This study presents an innovative technique for the in situ analysis of aquatic biochemical elements detected through wet chemical processes. A new compact in situ phosphate analyzer based on sequential injection analysis, liquid waveguide capillary flow cell and spectrophotometry was developed, and a safe and modular electronics-chemical separation mechanical structure was designed. The sequential injection system of this analyzer was optimized, and the major functions of this analyzer were studied and estimated. With a 10 cm liquid waveguide capillary flow cell and a 6.3 min time cost of detection, the analyzer reaches a detection limit of 1.4 μg·L⁻¹ (≈14.7 nM, [PO₄³⁻]) and a consumption of 23 μL at most for each reagent. This analyzer was operated in situ and online during two scientific research cruises in the Pearl River Estuary and northern South China Sea. The advantages of this analyzer include its simple versatile manifold, full automation, low chemical consumption and electronics-chemical separate safe structure. Long-term in situ performance of this analyzer will be validated in the future.

The diffuse attenuation coefficient for the downwelling irradiance $K_d\left(z,\lambda \right)$ is a critical parameter in terms of the optical properties of the ocean. In the northwestern South China Sea, there are complex physical processes, and the accurate estimation of $K_d\left(z,\lambda \right)$ in the northwestern South China Sea is critical for the study and application of the underwater light field and water constituents. In this study, using Hydrolight 6.0 (HL60) software, $K_d\left(z,\lambda \right)$ was simulated based on the inherent optical properties (IOPs) and chlorophyll a concentration dataset in the northwestern South China Sea. The simulations were in good agreement with the results calculated by the model of Lee (2005), and the spectral characteristics of $K_d\left(z,\lambda \right)$ were consistent with several oceanic types according to Jerlov’s classification. The horizontal and vertical distribution characteristics of $K_d(z,490)$ were studied in the two typical upwelling areas of eastern Hainan Island and eastern Vietnam. $K_d(z,490)$ in eastern Hainan Island exhibited an overall decreasing trend from west to east at the same depth, while the vertical depth of the maximum value of $K_d(z,490)$ in eastern Hainan Island was found to increase from west to east, which was significantly associated with the distribution trend of the temperature and salinity. $K_d(z,490)$ in eastern Vietnam exhibited unique horizontal and vertical distribution characteristics due to upwelling, with a low temperature and high salinity. A satisfactory linear relationship between $K_d(z,490)$ and $K_d\left(z,\lambda \right)$ was found from $420 \mathrm{t}\mathrm{o} 580 \mathrm{n}\mathrm{m}$ with $R^2$ > 0.76, root mean square (RMSE) $\le$ 0.010 ${\mathrm{m}}^{-1}$, and mean absolute percentage error (MAPE) < 9%, and this result indicated that $K_d\left(z,\lambda \right)$ from $420~580 \mathrm{n}\mathrm{m}$ could be estimated with $K_d(z,490)$. The regression accuracy sharply decreased after 580 nm, indicating that $K_d\left(z,\lambda \right)$ estimation based on $K_d(z,490)$ can be more suitably achieved from 420~580 nm and becomes inaccurate after 580 nm. Based on the simulations, an empirical relationship for $Z_{SDD}$ estimation involving $K_{d,10\%}\left(490\right)$ was developed, and $Z_{SDD}$ in the northwestern South China Sea was calculated, with a range of 5–23 m and a suitable agreement with $Z_{SDD}$ obtained via the method of Lee (2018).

Shallow waters hold ecological and economic importance. Traditional reflectance forward models, such as Hydrolight and Monte Carlo (MC), are difficult to access or time-consuming. Two-stream models show great performance in radiative transfer simulation, but most of them are applied in other mediums, or ignore the asymmetry scattering characteristic of shallow water. In this article, we propose a general multilayer analytical radiative transfer (GMART)-based model considering the realistic volume scattering function (VSF) of water bodies, which is theoretically applicable to various shallow waters. GMART results are validated against Hydrolight and MC. The mean absolute percentage error (MAPE) of GMART and Hydrolight remote sensing reflectance ( $R_{\text {rs}}$ ) among different combinations of the bottom depth, solar zenith angle, and wind speed spans in ranges of 5.37%–28.86%, 3.37%–39.28%, and 4.91%–26.73%, with root mean square error (RMSE) values from $1.73\times 10^{-4}$ to $4.49\times 10^{-3}$ , from $2.68\times 10^{-4}$ to $1.85\times 10^{-2}$ , and from $1.59\times 10^{-4}$ to $2.92\times 10^{-3}\,\,sr^{-1}$ , respectively, for the coral, sand, and seagrass bottom. Comparisons of the bidirectional reflectance function (BRF) with Hydrolight and MC indicate the capacity of GMART to compute bidirectional reflectance. In addition, a comparison with field measurements is carried out. The minimum MAPE and RMSE between the modeled and measured normalized $R_{\text {rs}}$ among several stations in the Sanya coastal area are 13.99%, and 0.061, respectively. The mean MAPE and RMSE among the six stations are 21.38% and 0.083, respectively. The field validation underscores the feasibility of the model in shallow water radiative transfer. Compared with MC, the computational efficiency of GMART is much higher. The new model is shown to be an alternative and efficient tool capable of addressing shallow water-related challenges across diverse environmental conditions.