Photosynthetic production at subsaturating light intensities

In this section, we focus on the adaptive mechanisms which phytoplankters use to optimise their carbon fixation under irradiance fluxes that markedly undersaturate the capacity of the individual light-harvesting centres. To do this, it is necessary to relate the light-harvesting capacity to the cell rather than to chlorophyll per se, as we invoke the number of centres in existence within the cell and their intracellular distribution as adaptive variables, as well as the role played by accessory pigments. It is also necessary to adopt a more quantitative appreciation of the diminution of harvestable light as a function of increasing water depth and the contribution to vertical light attenuation that the plankton makes itself. Finally, water movements entrain and transport phytoplankton through part of this gradient, frequently mixing them to depths beyond the point where photosynthesis is entirely compensated by respirational loss. This theme is continued in Section 3.3.4.

Characterising the photosynthetic impact of the underwater light intensity

Taking first the condition of phytoplankton at a fixed or relatively stable depth in the water column, the light energy available to them will, in any case, fluctuate on a diel cycle, in step with daytime changes in solar radiation, and also on a less predictable basis brought about by changes in cloud cover and changes in surface reflectance owing to surface wind action. Besides variance in I0, the instantaneous incident solar flux of PAR on the water surface, there is variance in the irradiance in the flux passing to just beneath the air-water interface, ¡0. Reflectance of incoming light is least (about 5%) at high angles of incidence but the proportion reflected increases steeply at lower angles of incidence, especially <30o. Wind-induced surface waves modify the reflectance, both reducing and amplifying penetration into the water at the scale ofmilliseconds. Near sunrise and sunset, waves are important to maintaining a flux of light across the water surface (Larkum and Barrett, 1983).

Variability in the light income to a water body is represented in the examples in Fig. 3.7. Measured daily integrals of the solar insolation input across the surface of Esthwaite Water, UK (a temperate-region lake, experiencing a predominantly oceanic climate) through a period of just over one year are shown in the main plot. Besides showing a 100-fold variation between the highest (56.4 mol photons m-2d-1) and lowest light inputs (0.5 mol photons m-2 d-1), the plot reveals that part of this owes to the substantial annual fluctuation in the maximum possible insolation (based on the solar constant, its latitudinal correction, the deduction of non-PAR and with allowance for albedo scatter in the atmosphere) due to the location of the lake (54 oN). Superimposed upon that are the near chaotic fluctuations that are due to day-to-day variations in the extent and thickness of cloud cover and which cut the light income by anything between 2% or as much as 94% of the theoretical maximum (Davis et al., 2003). The inset in Fig. 3.7 presents two day-long

Figure 3.7

Main plot: Measured daily insolation at the surface of Blelham Tarn (August 1999 to October 2000), compared to the theoretical maximum insolation, as calculated from the latitude and assuming a clear, dry atmosphere (redrawn with permission from the data of Davis et al., 2003). The insets show the diel course of solar radiation intensity, measured at the same location on an overcast January day and a cloudless day close to the summer solstice. Based on data presented originally by Talling (1973) and redrawn with permission from Reynolds (1997a).

time courses of insolation, measured at the same latitude by Talling (1971). Despite coming from quite different data sets collected at quite different times, these two extremes amplify the variation shown in the main plot of Fig. 3.7. The first corresponds to a rainy, overcast winter day on which the aggregate insolation is below 1 mol photons m-2 d-1; the second is a cloudless near-solstice day when the maximum instantaneous insolation reached over 1.6mmol photons m-2 s-1.

Downwelling radiation is subject to absorption and scatter beneath the water surface, where it is more or less attenuated steeply and exponentially with depth, as shown in Fig. 3.3d. This attenuation can usefully be expressed by the coefficient of vertical light extinction, e. On the basis of the spectral integration used to translate the P vs. depth to P vs. I, the coefficient may be estimated on the basis of the equation

where e is the natural logarithmic base, and whence e = -ln(Iz/10)/h(0 - z)

where h(0 — z) is the vertical distance from the surface to depth z.

From Fig. 3.3d, £(0.5 m to 1.5 m) is solved from (lnI0.5 — lnI15) as £ = 1.516 m—1. We may now apply this spectral integral of the attenuating light to characterise the daytime changes in the underwater light field (see Fig. 3.8). The plot in Fig 3.8a shows the reconstructed time course of the irradiance at the top of the water column of Crose Mere on 25 February 1971. Beneath it are marked the values of Ik and 0.5Ik derived in the original experiment (Fig. 3.3). For clarity, the same information is plotted on to Fig. 3.8b, now against a natural logarithmic scale. Assuming no change in any component save the incoming radiation through the day, Fig. 3.8c represents the diurnal time track between sunrise and sunset of the depths of Ik and 0.5Ik. Other factors being equal (including the coefficient of vertical extinction of light, £), the depth at which chlorophyll-specific photosynthesis can be saturated increases to a maximum at around the diurnal solar zenith, as a function of the insolation

Figure 3.8

(a) Hypothetical plot of the time-course of immediate subsurface irradiance intensity (10) on 25 February 1971 (the date of the measurements presented in Fig. 3.3); (b) the same shown on a semilogarithmic plot. In (a) and (b), the contemporaneous determinations of Ik and 0.5 Ik are inserted. In (c), the time courses of the water depths reached by irradiance intensities respectively equivalent to Ik and 0.5 Ik are plotted SR, sunrise; SS, sunset. Redrawn from Reynolds (1984a).

