The scattering properties of phytoplankton

Phytoplankton cells and colonies scatter, as well as absorb, light and can make a significant contribution to the total scattering behaviour of the aquatic medium, but to an extent that varies from one species to another: this has been studied in detail, both experimentally and theoretically, by Morel, Bricaud and coworkers.160,162,943,948 A convenient parameter in terms of which to compare the scattering propensities of different species is the specific scattering coefficient, bc, which is the scattering coefficient that would be exhibited by cells of a given species suspended at a concentration corresponding to 1 mg chlorophyll a m~3; it has the units m2mgchl a-1. Table 4.3 lists values for

Table 4.3 Specific scattering coefficients for phytoplankton.

Wavelength

Organism (nm) bc (m2mgchl a-1) Reference

Marine

Tetraselmis maculata

590

0.178

948

Hymenomonas elongata

590

0.078

948

Emiliania huxleyi

590

0.587

948

Platymonas suecica

590

0.185

948

Skeletonema costatum

590

0.535

948

Pavlova lutheri

590

0.378

948

Prymnesium parvum

590

0.220

948

Chaetoceros curvisetum

590

0.262

948

Isochrysis galbana

590

0.066

948

Synechocystis sp.

590

0.230

943

Synechococcus

450

0.40

945

550

0.30

945

665

0.180

945

Prochlorococcus

450

0.04

945

550

0.030

945

665

0.015

945

Fresh water

Scenedesmus bijuga

550

0.107

284

Chlamydomonas sp.

550

0.044

284

Nostoc sp.

550

0.113

284

Anabaena oscillarioides

550

0.139

284

Natural (freshwater) populations

Irondequoit Bay, L. Ontario (USA)

Av. mixed population

400-

700

0.08

1445

Cyanobacterial bloom

400-

700

0.12

1445

L. Hume, Murray R., Australia

400-

700

0.11

1014

Melosira granulata dominant

L. Mulwala, Murray R., Australia

400-

700

0.22

1014

M. granulata dominant

bc measured on laboratory cultures of a range of marine and freshwater phytoplankton species, and on some natural populations. Algae such as diatoms (S. costatum) and coccolithophores (E. huxleyi) in which a substantial proportion of the total biomass consists of mineralized cell walls, or scales, scatter more light per unit chlorophyll than, for example, naked flagellates (I. galbana). Also, blue-green algae with gas vacuoles scatter light much more intensely than those without.433

Like the mineral and detrital particles, which carry out the greater part of the scattering in most natural waters, algal cells have a scattering phase function that is strongly peaked at small forward angles,1308 but the back-scattering ratio (bb/b, the proportion of the total scattering that is in a backwards direction, 0 > 90 °) is much lower (0.0001-0.004) for the living cells162,1308 than for the mineral and detrital particles (—0.019). This is a consequence162 of the low refractive index (relative to water) of the living cells (1.0 1 5-1.07 5)7,205,943 compared with that of the inorganic particles (1.15-1.20).636 The backscattering ratio is greater in the small (picoplankton) cells, such as cyanobacteria, than in the larger eukaryotic cells.1308

Volten et al. (1998) measured the angular distribution of light scattering over the range 20 to 160 ° by laboratory cultures of 15 phytoplankton species and two types of estuarine sediments. In most of the phytoplank-ton species, scattering, which was in every case predominantly in the forward direction, declined to a minimum at —120 ° and then increased somewhat over the 120 to 160 ° range. Scattering by the estuarine silt samples, by contrast, remained essentially flat above 120

In the ocean, satellite remote sensing shows that blooms of coccolitho-phores (haptophyte algae with spherical cells covered with circular calcareous plates called coccoliths, Fig. 4.10) have a high reflectance, indicating efficient upward, and therefore backward, scattering, which might seem to contradict the generalization that phytoplankton are weak backward scatterers. As coccolithophore cells age the coccoliths become detached and it is thought that the intense upward scattering from these blooms originates mainly from the numerous detached coccoliths, rather than from the living cells themselves.579,943,65 Balch et al. (1996a) found the backscattering ratio in the most turbid parts (b = 1-3 m—1) of a very large (—0.5 x 106 km2) bloom of Emiliania huxleyi in the North Atlantic to be —0.01 to 0.02 at 440 and 550 nm. For laboratory cultures of E. huxleyi, Voss et al. (1998) found that the backscattering coefficient varied inversely with wavelength, in accordance with 1—14 for coccoliths, and 1—12 for the cells with coccoliths attached. Calculations using anomalous diffraction theory64 show that for spherical particles, calcite-specific

Fig. 4.10 Cell of the common coccolithophore species, Emiliania huxleyi, showing the attached circular calcareous plates (coccoliths), which are mainly responsible for the intense scattering properties of this phytoplankter. (Courtesy Dr Susan Blackburn, CSIRO Marine and Atmospheric Research.)

scattering (m2 mg calcite C-1) is at a maximum at 1 to 3 mm, which is about the diameter of the individual coccoliths, but is much lower for spheres of the typical diameter (~10 mm) of the coccolithophores themselves. Using an inversion algorithm developed by Gordon and Boynton (1998), Gordon et al. (2009) used in situ radiance/irradiance profiles measured in an E. Huxleyi bloom off the coast of Plymouth, UK, to calculate spectral values of the backscattering coefficient of the scattering particles - predominantly detached coccoliths - present in the water. On the basis of estimates of coccolith numbers obtained by flow cytometry (see below), they calculated a backscattering cross-section (strictly speaking, the upper limit) of individual coccoliths to be 0.123 ± 0.039 mm2/coccolith at 500 nm.

Figure 4.11 shows coccolithophorid blooms in the Atlantic Ocean, west of Ireland and of Cornwall, and also in the Celtic Sea, as observed by the SeaWiFS sensor from space.

Fig. 4.11 Coccolithophorid blooms in the Atlantic Ocean, west of Ireland and of Cornwall, and also in the Celtic Sea, Monday 18 May 1998. Image collected at NERC Satellite Receiving Station, Dundee, Scotland. (Courtesy of the SeaWiFS Project, NASA Goddard Space Flight Center.) See colour plate.

For a single-angle scattering meter measuring backscattering at ^140 Vaillancourt et al. (2004) found the conversion factor (w(0)) for obtaining bb from b(y) (using bb = 2p w($) b($), see above) averaged 0.82 for cultures of nine phytoplankton species. This may be compared with values in the region of 1.2, which have been found for coastal sea waters.139,216

The powerful technique of flow cytometry, which makes use of light scattering from individual cells, has been adapted for the study of phyto-plankton populations.7,224,809,1048,1268,1280,1485 The sample fluid, e.g. ocean water, is injected coaxially into a stream of particle-free sheath fluid. The liquid passes through a capillary flow chamber which is traversed by an intense argon-ion laser beam operating typically at 488 or 514 nm. The dimensions of, and rate of flow through, the flow chamber are such that the individual cells pass through the laser beam one at a time, and as they do so they scatter the incident light, and also exhibit fluorescent emission in response to absorption of light by their photosynthetic pigments. Light scattered forward (in the range 1.5-19 °) and at ~90 °, and fluorescent emission in the orange (530-590 nm) and/or red (>630 nm) wavebands, are measured. Different sizes and pigment classes of phytoplankton have different combinations of scattering and fluorescence signals, and so the technique can be used for enumerating, characterizing and following the development of natural phytoplankton populations.

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