Coastal and estuarine environments often present complex optical conditions due to high turbidity, strong riverine influence, and diverse phytoplankton assemblages. Remote sensing reflectance (R_rs) measured from above the water’s surface is crucial for characterizing these waters and retrieving key bio-optical variables, such as suspended particulate matter (SPM) and chlorophyll-a (Chl-a). However, the accuracy and stability of R_rs retrievals can be hindered by various factors, such as skylight reflection, sun glint, whitecaps, and fluctuations in environmental conditions like wind speed and viewing geometry. In this study, we aim to investigate the performance of shipborne apparent optical properties observation system (AOP-Cruise) in the Yangtze River estuary and adjacent waters, and conduct a systematic comparison of four commonly used on-water spectral correction methods (RSOA, G01, M99, and J20) across varying water types, wind speeds, and observation angles.

Field observations were carried out in July 2023 in the Yangtze River estuary and adjacent coastal waters, covering salinities from 16 to 31 psu and a wide range of turbidity levels. Thirty-two stations are sampled, and over 2000 hyperspectral measurements are obtained during daytime cruises using the AOP-Cruise system. The system continuously measures three above-surface radiometric quantities [L_t(λ), L_s(λ), and E_s(λ)] with high spectral resolution. Before deriving R_rs(λ), the measurements are interpolated onto a 1 nm grid from 320 nm to 950 nm. Four spectral correction methods are applied: M99 (fixed reflectance ρ≈0.028 and near-infrared residual correction), G01 (ρ≈0.021 with specific near-infrared channels), J20 (residual skylight removal near 810 nm), and RSOA (a spectral optimization approach modeling ρ(λ) and minimizing residual biases). After spectral calibration, quality checks (e.g., filtering out high sun zenith angles), and Savitzky-Golay smoothing, the resulting R_rs spectra from each method are used in empirical single-band (555 nm) and fluorescence-based retrievals of SPM and Chl-a, respectively. The SPM and Chl-a measurements taken at each station are used for model validation and comparison.

Over 80% of the processed R_rs data have high spectral quality scores, which demonstrates that the four correction methods yield plausible R_rs under favorable conditions (wind speed <5 m·s^-1, viewing azimuth near 135°, and solar zenith angle <60°). Differences emerge in the blue-green region (412?555 nm), where G01 and J20 tend to overestimate R_rs, whereas M99 exhibits a closer alignment with RSOA. The linear comparison with RSOA indicates that G01 and J20 have higher slopes (~1.18 and ~1.17, respectively), while M99 has a slope near unity and a lower mean absolute percentage deviation (~27%). When the wind speed exceeds 5 m·s^-1 or the viewing azimuth deviates more than ±10° from 135°, the derived R_rs display larger variances due to increased surface roughness, whitecaps, and greater sky-glint contamination. Under these conditions, RSOA’s adaptive spectral approach and M99’s near-infrared correction remain relatively robust, while G01 and J20 show more pronounced biases. Retrievals of SPM and Chl-a from the four methods show a good correlation with in situ measurements (mean R2 around 0.74?0.77 for SPM and ~0.75 for Chl-a), though higher SPM (>20 mg·L?1) introduce larger scatter, with G01 and J20 frequently overestimating and M99 slightly underestimating. For Chl-a, all four approaches are relatively consistent across low-to-moderate concentrations. The spatial distributions of SPM and Chl-a show nearshore maxima and offshore decreases, which highlights both natural gradients and method-dependent differences. The analysis by water type (clear vs. turbid) indicates that RSOA achieves lower variability in clearer waters, whereas M99 performs better in more turbid areas, which reflects each method’s sensitivity to wind speed, geometry, and water optical properties.

In summary, we confirm the applicability of RSOA, G01, M99, and J20 for on-water spectral correction and subsequent SPM/Chl-a retrievals in the Yangtze River estuary and adjacent regions. Under near-ideal conditions, all methods produce consistent R_rs results. However, G01 and J20 tend to overestimate in the blue-green domain, while M99 aligns more closely with RSOA. Higher wind speeds or non-standard viewing angles exacerbate the differences between methods, which highlights the complexities of skylight reflection and whitecap effects. While all methods demonstrate good overall performance in retrieving SPM and Chl-a, RSOA generally provides greater stability in clear waters under moderate wind conditions, whereas M99 shows stronger robustness in higher turbidity or wind speeds. G01 and J20 are found to be sensitive to geometric or surface perturbations. We underscore the potential of the domestic AOP-Cruise system for real-time hyperspectral observations and stress the importance of choosing suitable correction methods based on local water types and environmental conditions. Future efforts could focus on refining site-specific calibration or extending the comparison to a wider range of coastal and inland water environments to further improve measurement reliability and accuracy.