The nature of the underwater light field
6.1 Downward irradiance – monochromatic
The nature of the underwater light field
Having in earlier chapters considered the nature of the solar radiation flux incident on the surface of the aquatic ecosystem, and the influences to which the light is subject once it enters the water, we shall now discuss the kind of underwater light field that results.
6.1 Downward irradiance – monochromatic
As a result of absorption and scattering of the solar flux, the downward irradiance, Ed, of the light field diminishes with depth. In Fig. 6.1, Ed
for greenish-yellow light, expressed as a percentage of the value just below the surface, is plotted against depth in a freshwater impoundment.
Irradiance diminishes in an approximately exponential manner in accord-ance with
EdðzÞ ¼Edð0ÞeKdz ð6:1Þ or
lnEdðzÞ ¼ KdzþlnEdð0Þ ð6:2Þ whereEd(z) andEd(0) are the values of downward irradiance atzm and just below the surface, respectively, and Kd is the vertical attenuation coefficient for downward irradiance.
In Fig. 6.2a–d, the logarithm of Ed is plotted against depth for red, green and blue light in one oceanic and three inland waters. The graphs are approximately linear, in accordance with eqn 6.2, but significant divergence from linearity is in some cases apparent. In the oceanic water, for example, it can be seen that the green and the blue light are
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attenuated more rapidly below about 10 m than above. This may be attributed to the downward flux becoming more diffuse, less vertical in its angular distribution with depth, with a consequent increase in attenu-ation (see}}6.6,6.7).
The relative rates of attenuation in the different wavebands are deter-mined largely by the absorption spectrum of the aquatic medium. In the tropical Pacific Ocean (Fig. 6.2a), and in non-productive oceanic waters generally, where water itself is the main absorber, blue and green light both penetrate deeply and to about the same extent, while red light, which water absorbs quite strongly, is attenuated much more rapidly. In the productive waters of oceanic upwelling areas, blue light is attenuated more strongly than green light, due to absorption by phytoplankton pigments,942 but still not as strongly as red light. In coastal waters, which contain more yellow substances and phytoplankton than normal oceanic waters, green is again the most penetrating waveband. Only in the most coloured coastal waters, influenced by major river discharge (Jerlov’s types 7–9), however, is blue light attenuated as strongly as red light.
In contrast to the sea, in fresh water the blue waveband is usually the most strongly attenuated (Fig. 6.2b–d), because of the higher levels of yellow substances that typically occur in inland waters. Green is
Fig. 6.1 Exponential diminution of downward irradiance of greenish-yellow (580 nm) light with depth in a freshwater body (Burrinjuck Dam, NSW, Australia, 8 December 1977). (Kirk, unpublished data.)
usually the most penetrating waveband in inland waters, followed by red (Fig. 6.2b). When the concentration of yellow materials is high, however, red light may penetrate as far as green (Fig. 6.2c), and in very yellow waters red light penetrates best of all (Fig. 6.2d).
Fig. 6.2 Depth profiles for downward irradiance of blue (○), green (●) and red (D) light in a tropical oceanic and three inland freshwater systems.
(a) Pacific Ocean, 100 km off Mexican coast (plotted from data of Tyler and Smith, 1970). (b) San Vicente Reservoir, California, USA (plotted from data of Tyler and Smith, 1970). (c) Corin Dam, ACT, Australia (Kirk, unpublished data). (d) Georgetown billabong, Northern Territory, Australia (plotted from unpublished data of P. A. Tyler). All measurements were taken under sunny conditions. Note the expanded depth scale in (d). The wavelength (nm) at which measurements were made is indicated alongside each graph.
6.1 Downward irradiance – monochromatic 155
Figure 6.3 compares the spectral variations of the vertical attenua-tion coefficient for irradiance across the photosynthetic range in an unproductive oceanic water, a productive (upwelling) oceanic water and an inland impoundment. Information on the extent to which solar radiation penetrates sea ice is important in global climate modelling.
