## Proportion of incident light captured by phytoplankton

A major factor limiting conversion of solar energy to chemical energy by phytoplankton is, as was pointed out by Clarke (1939), the competition for radiant energy by all the non-living components of the water. We saw in Chapter 3 that the different components of the aquatic medium -water, soluble colour, tripton and phytoplankton - each account for a proportion of the total light absorbed by the water body. We also saw that to obtain an accurate estimate of the amount of PAR captured by each component separately, calculations should be carried out using the absorption coefficients for a series of narrow wavebands followed by summation across the photosynthetic spectrum. A useful approximation, however, is to first consider the total PAR and then assume that the relative rates of absorption of light by the different components of the aquatic medium are in proportion to their individual contributions to the total vertical attenuation coefficient for downward irradiance of PAR, in accordance with eqn 9.10

For many, perhaps most, waters of interest to limnologists and marine biologists, this assumption will not be unacceptably far from the truth but it does presuppose that absorption rather than scattering is the dominant contributor to each of the partial attenuation coefficients on the right-hand side of eqn 9.10. For Kw (water), KG (gilvin) and KPH (phytoplankton) this will be true, and when the tripton fraction is strongly coloured (e.g. by insoluble humic material) it will also be true for KTR. If, however, the tripton fraction is high in concentration so that KTR is high, but consists of mineral particles low in intrinsic colour so that Ktr is made up mainly of the scattering contribution (see § 6.8, eqn 6.31), then the assumption that relative rates of absorption are proportional to the partial vertical attenuation coefficients will be seriously in error.

Nevertheless, for waters other than the kind we have just described, the fraction of the total absorbed light that is captured by the phytoplankton is, with this approximate treatment, given by

Kd(PAR) = Kd(PAR) = Kw + Kg + Ktr + [Chl\kc = Knp + [Chl\kc ( ' )

where KNP (which is equal to Kw + KG + KTR) is the vertical attenuation coefficient due to all non-phytoplankton components, [Chl] is the phytoplankton concentration (mg chl a m-3) and kc is the specific vertical attenuation coefficient per unit phytoplankton concentration. Thus the extent to which the phytoplankton succeed in competing with other components of the medium for the available quanta depends on the relative size of [Chl ]kc and KNP. The range of possibilities is limitless but we shall consider a few specific examples. We shall take 0.014m2mg-1 as a typical mid-range value of kc (see Table 9.1). Table 10.2 lists some values for the proportion of absorbed PAR captured by phytoplankton in a number of hypothetical (but typical) water bodies ranging from very pure oceanic water with KNP not much greater than that due to pure water (KW = 0.03-0.06 m-1), to a quite highly coloured,

 Type of water body knp (m-1) Phytoplankton (mg chl a m-3) Proportion of absorbed PAR captured by phytoplankton (%) Proportion of absorbed PAR captured by non- phytoplankton material (%) Clear oceanic 0.08 0.2 3.4 96.6 0.5 8.0 92.0 1.0 14.9 85.1 Coastal 0.15 1.0 8.5 91.5 2.0 15.7 84.3 4.0 27.2 72.8 Clear lake, 0.4 4.0 12.3 87.7 limestone 8.0 21.9 78.1 catchment 12.0 29.6 70.4 Productive 1.0 8.0 10.1 89.9 lake, 16.0 18.3 81.7 coloured 32.0 30.9 69.1 water 64.0 47.3 52.7 Oligotrophic 2.0 1.0 0.7 99.3 lake, 2.0 1.4 98.6 coloured 4.0 2.7 97.3 water

but productive, inland water. Very approximate though these calculations are, they do show that the share of the available quanta collected by the phytoplankton can vary from a few per cent in the less productive waters, to well over 40% in highly productive systems. They also emphasize the point that quite dilute algal populations can collect a substantial proportion of the quanta, provided the background absorption is low.

Some calculations of this type have been carried out for real water bodies. Dubinsky and Berman (1981) estimated that in the eutrophic Lake Kinneret (Sea of Galilee) the proportion of the absorbed quanta captured by phytoplankton (mainly the dinoflagellate Ceratium) varied from about 4 to 60% as the algal concentration rose from about 5 to 100 mg chl a m-3. For the eutrophic, blue-green-algal-dominated Halsted Bay, Lake Minnetonka, USA, Megard et al. (1979) calculated that the proportion of absorbed PAR collected by the algae rose from about 8 to 80% as the phytoplankton concentration increased from about 3 to 100mgchl am~3. From the data of Talling (1960) these workers estimated that for Lake Windermere (Asterionella-dominated), England, which has relatively low background absorption, the proportion of absorbed PAR collected by the algae rose from about 5 to 25% over the population density range of 1 to 7mgchl am~3. For the eutrophic, shallow (and therefore turbid) Lough Neagh, Ireland, Jew-son (1977) estimated that phytoplankton accounted for about 20% of the absorbed light at the lowest population level (26.5mgchl a m~3) and 50% at the highest (92mgchl am~3). In mesotrophic Lake Constance (Germany), Tilzer (1983) calculated fractional light absorption by phytoplankton to vary between about 4 and 70% over a two-year period in which chlorophyll a levels varied between about 1 and 30mgm~3 (Fig. 11.8).

As we saw in the previous chapter, in the sea, where the waters are deep and usually with little dissolved yellow colour, and the light field with increasing depth becomes increasingly confined to the blue-green (400-550 nm) spectral region, the specific effective absorption coefficient of the phytoplankton for PAR, äf*(z) (§9.4 and below), also increases with depth. Where, as is normally the case in the ocean, there is a layer of increased phytoplankton concentration (the deep chlorophyll maximum, §11.1) near the bottom of the euphotic zone, the combination of increased pigment concentration and enhanced light-harvesting efficiency leads to a great increase in the proportion of the total light absorption that is carried out by the phytoplankton. In the Pacific Ocean, off southeastern Japan, Kishino et al. (1986) found the fractional light absorption by phytoplankton to increase from 1.7% at the surface to 40% in the middle of the deep chlorophyll maximum at 75 m.

As we have just discussed, the usefulness of a given light field for photosynthesis is not simply a function of the total intensity of PAR, but is very much determined by how well the spectral distribution of the PAR matches the absorption spectrum of the phytoplankton or other aquatic plants. Morel (1978, 1991) has introduced the concept of photo-synthetically usable radiation, or PUR, which may be thought of as a modified PAR obtained by weighting the actual PAR across its spectrum for absorbability by the phytoplankton. This can be achieved by multiplying the PAR in each narrow waveband by some dimensionless quantity proportional to phytoplankton absorption in that waveband, and then summing across the spectrum. As a suitable dimensionless quantity, Morel in fact chose the ratio of the phytoplankton absorption coefficient in any given waveband to the maximum absorption coefficient - which is commonly the value it has at about 440 nm. Photosynthetically usable radiation can thus be defined by

where ap(1, z) and ap(1max, z) are the absorption coefficients at wavelengths 1 and 1max (wavelength of maximum phytoplankton absorption), respectively, of the phytoplankton population existing at depth z m, and E0(1, z) is the scalar irradiance per unit bandwidth (nm-1) at wavelength 1 and depth z m. From the definition of the effective absorption coefficient, ap(z), of the phytoplankton for the whole PAR waveband (eqn 10.16, see below) it follows that we can also write

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