A model based on stacked-constraints approach for partitioning the light absorption coefficient of seawater into phytoplankton and non-phytoplankton components

Abstract

Partitioning of the total non-water absorption coefficient of seawater, anw(λ) (i.e., the light absorption coefficient after subtraction of pure water contribution), into phytoplankton, a ph(λ), and non-phytoplankton, adg(λ), components is important in the areas of ocean optics, biology, and biogeochemistry. We propose a partitioning model based on stacked-constraints approach, which requires input of anw(λ) at a minimum of six specific light wavelengths. Compared with existing models, our approach requires much less restrictive assumptions about the spectral slope of a dg(λ) and the spectral shape of aph(λ). Our model is based on several inequality constraints determined from an extensive, quality-verified set of field data covering oceanic and coastal waters from low to high-latitudes. With these constraints, the model first derives a wide range of speculative solutions for adg(λ) and a ph(λ) and then identifies feasible solutions. Final model outputs include the optimal solution and a range of feasible solutions for adg(λ) and aph(λ). The optimal solutions agree well with measurements. For example, the median ratio of the model-derived optimal solutions to measured adg(λ) and a ph(λ) at 443 nm is very close to 1, i.e., 1.004 and 0.988, respectively. The median absolute percent difference between the optimal solutions and measured values of adg(443) and aph(443) is 6.5% and 12%, respectively. The range of feasible solutions encompasses the measured adg(λ) and aph(λ) with a probability >90% at most wavelengths. Our results support the prospect for the applications of the partitioning model using the input data of a nw(λ) collected from various oceanographic and remote-sensing platforms. Key Points A model for partitioning light absorption coefficient of seawater was developed. The model makes weakly restrictive assumptions involving inequality constraints. Phytoplankton and non-phytoplankton absorption is derived with good accuracy. ©2013. American Geophysical Union. All Rights Reserved.