Simulated Inherent Optical Properties of Aquatic Particles using The Equivalent Algal Populations (EAP) model

Abstract

Paired measurements of phytoplankton absorption and backscatter, the inherent optical properties central to the interpretation of ocean colour remote sensing data, are notoriously rare. We present a dataset of Chlorophyll a (Chl a) -specific phytoplankton absorption, scatter and backscatter for 17 different phytoplankton groups, derived from first principles using measured in vivo pigment absorption and a well-validated semi-analytical coated sphere model which simulates the full suite of biophysically consistent phytoplankton optical properties. The optical properties of each simulated phytoplankton cell are integrated over an entire size distribution and are provided at high spectral resolution. The model code is additionally included to enable user access to the complete set of wavelength-dependent, angularly resolved volume scattering functions. This optically coherent dataset of hyperspectral optical properties for a set of globally significant phytoplankton groups has potential for use in algorithm development towards the optimal exploitation of the new age of hyperspectral satellite radiometry.

Fig. 1

Complex Refractive Indices of core and shell spheres, and their homogenous sphere equivalents.

Fig. 2

Chl a-specific absorption and backscatter for 3 eukaryotic phytoplankton groupings displaying distinct diagnostic pigments (fucoxanthin & peridinin, phycoerythrin and phycocyanin for Dinoflagellates, Synechococcus and Cyanobacteria respectively). These examples represent theoretical variability in assemblage effective diameter (D_eff) and c_i. The impact of c_i variability on the IOPs is particularly remarkable for small cells i.e. Synechococcus.

Fig. 3

EAP Chl a-specific Inherent Optical Properties for M. aeruginosa, for a measured size distribution with a D_eff of 5.1 um.

Fig. 4

The 16 PG collections of the absorption measurements.

Fig. 5

For each PG, a mean absorption spectrum was calculated from the absorption measurements, and this is used to represent the shape of the chloroplast imaginary refractive index input into the model to represent a generalised RI for each type (remembering that the magnitude is dictated by the parameterisation of pigment density). The greyed areas represent the range of variability in the original measurements, with the mean represented by the black line.

Fig. 6

EAP modelled Chl a-specific absorption vs measured Chl a-specific absorption for individual species with corresponding ESD and c_i measurements available. The grey ranges represent variability in the model driven by small variations in size and c_i.

Fig. 7

EAP modelled Chl a-specific scatter vs measured Chl a-specific scatter for individual species with corresponding ESD and c_i measurements available. The grey ranges represent variability in the model driven by small variations in size and c_i.

Fig. 8

EAP modelled Chl a-specific backscatter vs measured Chl a-specific backscatter for individual species with corresponding ESD and c_i measurements available. The grey ranges represent variability in the model driven by small variations in size and c_i.