticipated by the original study — a reason this is a
qualification, not a contradiction.
2. Site and period change. This is a Replication, not a Reproduction: the Sado
in situ data are not public (paper Data Availability Statement), so we validated
in the Westerschelde (2016–2026), a different and more turbid mesotidal estuary.
A different optical regime tests generalisability, not the correctness of the
original analysis. We report both the full record and the 2018–2020 subset to
separate the period change from the site change as far as the data allow.
3. Sample size. N = 33 (14 in the original window) over six stations is small — as
in the original (N = 19–21) — so every R² carries wide uncertainty. The
direction of the result (no useful agreement) is robust; its precise magnitude
is not.
A grid-sensitivity check on a HEALPix / DGGS (GRID4EARTH) representation of the
corrected fields is deliberately out of scope here and deferred to a separate
follow-up chain that will extend this one, so the primary validation stays on the
native Sentinel-2 grid.""";
<https://w3id.org/sciencelive/o/terms/hasOutcomeRepository> <https://doi.org/10.5281/zenodo.21561994>;
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<https://orcid.org/0000-0002-1784-2920> <http://xmlns.com/foaf/0.1/name> "Anne Fouilloux" .
this: dc:created "2026-07-25T16:23:44.374Z"^^xsd:dateTime;
dc:creator <https://orcid.org/0000-0002-1784-2920>;
dc:license <https://creativecommons.org/licenses/by/4.0/>;
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<http://www.w3.org/2000/01/rdf-schema#label> "Open-source Sentinel-2 chlorophyll-a retrieval in the Westerschelde qualifies the Sado weak-Chl-a finding";
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