Partitioning particulate absorption coefficient into contributions of phytoplankton and nonalgal particles: A case study in the northern South China Sea

We measured the absorption coefficients of suspended particles (a_p(λ)) during three cruises from coastal waters to open ocean in the northern South China Sea (NSCS). The absorption contributions of phytoplankton (a_ph(λ)) and nonalgal particles (a_NAP(λ)) were determined using the methanol extraction method. Based on the dataset of about 360 samples, we examined the spectral relationships of the particle absorption coefficients. The results show that a_p(λ) spectra are well linearly correlated with a_p(443) over the wavebands between 420–650 nm; a_ph(λ) could be well expressed as the second-order quadratic equations of a_ph(443) among the blue-green wavebands, and a_NAP(λ) follows the general exponential function. Based on these spectral relationships, a model was proposed for partitioning the total particulate absorption coefficients into the contributions of phytoplankton and nonalgal particles using the nonlinear optimization method. The model was validated by comparing the computed results with in situ absorption coefficients. In some wavebands, such as 412 nm, 443 nm, 490 nm and 683 nm, we obtained good correlations with the percentage root mean square error (RMSE) values being controlled within 25% and the slopes being closer to 1.0. For samples from coastal waters, the discrepancy was a little large, which might be due to the higher absorption contributions from certain pheopigments. Overall, this model provides us much insight into phytoplankton absorption retrieval from in situ measurements and remote sensing ocean color data.