F f Ek

The consequence in natural water bodies of the decrease in quantum yield with increasing light intensity is that quantum yield and conversion efficiency vary markedly with depth, the general tendency being, as might be expected, for f and ec to increase with depth.330,940 Quantum yields in the surface layer in the middle period of the day, when irradiance values are generally above the range corresponding to the linear part of the P versus Ed, curve, are usually below fm.

Morel (1978) calculated the quantum yield at a series of depths from 14CO2 fixation, chlorophyll and light data for a variety of oceanic waters from the highly oligotrophic Sargasso Sea to the productive waters of the Mauritanian upwelling area. In most cases f increased with depth, i.e. with decreasing irradiance. On average, the f values for green eutrophic waters were higher than those observed in blue oligotrophic waters. In the surface layers the f values were mainly in the range 0.003 to 0.012 (equivalent to ec values of 0.6-2.4%). Kishino et al. (1986) found, in the Pacific Ocean southeast of Japan, that quantum yields of photosynthesis were 0.005 to 0.013 at the surface, 0.013 to 0.033 at that depth (10-20 m) where photosynthesis reached its maximum rate in the surface mixed layer, and 0.033 to 0.094 in the deep chlorophyll maximum (^70 m). If, as these authors suggest, we reduce all these values by 20%, as an approximate correction factor for the fact that they are based on measured irradiance, rather than scalar irradiance, values then we obtain quantum yields of 0.004 to 0.01, 0.01 to 0.026, and 0.026 to 0.075, respectively.

In oligotrophic Lake Superior, Fahnenstiel et al. (1984) found quantum yields to be very low, ^0.003, at the surface, and to increase with depth, reaching maximum values of 0.031 to 0.052 (corrected for scalar irradiance) at 15 to 25 m. Dubinsky and Berman (1981) estimated that during the spring Peridinium bloom in Lake Kinneret (Sea of Galilee), ec rose from about 5% at the surface to 8.5% at 3 m. In late summer, with a much lower population of different (green) algae, ec was 2.5% at the surface but rose to about 12% at 5 to 7 m.

The maximum quantum yield that a phytoplankton population, or aquatic macrophyte, can exhibit, fm, is a parameter of considerable theoretical and ecological interest. On the assumption (see above) that in the linear region of the P versus E curve the cells are achieving their maximum efficiency, estimates of fm are normally obtained from the observed slope, P/E, commonly referred to as a, in this region of the curve.

From the equations for f it follows that fm should be proportional to a. For example, if E is E0, then from eqns 10.29, 10.25 and 10.18 it follows that fm = a/df*. If E is Ed, then from eqns 10.29, 10.25 and 10.24 it follows that fm = a/kc. The photosynthetic process imposes by its essential nature a maximum value for fm of ^0.1, for all photosynthetic systems. On the basis of a critical analysis of literature data, Bannister and Weidemann (1984) have concluded that published values of in situ fm in excess of 0.10 are almost certainly in error. The fact that a fm value ~0.1 is one which any plant species might in principle achieve, and also that a is linearly related to fm (since a = df*fm or kc fm), has aroused expectations that a should not vary markedly for a given species in different environments, or from one species to another. Such expectations, however, ignore the extent to which the proportionality factor, df* or kc, can vary. We saw earlier (Chapter 9) how markedly df* varies with the size and shape of the cells or colonies, for example decreasing as the absorbing units become larger or more intensely pigmented. Taguchi (1976), from studies on seven species of marine diatom, found, as might be predicted, that a decreased with increasing size of the cells. Also, since df* and kc are expressed per unit chlorophyll a, then they can vary markedly in value in accordance with variation in the type of accessory photosynthetic pigments present, and their ratio to chlorophyll a (§ 9.5): the resulting changes in shape of the absorption spectrum markedly influence the rate of energy capture from the white light fields normally used in the determination of a. Welschmeyer and Lorenzen (1981), in a comparison of six species of marine phytoplankton growing exponentially under identical conditions, found that there were no significant differences in f, but there were significant differences in a: these they attributed to differences in the light absorption efficiency per unit of chlorophyll.

Another problem in the determination of a values, particularly when we wish to compare different data sets from the literature, is that there is no generally accepted standard for the light source to be used in its measurement. Some workers use natural sunlight, others use tungsten-halogen lamps, while some will use lamps fitted with blue filters. The spectral distribution of the incident light will be quite markedly different in all three cases, with the result that for any given phytoplankton population, with a given absorption spectrum, a will have a different value for each light source because the effective specific absorption coefficient, df*, for the incident PAR will be different for each light source.

