Effect of aquatic plants on the underwater light field

We have concerned ourselves so far in this chapter with the ability of phytoplankton and macrophytes to make use of the underwater light field. By harvesting light from the field, however, the plants in turn modify the light climate for any other plants below them in the water column. Within any substantial stand of aquatic macrophytes, such as a kelp forest, or a bed of seagrass or freshwater aquatic higher plants, the intensity of PAR is greatly reduced. Phytoplankton also increase the rapidity of attenuation of light with depth, and in productive waters may do so to such an extent that by self-shading they become a significant factor limiting their own population growth. The contribution of phyto-plankton to vertical attenuation of PAR must therefore be taken into account in any consideration of the extent to which light availability limits primary production in the aquatic biosphere.

We saw in §6.8 that the total vertical attenuation coefficient for monochromatic irradiance at a given depth can be regarded as the sum of a set of partial attenuation coefficients, each corresponding to a different component of the medium. This proposition does not, strictly speaking, hold for the average vertical attenuation coefficient throughout the euphotic zone, and even less does it hold for the whole photosynthetic waveband, 400 to 700 nm. Nevertheless, the assumption that the average

Kd(PAR) for the euphotic zone can be partitioned in this way is so useful that, despite its only approximate truth, it is commonly made, and may be given approximate expression through the equation

where KW, KG, KTR and KPH are the partial attenuation coefficients (for PAR) due to water, gilvin, tripton and phytoplankton, respectively. The contribution of phytoplankton to vertical attenuation of PAR can therefore be expressed in terms of the contribution of KPH to the value of Kd(PAR) in eqn 9.10.

To arrive at a quantitative estimate of the contribution of phytoplank-ton to Kd (PAR) we must determine KPH. To do this we make use of a further commonly made assumption, which as we saw in §6.7 is also only approximately valid, namely that the contribution of any component of the medium to Kd (PAR) is linearly related to the concentration of that component. Applying this to the phytoplankton we assume that

Where [Chl] is the phytoplankton biomass concentration expressed in terms of mg chlorophyll a m-3, and kc is the specific vertical attenuation coefficient (units m2mgchl a-1) per unit phytoplankton concentration. We may now write

If, for any given water body, an estimate of the specific attenuation coefficient, kc, is available, then from a measurement of the phytoplank-ton concentration, [Chl], we obtain a value for KPH.

The value of kc has been calculated for each of the four kinds of blue-green algal colonies to which the data in Fig. 9.2 apply: kc was 0.0063 m2mg-1 for the 58 mm spheres, 0.0084 m2mg-1 for the 230 x 29 mm prolate spheroids, 0.0133 m2mg-1 for the 3500 x 6 mm cylinders and 0.0142 m2mg-1 for the 6 mm spheres.695 This marked variation of kc among the different types of algae confirms the importance of the package effect in light capture by phytoplankton. These values were all obtained by calculation for idealized algae: kc can also be determined experimentally for real phytoplankton populations by measuring Kd for PAR in natural water bodies at different times as the algal population waxes and wanes, and determining the linear regression of Kd(PAR) with respect to phytoplankton chlorophyll a concentration. Not many measurements of kc covering the whole photosynthetic waveband have in fact been made. There are, however, other data in the literature, on the effect of varying phytoplankton concentration on the value of Kd in particular spectral bands within the 400 to 700 nm range, obtained with irradiance meters fitted with broad-band filters. Talling (1957b) found that for various natural waters an approximate value for Kd (PAR) could be obtained by multiplying the minimum value of Kd(A) for the water body concerned (usually that in the green waveband in inland waters) by 1.33; for Lough Neagh, N. Ireland, a factor of 1.15 was found to be more suitable. Using this sort of relation, measurements of Kd in wavebands within the photo-synthetic range can be used to provide estimates of kc. Table 9.1 lists some values of kc determined in various natural water bodies, from measurements either of irradiance for total PAR or for a particular waveband. It can be seen that kc varies widely - by a factor of four between the lowest and the highest value - from one alga, one water body, to another.

