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Figure 3.12

Carbon-specific area of freshwater planktic algae (ka) plotted against the dimensionless shape index, msv-1. Near-spherical algae line up close to msv-1 = 6, smaller species tending to have greater interception properties than larger ones. Shape distortions increase msv-1 without sacrifice of ka. The algae are Ana, Anabaena flos-aquae; Aphan, Aphanizomenon flos-aquae; Ast, Asterionella formosa; Chla, Chlamydomonas; Chlo, Chlorella; Cry, Cryptomonas ovata; Eud, Eudorina unicocca; Fra, Fragilaria crotonensis; Lim red, Limnothrix redekei; Mic, Microcystis aeruginosa; Monod, Monodus; Monor, Monoraphidium contortum; Per, Peridinium cinctum; Pla ag, Planktothrix agardhii; Plg, Plagioselmis; Sc q, Scenedesmus quadricauda; Tab, Tabellaria flocculosa. Redrawn with permission from Reynolds (1997a).

the cells, relative to (say) cell carbon. Plotting ka, the area projected per mol of cell carbon, against an index of shape (msv-1 is the product of the maximum dimension and the surface-to-volume ratio), shows the package effect to be upheld for quasi-spherical units (from Chlorella to Microcystis; msv-1 = [d4n(d/2)2/4/3n(d/2)3] = 6), while distortion from spherical form usually enhances the area that the equivalent sphere might projection (Reynolds, 1993a) (see also Fig. 3.12). Note that the individual carbon-based projections are less liable to variation than the chlorophyll-based derivations.

The attenuation components self-compound (Eq. 3.14) to influence the diminution of the depth to which incident radiation of given wavelength penetrates (Eq. 3.11). In some extreme instances of attenuation with depth, one of the components may well dominate over the others. In Fig. 3.13, instances of high attenuation due to a relatively high sw (in the humic Mt Bold Reservoir, Australia), sp (in the P. K. le Roux Reservoir, South Africa) and Nea (in an Ethiopian soda lake) are compared with the transparency of a mountain lake (2800 m a.s.l.) in the middle Andes. In each of these relatively deep-water examples, the gradient in Iz is exclusively related to the depth and to the coefficient of attenuation. The same principles apply in shallow waters but, because light can reach the bottom and be reflected back into the water column, the gradient of Iz may not be so steep or so smooth as that shown in Fig. 3.13 (discussion in Ackleson, 2003). Nevertheless, the present examples give a feel for the column depth in which diurnal photosynthesis can be light-saturated (Iz > Ik > (say) 0.1 mmol photons m-2 s-1) and, more to the point, the vertical extent of the water column where phytoplank-ters need to adapt their light-harvesting potential to be able to match their capabilities for carbon fixation.

Phytoplankton adaptations to sub-saturating irradiances

Several mechanisms exist for enhancing cell-specific photosynthetic potential at low levels of irradiance. One of these is simply to increase the cell-specific light-harvesting capacity, by adding to the number of LHCs in individual cells. This may be manifest at the anatomical level in the synthesis of more chlorophyll and deploying it in more (or more extensive) plastids placed to intercept more of the available photon flux falling on the cell. Most phytoplankton are able to adjust their chlorophyll content within a range of ±50% of average and to do so within the timescale of one or two cell generations. For example, Reynolds (1984a) made reference to measurements of the chlorophyll content of Asterionella formosa cells taken at various stages of the seasonal growth cycle in a natural lake, during which the cell-specific quota fluctuated between 1.3 and 2.3 mg chla (109 cells)-1, i.e. 1.3-2.3 pg chla (cell)-1. Supposing the capacity of the Asterionella to fix carbon to have been as measured in Fig. 3.3, equivalent to ~0.8 mg C (mg chla)-1 h-1,