Figure 3.8

(a) Hypothetical plot of the time-course of immediate subsurface irradiance intensity (10) on 25 February 1971 (the date of the measurements presented in Fig. 3.3); (b) the same shown on a semilogarithmic plot. In (a) and (b), the contemporaneous determinations of Ik and 0.5 Ik are inserted. In (c), the time courses of the water depths reached by irradiance intensities respectively equivalent to Ik and 0.5 Ik are plotted SR, sunrise; SS, sunset. Redrawn from Reynolds (1984a).

Figure 3.9

Depth-integrated community photosynthetic rates (ENP) for selected times through the day (07.00, 09.00, etc.) predicted interpolated values of (10) and the time track of the depth of Ik developed in Fig. 3.8. Redrawn from Reynolds (1984a).

Figure 3.9

Depth-integrated community photosynthetic rates (ENP) for selected times through the day (07.00, 09.00, etc.) predicted interpolated values of (10) and the time track of the depth of Ik developed in Fig. 3.8. Redrawn from Reynolds (1984a).

( 1,0). Outside the Ik perimeter, chlorophyll-specific fixation rates are light-limited, as predicted by the contemporaneous P vs. I curve. For instance, extrapolation of relevant data from the experiment shown in Figure 3.3 allows us to fit selected reconstructions of P vs. z curves applying at various times of the day (see Fig. 3.9).

In reality, matters are more complex than that, especially if cloud cover (and, hence, the insolation, I0), or the extinction coefficient is altered during the course of the day. The greatest depth of Ik need not be maximal in the middle of the day. Modern in-situ recording and telemetry of continuous radiation make it possible to track minute-to-minute variability in the underwater light conditions. Moreover, it is now relatively simple to translate light measurements to instantaneous photosynthetic rates, from the P vs. I curve, and to integrate them through depth (ENP, in mg C m—2 h—1) and through time

(EENP, in mg C m-2 d-1). A. E. Walsby and colleagues have been particularly successful in developing this approach (using Microsoft Excel 7 software), details of which they make available on the World Wide Web (Walsby, 2001; see also Bright and Walsby, 2000; Davis et al., 2003). The technique is helpful in establishing the environmental requirements and limitations on algal growth.

The impact of optical properties of water on the underwater light spectrum As Kirk's manual (1994) and several other of his contributions (see, for instance, Kirk, 2003) have powerfully emphasised, the attraction and usefulness of a single average vertical attenuation coefficient (e) remains an approximation of the complexities of the underwater dissipation of light energy. First, as already established, light is not absorbed equally across the visible spectrum, even in pure water (see Fig. 3.10). Photons travelling with wavelengths of about 400-480 nm are least likely to be captured by water molecules; those with wavelengths closer to 700 nm are 30 times more likely to be absorbed. For this reason, water cannot be regarded as being colourless.

Figure 3.10

The absorption of visible light (375-725 nm) by pure water. Drawn from data in Kirk (1994).

Figure 3.10

The absorption of visible light (375-725 nm) by pure water. Drawn from data in Kirk (1994).

waters of high clarity, a second source of error is encountered. This relates to another fundamental of attenuation: besides being absorbed and scattered, according to the properties of the water, attenuation is modified perceptibly by the angular distribution of the light, which becomes increasingly diffuse with increasing depth. Kirk (2003) proposed the use of an irradiance weighting of the wavelength-specific gradient, by integrating light measurements over the entire water column, so that fjzlzdZ]/[f

where is the weighted average attenuation

The selective absorption in the red leaves oceanic water of high clarity distinctly blue-green in colour. This effect is strongly evident at increasing water depths beneath the surface, where the most penetrative wavelengths come increasingly to dominate the diminishing light field. In lakes, there is a tendency towards higher solute concentrations, including, significantly, of plant or humic derivatives. These absorb wavelengths in the blue end of the spectrum, to leave a yellow or brownish tinge to the water, as the older names ('Gelbstoff, 'gilvin') might imply. Under these circumstances, the averaged extinction coefficient (distinguished here as eav) becomes less steep with depth and cannot strictly be normalised just by logarithmic expression. eav or e as used generally, calculated as in Eq. (3.12) from the slope of Iz on z, eav really has only a local value, applying to relatively restricted depth bands.

To express attenuation of light of a given wavelength or within a narrow waveband overcomes part of the difficulty but, especially in coefficient and ez and Iz are the attenuation coefficient and residual light values at each depth increment.

The use of this integral improves resolution of the subsaturating light levels in clear waters but the simpler, less precise attenuation coefficient derived in Eq. (3.12) remains adequate for describing the underwater light field in most lakes and many coastal waters, where there may be more humic material in solution and there may be more particulate material in suspension. Moreover, greater concentrations of algal chlorophyll also contribute to the rapid relative attenuation of light with depth. The average attenuation coefficient comprises:

ew is the attenuation coefficient due to the water. As already indicated, there is no unique and meaningful value that applies across the visible spectrum. The minimum absorption in pure water, equivalent to 0.0145 m-1, occurs at a wavelength of ~440 nm. The clearest (least absorbing) natural waters are in the open ocean (Sargasso, Gulf Stream off Bahamas), where the coefficients of vertical attenuation at 440 nm (ew 440) of ambient visible insolation (10) are <0.01 m-1 (various sources tabulated in Kirk, 1994). Values quoted for the open Atlantic, Indian and Pacific Oceans range between 0.02 and 0.05 m-1. Among the clearest lakes for which data are to hand, Crater Lake, Oregon has an exceptionally low coefficient of incident light attenuation (e ~ 0.06 m-1: Tyler, in Kirk, 1994). High-altitude lakes in the Andes

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