There are several possible reasons for this variability. One is the influence of cell size and geometry. We noted above that the package effect - within the range that might occur in nature - can vary kc by a factor of more than two in blue-green algae of identical pigment composition. We may reasonably attribute the low kc values for the large dinoflagellates in Table 9.1 to the low efficiency of light collection by large pigmented particles. The package effect also, as outlined earlier (§9.2), increases the more strongly the particles absorb (at constant size and shape). Thus, even for algae of similar size, shape and pigment type, kc will decrease as total pigment content increases. In addition, since kc is expressed per unit chlorophyll a, there can be marked variation in specific attenuation from one alga to another, due to differences in the type of other photosynthetic pigments present and their ratio to chlorophyll a. Calculations for model cells having the same chlorophyll a content indicated that kc for diatoms would be about 70% higher than that for green algae because of the increased absorption in the 500 to 560 nm region due to fucoxanthin; kc for blue-green algae with substantial levels of the biliprotein phycocyanin, absorbing in the 550 to 650 nm region, was calculated to be about twice that for diatoms.695

The colour of the aquatic medium in which the cells are suspended can also have a marked influence on the values of kc. Green algal cells, for example, absorb strongly in the blue region (Fig. 9.4a). In typical inland waters, however, with high levels of yellow substances, the contribution of the blue spectral region to the underwater light field is greatly diminished, and so in such waters, green cells have a low value of kc.694 The kc values we have considered so far have been the average values over some considerable optical depth, i.e. a depth in which the downward irradiance

Table 9.1 Values of specific vertical attenuation coefficient for PAR per mg phytoplankton chlorophyll a, obtained from in situ measurements of irradiance.

Water body Phytoplankton type kc (m2mg 1) Reference

Table 9.1 Values of specific vertical attenuation coefficient for PAR per mg phytoplankton chlorophyll a, obtained from in situ measurements of irradiance.

Water body Phytoplankton type kc (m2mg 1) Reference

L. Windermere,

Asterionella (diatom)

0.027a

1335

England

Esthwaite Water,

Ceratium (large

~0.01a

1337

England

dinoflagellate)

L. George, Uganda

Microcystis (blue-green)

0.016-0.021a

430

Loch Leven, Scotland

Synechococcus (blue-green)

0.011a

117

L. Vombsjon, Sweden

Microcystis

0.021a

444

(blue-green)

0.014b 0.008b

0.012-0.013b

Lough Neagh, Ireland

Melosira (diatom) Stephanodiscus (diatom)Oscillatoria (blue-green)

642

L. Minnetonka, Minn.,

Mixed blue-green

0.022c

896

USA

(Aphanizomenon etc.)

L. Tahoe, Calif.-

Small diatoms

0.029

1363

Nevada, USA

(mainly Cyclotella)

Irondequoit Bay,

Mixed blue-green

0.019

1445

L. Ontario, USA

L. Constance, Germany

Mixed

0.015c

1361

L. Zurich

Mixed

0.012

1182

(0-5 m depth),

Switzerland

Sea of Galilee

Peridinium (large

0.0067

331

(L. Kinneret), Israel

dinoflagellate)

Various oceanic and

Mixed

0.016

1240

coastal waters

Obtained by multiplying Kd(1)min by 1.33. Obtained by multiplying Kd(l)min by 1.15.

Derived from measurements of scalar, rather than downward, irradiance of PAR.

falls to some small fraction of that at the surface. In fact, since the spectral distribution of the light changes progressively with depth, so the value of kc calculated over a small increment of depth also changes.38 In the case of blue-green algae which, due to the presence of ample levels of bilipro-tein, absorb strongly in the green, as well as in the blue and red regions, kc does not vary markedly with depth in any water type. In the case of green algae, however, in an inland water absorbing strongly in the blue, calculations by Atlas and Bannister (1980) indicate that kc diminishes from about 0.012m2mg_1 in the surface layer to about 0.005m2mg_1 at the a c

Fig. 9.5 Mixed coastal phytoplankton, Tasmania, Australia. (Courtesy Ian Jameson, CSIRO Marine & Atmospheric Research.) See colour plate.

bottom of the euphotic zone. In the ocean we would expect the same sort of changes in kc with depth as are found for the effective specific absorption coefficient, a*(z) (see previous section).

One further possible cause of variation in the value of kc from one kind of phytoplankton to another is variation in their light-scattering properties. Scattering, as we saw earlier (§6.7), contributes in various ways to the vertical attenuation of irradiance. In dense algal blooms the contribution of the algal population to total scattering could significantly increase kc. The amount of scattering - especially per unit chlorophyll a - can vary markedly from one species to another (see Table 4.2). Coccolithophores and diatoms, for example, scatter light more intensely than algae enclosed within less refractile integuments.

The effects of macrophytes on the underwater light field vary so much with the growth habit of the plants and the morphology of the leaves or thallus that a general theoretical account is not feasible. According to Westlake (1980c) the specific vertical attenuation coefficient per mg chlorophyll is lower for macrophytes than for phytoplankton. Dense stands of emergent and floating macrophytes can make the whole water column virtually aphotic. Within submerged weed beds the spectral distribution of irradiance is predominantly green.1457

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