Figure 3.13

Some examples of light penetration and (inset) components of absorption due to water and solutes (unshaded), non-living particulates (hatched) and phytoplankton (solid). Laguna Negra, Chile (a), is a very clear mountain lake; (b) Mount Bold Reservoir, Australia, is a significantly coloured water; (c) P. K. le Roux Reservoir, South Africa is rich in suspened clay; (d) Lake Kilotes, Ethiopia is a shallow, fertile soda lake, supporting dense populations of Spirulina. Various sources; redrawn with permission from Reynolds (1987b).

then the cells with the lower chlorophyll a complement would have fixed 1.04 mg C (109 cells)-1 h-1, or 0.012 mg C (mg cell C) h-1. Other things being equal, the cells with the higher chlorophyll complement might have been capable of fixing 1.84 mg C (109 cells)-1 h-1, or 0.022 mg C (mg cell C) h-1. The main point, however, is that the cells with the higher chlorophyll content are capable of fixing the same amount of carbon as those with the lower complement but at a lower photon flux density: in this instance, the highchlorophyll cells achieve 1.04 mg C (109 cells)-1 h-1 not at >48 |imol photons m-2 s-1 but at >27 |imol photons m-2 s-1. The extra chlorophyll a increases the steepness of biomass-specific P on I (the slope a).

The measurements of biomass-specific chlorophyll a referred to in Section 1.5.4 range over an order of magnitude, between 0.0015 and 0.0197 pg |im-3 of live cell volume. This corresponds approximately to 3 to 39 mg g-1 of dry weight (0.3 to 3.9%) or, relative to cell carbon, 6.5 to 87 mg chla (g C)-1. Many of the lowest values come from marine phytoplankters in culture (Cloern et al., 1995); most of the highest come from cultured or natural material, but grown under persistent low light intensities (Reynolds, 1992a, 1997a). The data show that the frequently adopted ratio of cell carbon to chlorophyll content (50:1, or 20 mg chla (gC)-1] must be applied with caution, though it remains a good approxi mation for cells exposed to other than very low light intensities.

It appears that, over successive generations, phytoplankton vary the amount of chlorophyll, both upwards (in response to poor photon fluxes) and downwards (when similar cells of the same species are exposed to saturating light fluxes). This represents an ability to optimise the allocation of cellular resources in response to the particular internal rate-limiting function, bearing in mind that the synthesis and maintenance of the light-harvesting apparatus carries a significant energetic cost and no more of it will be sponsored than a given steady insolation state may require (Raven, 1984). Moreover, under persistently low average irradiances, there is plainly a limit to the extra light-harvesting capacity that can be installed before the returns in cell-specific photon capture diminish to zero. If all the photons falling on the cell are being intercepted, more harvesting centres will not improve the energy income. On the other hand, this logic indicates the advantage (preadaptation?) of having a relatively large carbon-specific area of projection. Most of the species indicated towards the top of Fig. 3.12 are able to operate under relatively low photon fluxes, in part, through enhancement of the chlorophyll deployment across the light field available. Many of the slender or filamentous diatoms, as well as the solitary filamentous Cyanobacteria (Limnothrix, Planktothrix), which project perhaps 10-30 m2 (mol cell C)-1, perform well in this respect. The quasi-spherical colonies of Microcystis show the opposite extreme (ka: 2-3 m2 (mol cell C)-1).