Light et al. (2008) measured depth profiles of spectral irradiance in sea ice in the Beaufort Sea (Arctic Ocean) and found that Kd(l) showed a
Fig. 6.3 Spectral variation of vertical attenuation coefficient, Kd(l), for downward irradiance in (a) the Gulf Stream (Atlantic Ocean) off the Bahama Islands (plotted from data of Tyler and Smith, 1970). (b) The Mauritanian upwelling area (8.0–9.9 mg chl am3) off the West African coast (plotted from data of Morel, 1982). (c) Burrinjuck Dam, southern tablelands of NSW, Australia (plotted from data of Kirk, 1979).
broad minimum near 500 nm and a strong increase at near-infrared wavelengths: Kd(600 nm) was 0.8 m1 for bare ice and 0.6 m1 for ponded ice.
As the solar altitude decreases, so the pathlength of the unscattered solar beam within the water, per metre depth, increases in proportion to (1/m0), where m0¼cosyw, the cosine of the zenith angle of the refracted solar beam within the water. We may therefore expectKdto increase with diminishing solar altitude. Such an effect is observed but is small:636it is only of significance in the upper layer of very clear waters. Baker and Smith (1979) found that in an inland impoundment,Kdat several wave-lengths varied by not more than 5% between solar altitudes of 80 and 50, and by not more than 18% between 80 and 10. In the clear water of the Sargasso Sea (Atlantic Ocean), Stramska and Frye (1997) found thatKd(l) at wavelengths (412, 443, 490, 510 and 555 nm) in the blue to green region of the spectrum decreased by 18 to 30% as solar altitude increased from 17 (1/m0¼1.45) to 60 (1/m0¼1.08). Kd(PAR) also decreased, by about 25%. At the red end of the spectrum (665 and 683 nm), by contrast,Kd(l) increased as solar altitude increased. Irradi-ance reflectIrradi-ance also increased dramatically (>100%) in the red, but changed only slightly at the other wavelengths. The authors attribute the anomalous behaviour ofRandKdat the long-wavelength end of the spectrum to the presence in the radiation field of additional red light generated by Raman scattering and chlorophyll fluorescence.
The ultraviolet component (l <400 nm) of the solar flux is respon-sible for a large part of the photoinhibition of photosynthesis that occurs in aquatic ecosystems (see Chapter 10), and so measurements in this spectral region are highly relevant for an understanding of primary production. For most inland water bodies, because of their high levels of yellow substances, which absorb even more strongly in the UV than in the visible region, this is less important than it is in the ocean. The ultraviolet radiation reaching the Earth’s surface is, for convenience, considered as being made up of two spectral regions:
UVA 320 to 400 nm, UVB 280 to 320 nm. UVB has a much greater photoinhibitory effect than UVA, and so it is on this waveband that most attention is focused. Very little solar radiation at wavelengths
<300 nm penetrates the Earth’s atmosphere, so in fact we need to consider only the 300 to 320 nm band. To characterize UVB penetra-tion, measurements at 310 nm are commonly carried out. Table 6.1 shows a selection of Kd(310) and Kd(305) values for a number of marine and inland waters.
6.1 Downward irradiance – monochromatic 157
Table 6.1Vertical attenuation coefficients for downward irradiance of UVB in some marine and fresh waters.
Water body Kd(310 nm) Reference
I. Marine
Central Equatorial Pacific Ocean 0.150 1242
Sargasso Sea, Atlantic Ocean 0.116 1242
Eastern Mediterranean 0.150 632
Western Mediterranean 0.16–0.43 578
Western Greenland 0.17–0.21 578
Shetland-Faeroe 0.19 578
North Sea (Fladen Ground) 0.37–0.67 578
German Bight 0.53–5.0 578
Baltic Sea proper 3.0–3.5 578
Yellow Sea 31.8–37.3 578
Gulf of Aqaba, Red Sea 0.19 Illuz, D., pers.
comm.,1993 Gulf of Mexico, N–S transect across
Louisiana Shelf, February 1988
Gulf of St Lawrence 0.7 757
Kongsfjord, Spitsbergen
L. Ontario, N. Shore station 2.89 1201
L. Ontario, deep (178 m) station 2.30 1201
Pennsylvania, USA, lakes
Giles 1.38 1201
Lacawac 10.06 1201
Kd(305 nm) Mountain lakes, Europe (Alps, Pyrenees)
Predominantly rock catchments (10 lakes) 0.51 av. 776 Alpine meadow catchments (5 lakes) 1.89 av. 776
Forested catchments (9 lakes) 2.89 av. 776
L. Biwa, Japan
North Basin, clear station EW6 1.74 95
South Basin, turbid site NS9 8.37 95