Thus, even if we did not expect fm to vary dramatically with species or environment, we should not expect the same constancy of a, because of the variability of df* and kc. Unfortunately this variability in df* and kc, and the consequent uncertainty in their values, makes the determination of fm in natural populations, from a, difficult. The best estimates are undoubtedly those based on full spectral data for phytoplankton absorption and the underwater light field, and combining eqn 10.29 with w(z) values from eqns 10.15, 10.19 or 10.23.

Some typical values of a for natural phytoplankton populations, in the units mgCmgchl a_1h_1 (mmolesphotonsm-2s-1)-1, are: 0.05 (range 0.007-0.15) for Nova Scotia coastal waters,1063 0.06 for nano-plankton (<22 mm) in the lower Hudson estuary, USA,859 0.03 3 to 0.056 for picoplankton in the Celtic Sea,649 and 0.024 for diatoms and 0.034 for blue-green algae in eutrophic Lough Neagh, N. Ireland.641 In the Arabian Sea, Johnson et al. (2002) found that a was 0.025 ± 0.011 in the spring and fall intermonsoon seasons, but rose to 0.040 ± 0.016 in the early Southwest Monsoon, when nitrate levels were elevated.

For reasons that are not fully understood, the maximum quantum yield of phytoplankton populations in the ocean is extremely variable. For example, Prezelin et al. (1991) found that in a 200 km transect of a hydrographically variable region of the Southern California Bight, the value of fm varied over the range ~0.01 to 0.06, the variation being found both with distance along the transect through different water masses, and with depth at any given station. Some of this spatial variation may have a genetic base, i.e. it may be due to the presence of taxonomically different phytoplankton populations: diatom-dominated communities in the Southern California Bight had fm values twice as high as the cyanobac-terial picoplankton in the deep chlorophyll maximum.1190 Some of it may have a physiological basis, due to populations differing in nutritional status, or recent light exposure history. For phytoplankton photosynthesis in the Sargasso Sea, Cleveland et al. (1989) found an inverse relationship between fm - which varied from 0.033 to 0.102 - and distance from the top of the nitracline. Similar results were obtained by Kolber, Wyman and Falkowski (1990) in the Gulf of Maine, and in both cases the conclusion was reached that quantum yield was related to nitrogen flux. Also, in the Arabian Sea, Johnson et al. (2002) found that fm was 0.020 ± 0.009 in the spring and fall intermonsoon seasons, and 0.051 ± 0.024 in the early Southwest Monsoon, when nitrate concentrations were higher. The widespread, and successful prymnesiophyte species, Phaeocystis pouchetii, appears to be able to achieve quantum yields close to the maximum. Samples from a massive bloom of this phytoplankter in the northern Greenland Sea achieved values of fm averaging 0.102.260

Babin et al. (1996) measured phytoplankton carbon fixation at three sites in the northeast tropical Atlantic, representing typical eutrophic, mesotrophic and oligotrophic regimes. In the eutrophic and mesotrophic sites the mixed layer extended deeper than the euphotic zone, and photo-synthetic parameters were nearly constant with depth, fm averaging 0.05 and 0.03 in the eutrophic and mesotrophic waters, respectively. At the oligotrophic site, there was a deep chlorophyll maximum (DCM), and fm varied from ^0.005 in the upper, nutrient-depleted, mixed layer to 0.063 below the DCM in this stratified water. Across all the sites fm was found to roughly covary with nitrate concentration, this factor accounting overall for a two-fold variation in quantum yield. fm varied inversely with increasing relative concentrations of non-photosynthetic carotenoids, to an extent that could give rise to a three-fold variation in quantum yield.

As well as the spatial variation, there is at any given location in the ocean, a diurnal variation in fm, , , often, but not invariably, involving a decrease in the afternoon. In Lake Constance (Germany), Tilzer (1984) found maximum quantum yield at midday to vary over the range 0.022 to 0.092 throughout the year, but on any given day fm could vary by up to three-fold, the tendency again being for a diminution in the afternoon.

In five biogeochemical ocean provinces extending across the North Atlantic from the east coast of Canada to the Canary Islands, Kyewa-lyanga et al. (1998) found that the maximum quantum yield of photosynthesis (mol C [mol photons]-1), averaged over all provinces, in the fall of

1992 (fm = 0.032 ± 0.015) was about twice that in the following spring of

1993 (fm = 0.017 ± 0.007). The photosynthetic parameters of the phyto-plankton were found to be more variable between seasons than between provinces. The authors attributed the seasonal differences to the different environmental conditions that prevailed.

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