Another physiological adaptation to persistent light limitation is to increase the complement of accessory photosynthetic pigments. In general, this assists photon capture by widening the wavebands of high absorbance, effectively plugging the gaps in the activity spectrum of chlorophyll a. In particular, the phy-cobiliproteins (phycocyanins, phycoerythrins of the Cyanobacteria and Cryptophyta) and the various xanthophylls (of the Chrysophyta, Bacillario-phyta and Haptophyta; see Table 1.1) increase the light harvesting in the middle parts of the visible spectrum. The close association of accessory pigments with the LHCs facilitates the transfer of excitation energy to chlorophyll a. The corollary of widening the spectral bands of absorption is a colour shift, the chlorophyll green becoming masked with blues, browns or purples. This is acknowledged in the term 'chromatic adaptation' (Tandeau de Marsac, 1977). Some of its best-known instances involve Cyanobacteria formerly ascribed to the genus Oscillatoria. Post et al. (1985) described the photosynthetic performance of a two- to three-fold increase in the chlorophyll content and a three- to four-fold increase in c-phycocyanin pigment in low-light grown cultures of Planktothrix agardhii. Photosynthetic attributes of pink-coloured, deep-stratified populations of P. agardhii were investigated by Utkilen et al. (1985b). A remarkable case of chromatic adjustment in a non-buoyant population of Tychonema bourrel-leyi, as it slowly sank through the full light gradient in the water column of Windermere, is given in Ganf et al. (1991). Chromatic adaptation reaches an extreme claret-colour in populations of Planktothrix rubescens, which stratify deep in the metalimnetic light gradient of alpine and glacial ribbon lakes (Meffert, 1971; Bright and Walsby, 2000). The early-twentieth-century appearance of Planktothrix at the surface of Murtensee, Switzerland, was popularly supposed to have come from the bodies of the army of the ruling dukes of Burgundy, defeated and slain in a battle at Murten in the fifteenth century. The connotation 'Burgundy blood alga' celebrates the colour of the alga as much as the independence that was won.

Reynolds et al. (1983a) described an analogous chromatic adaptation of a Cyanobacterium, now ascribed to Planktolyngbya, stratified in the metal-imnion of a tropical forest lake (Lagoa Carioca) in eastern Brazil. In each of those cases where measurement has been made, chromatic adaptation increased the chlorophyll-specific photosyn-thetic yield and the cell-specific photosynthetic efficiency (a). In the P. agardhii strain studied by Post et al. (1985), the efficiency (a) was ~7 times steeper in cultures grown at 20 °C on a 16 h : 8h light-dark cycle and a photon flux of 7 |imol photons m-2 s-1 than in material in similarly treated cultures exposed to >60 |imol photons m-2 s-1 (0.78 vs. 0.11 mgO2(mg dry weight)-1 (mol photon)-1 m2; or, in terms of carbon, approximately 0.54 vs. 0.08 mol C (mol cell C)-1 (mol photon)-1 m2). Supposing a basal (dark) rate of respiration for Planktothrix at 20 °C (derived from [Ra20 = 0.079 (s/v)0325]; see Sections 3.3.2 and 5.4.1) of 0.064 mol C (mol cell C)-1 d-1, or 0.74 x 10-6 mol C (mol cell C)-1 s-1, it is possible to deduce that compensation is literally achievable at ~1.4 |imol photons m-2 s-1. Realistically, allowing for faster respiration during photosynthesis and for the dark period (8 h out of 24 h), net photosyn-thetic gain is possible over about 3 to 4 | mol photons m-2 s-1.

Even this performance may be considerably improved on by stratified bacterial pho-tolithotrophs, with net growth being sustained by as little as 4-10 nmol photons m-2 s-1 (review of Raven et al., 2000). Chromatic photoadaptation also sustains net photoautotrophic production in cryptomonad-dominated layers in karstic dolines (solution hollows) at ambient photon flux densities of <2 |imol photons m-2 s-1 (Vicente and Miracle, 1988). This seems to be acceptable as a reasonable threshold for photoautotro-phy in phytoplankton. Deep chlorophyll maxima dominated by chromatically adapted cryptomon-ads have also been observed in somewhat larger lakes and reservoirs (Moll and Stoermer, 1982), sometimes close to the oxycline, from which short diel migrations, either upwards to higher light or downwards (to more abundant nutrient resources) are possible (Knapp et al., 2003).

