Retrieval of phytoplankton size from bio-optical measurements: theory and applications

Abstract

The absorption coefficient of a substance distributed as discrete particles in suspension is less than that of the same material dissolved uniformly in a medium-a phenomenon commonly referred to as the flattening effect. The decrease in the absorption coefficient owing to flattening effect depends on the concentration of the absorbing pigment inside the particle, the specific absorption coefficient of the pigment within the particle, and on the diameter of the particle, if the particles are assumed to be spherical. For phytoplankton cells in the ocean, with diameters ranging from less than 1 µm to more than 100 µm, the flattening effect is variable, and sometimes pronounced, as has been well documented in the literature. Here, we demonstrate how the in vivo absorption coefficient of phytoplankton cells per unit concentration of its major pigment, chlorophyll a, can be used to determine the average cell size of the phytoplankton population. Sensitivity analyses are carried out to evaluate the errors in the estimated diameter owing to potential errors in the model assumptions. Cell sizes computed for field samples using the model are compared qualitatively with indirect estimates of size classes derived from high performance liquid chromatography data. Also, the results are compared quantitatively against measurements of cell size in laboratory cultures. The method developed is easy-to-apply as an operational tool for in situ observations, and has the potential for application to remote sensing of ocean colour data.

Figure 1.

Bio-optical basis of combining cell diameter to specific absorption. (a) Variation of intracellular chlorophyll a concentration with cell diameter (red line, derived from Marañón et al. [18]); (b) phytoplankton absorption efficiency under assumptions of constant (black) and variable (red) intracellular pigment concentrations; (c) specific absorption of chlorophyll a*_c against diameter at λ = 676 nm under assumptions of constant (black) and variable (red) intracellular pigment concentration; the blue dots represent the chlorophyll-normalized absorption a*_p data of laboratory cultures taken from Sathyendranath et al. [14]. Possible error (percentage) in estimated diameter across the cell size owing to error in assigning (d) the exponent m; (e) the parameter c₀ and (f) the specific absorption of unpackaged chlorophyll inside the cell a*_ci; different thickness of lines correspond to 10% (light grey line), 20% (dark grey line) and 30% (solid black line) errors in the respective quantities.

Figure 2.

Computation and sensitivity of chlorophyll a-specific phytoplankton absorption: (a) in vivo specific absorption of chlorophyll a a*_c (676) as a function of chlorophyll a-normalized absorption a*_p (676) using equation (3.6) given in text; (b) error % in a*_c (676) estimation as a function of a*_c (676) values owing to error in parameter a^m—different thickness of black lines correspond to 10% (light grey line), 20% (dark grey line) and 30% (solid black line) errors in a^m; and (c) observed phytoplankton absorption normalized to chlorophyll a a*_p (red circle) and specific absorption of chlorophyll a a*_c (blue plus) at λ = 676 nm as functions of observed chlorophyll a concentrations.

Figure 3.

Comparison of the estimated diameter with in situ size class: (a) estimated diameter of phytoplankton population as a function of chlorophyll a concentration. Green, blue and red dots represent samples dominated, respectively, by picoplankton, nanoplankton and micro-plankton classified by diagnostic pigments; (b) estimated diameter as a function of chlorophyll a for samples dominated by diatoms (yellow dots), prymnesiophytes (red dots), Procholorcoccus (black dots), other cyanobacteria (blue dots) and green algae (green dots) as identified by pigments.

Figure 4.

Comparisons of the estimated diameters with laboratory culture data taken from Sathyendranath et al. [14]: (a) chlorophyll a specific absorption a*_c(676) values calculated from equation (3.2)–(3.3) using the diameters measured in laboratory culture against those calculated from equation (3.6) using a*_p(676) values measured in laboratory culture; (b) phytoplankton cell diameters computed by the current method against those measured in laboratory culture.