A semi-analytical model of the influence of phytoplankton community structure on the relationship between light attenuation and ocean color

A model was developed to examine the influence of phytoplankton community s tructure on the relationship between diffuse attenuation and ratios of upwe lling radiance. Shifts in phytoplankton communities were represented by cha nging mean optical properties as a function of chlorophyll (C, mg m(-3)), c onsistent with large data sets from the field and laboratory. The product o f cell size and internal pigment concentration, dc(i), governs; pigment pac kaging, which alters the specific absorption coefficients of phytoplankton ( a(ph)*, m(2) mgChl(-1)). Pigment packaging was parameterized as a functio n of C by combining the relationship between dc(i) and a(ph)* from phytopla nkton cultures with that between a(ph)* and C from the field, using data fo r 675 nm, where absorption by accessory pigments is low. Changes in accesso ry pigmentation were approximated by quantifying residual variability in a( ph)* at other wavelengths, as functions of C, once the variability with dc( i) was taken into account. Absorption by colored dissolved organic matter ( CDOM), detrital absorption, and scattering by particles were also parameter ized as functions of C, so that bio-optical relationships could be modeled as functions of trophic status. The model thus reconciled recognized relati onships between optical properties and C with ecologically interpretable sh ifts in phytoplankton communities. Empirical relationships between diffuse attenuation and ocean color were well reproduced at low (0.5 mg m(-3)) to m edium (10 mg m(-3)) C. Analysis of variability imposed by a range of dci su ggests that it may be possible to recognize phytoplankton communities with cell sizes and intracellular pigment concentration different from the centr al tendency, given a set of wavelengths which minimizes the influence of CD OM and detrital absorption.