Photosynthetic limits in lakes and seas

It is often convenient to subdivide the water column on the basis of its ability to sustain net photosynthesis or otherwise. The foregoing sections demonstrate three functional subdivisions based on the criterion of light availability. In the first (the uppermost), light is able to saturate photosynthesis (Iz > Ik); in the second, light is a constraint, being limiting to chlorophyll-specific photosynthesis (Iz < Ik), but whose effects may be photoadaptively offset in order to optimise the rate of biomass-specific photosynthesis. In the third, even biomass-specific photosynthesis is incapable of compensating the biomass-specific demands of respiration and maintenance (Iz < IP=R). The actual water depths for these irradi-ance thresholds are notionally simple to calculate from the I vs. z curve but, of course, they are not fixed in any sense. The immediate subsurface intensity through the solar day and it is subject to superimposed variability in cloud cover and atmospheric albedo, as well in the fluctuating surface reflectance and subsurface scattering by particulates, induced by wind action. Irradiance thresholds translate to given depths only on an instantaneous basis.

Despite the self-evident weakness of any depth-light threshold relationship, it is still valuable to intercompare various underwater light environments by reference to the impacts of their light attenuation properties. A commonly cited index used in connection with the ability of a water column to support phytoplankton growth is the average depth 'reached by 1% of surface irradiance'; this has also been used to define the depth of the so-called euphotic zone. Bearing in mind that the PAR flux at the surface at midday varies within at least an order of magnitude (200-2000 |imol photons m-2 s-1), the 1% irradi-ance boundary is approximated no more closely than 2-20 |imol photons m-2 s-1. Besides the temporal variability in its precise location in the water column, the quantity also suffers from its conceptual coarseness. Irradiances within this range could saturate the requirements of some species while simultaneously failing to compensate the respiration of others. The depth of the euphotic zone (hp) is not a general property of the underwater environment, although it remains valid as a species-specific statement of an individual plankter's position in the light gradient relative to its requirement to be able to compensate its respirational costs.

recognising that Ik is a property of the species of phytoplankton present and that its cell-carbon specificity is attributable to its carbon-specific photosynthetic efficiency; i.e.

where P is the carbon-specific rate of photosynthesis (in mol C (mol cell C) s-1) and a is the efficiency (in mol C (mol cell C)-1 (mol photon)-1 m2).

This development also infers the value of the attenuation coefficient, e, as a basis for intercom-paring aquatic environments. It has the advantage of being a property of the environment (albeit a transitory one) although care is necessary in citing the waveband being used (e440, e530, eav, etc.). Talling (1960, and many later publications) demonstrated a predictive robustness in the approximation of euphotic depth from the minimum attenuation coefficient as the quotient, 3.7/emin. In his examples, the least attenuation was in the green wavebands (X ~ 530 nm) but, as a rough guide to the depth in which photosynthesis is possible in the sea, the relationship holds quite well for other wavebands. Table 3.2 is included to contrast the photosynthetic limits of the clearest oceans and some of the most turbid estuarine waters, on the strength of the approximation that, for many phytoplankters and for much of the day-light period, positive net photosynthesis (Pg > Ra) is possible in the water column defined by hp = 3.7/emin. It is emphasised that almost all the net primary photoautotrophic production in the sea occurs within the top 100 m or so and, in lakes, within the top 60 m. In both cases, it is usually much more constrained than this.

Another, much more convenient measure of relative transparency of natural waters is available, the Secchi disk. A weighted circular plate, painted all-white or with alternate black and white quadrants, is lowered into the water and the depth beneath the surface that it just

Table 3.2 Comparison of the depth of water likely to be capable of supporting net photosynthetic production (hp) in some representative lakes and seas, supposing hp = 3.7/emjn (cf. Tailing, 1960). Values of £min, the minimum coefficient of attenuation across the visible spectrum are taken (1) from Kirk (1994) or (2) from sources quoted in Reynolds (1987b)

Water hp m

Source

Oceans

Table 3.2 Comparison of the depth of water likely to be capable of supporting net photosynthetic production (hp) in some representative lakes and seas, supposing hp = 3.7/emjn (cf. Tailing, 1960). Values of £min, the minimum coefficient of attenuation across the visible spectrum are taken (1) from Kirk (1994) or (2) from sources quoted in Reynolds (1987b)

Oceans

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