The most frequently and easily measured property of the underwater light field is irradiance. Knowledge of this parameter is valuable, not only because it provides information about how much light is available for photosynthesis, but also because irradiance plays a central role in the theory of radiation transfer in water. An irradiance meter, since it is meant to measure the radiant flux per unit area, must respond equally to all photons that impinge upon its collector, regardless of the angle at which they do so. With any given meter this can be tested by observing the way in which its response to a parallel light stream varies with its angle to that light stream.
As the angle of the radiant flux to the collector changes, the area of the collector projected in the direction of that radiant flux changes, and the proportion of the flux intercepted by the collector changes correspondingly (Fig. 5.1). Thus, the response of an irradiance meter to parallel radiant flux (wider than the collector) should be proportional to the cosine of the angle (0) between the normal to the collector surface and
the direction of the flux. An irradiance collector that meets this criterion is known as a cosine collector. The collector of an irradiance meter usually consists of a flat disc of translucent diffusing plastic, although opal glass may be used when measurements below 400 nm are required.638 It has been found that a more faithful cosine response to light incident at large values of 8 is obtained if the disc projects slightly up above the surrounding housing, allowing some collection of light through the edge.1238 Some of the light that penetrates into the diffusing plastic is scattered backwards and a part of this succeeds in passing up through the diffuser surface again. Because the difference between the refractive indices of plastic and water is less than that between those of plastic and air, there is less internal reflection of this light within the collector and therefore a greater loss of light when the collector is immersed in water than when it is in air: this is known as the immersion effect. To take account of this, if an irradiance meter has been calibrated in air, the underwater readings should be multiplied by the appropriate correction factor for that instrument. This factor is usually in the region of 1.3 to 1.4 in the case of a plastic collector.
Within the irradiance sensor, beneath the collecting disc, there is a photoelectric detector: a silicon photodiode, which has a broad spectral response, is commonly used. If a very narrow waveband of light is measured, as in certain spectroradiometers (see below), then a photo-multiplier may be used as the detector to provide the necessary sensitivity. The electrical signal from the photodetector is transmitted by cable to the boat where, after amplification if required, it is displayed on a suitable meter or digital readout.
In all natural waters most of the light at any depth is travelling in a downwards direction. Measurement of the irradiance of the upwelling flux is nevertheless worthwhile. This is partly because, in some waters, upwelling flux is a significant component (up to ^20%) of the total available radiant energy, but even more so because the ratio of upwelling to downwelling flux (irradiance reflectance, R = Eu/Ed) can provide information about the inherent optical properties of the water (see §§ 4.2, 6.4) and because the properties of the upwelling flux are of central importance to the remote sensing of the composition of natural waters. Thus, the more we know about this flux and its relation to other characteristics of the medium the better.
The holder in which the sensor is lowered into the water (Fig. 5.2) should thus be made so that the sensor can be facing upwards to measure downwards irradiance (Ed), or downwards to measure upward irradiance Eu. To minimize shading effects, the sensor is always lowered on the sunny side of the boat. For measurements in rivers, the sensor must be attached to a rigid support.
Measurements made with an uncorrected wide-band detector are unreliable because of the spectral variation of sensitivity: part of the diminution of irradiance with depth could be due to a shift of the wavelength of the predominating light to a part of the spectrum where the detector is less sensitive. It is therefore desirable to confine the measured light to a specific waveband with an appropriate filter in the sensor: narrow-band interference filters are best for this purpose. The alternative, and in the context of photosynthesis the best, solution (short of determining the complete spectral distribution of irradiance), is to use a meter designed to respond equally to all quanta within the photosynthetic range regardless of wavelength, but to be insensitive to quanta outside this range. In this way a measure of the total photosynthetically available radiation (PAR) is obtained.
It is better that the meter should be designed to respond equally to quanta regardless of wavelength rather than to respond equally to a given amount of radiant energy regardless of wavelength, because the usefulness of the prevailing light for photosynthesis is more closely related to the flux of quanta than to the flux of energy. This is because once a quantum has been absorbed by a cell, it makes the same contribution to photosynthesis regardless of its wavelength (see §3.l), although the probability of its being absorbed in the first place does, of course, vary with wavelength. An absorbed red quantum, for example, is just as useful as an absorbed blue quantum even though it may contain only two thirds of the energy.
An irradiance meter designed in this way is commonly referred to as a quanta meter. Since no existing photodetector in its normal state
responds equally to all quanta throughout the photosynthetic range, ways of adjusting the relative response in different parts of the spectrum must be found. Jerlov and Nygard (1969) used a combination of three colour filters, each covering a different part of the photodetector (Fig. 5.3). The filters were covered with an opaque disc with a number of small holes drilled in it: the number, size and position of these holes determined the amount of light reaching each filter. By a suitable combination of this variable attenuation with the differing absorption properties of the filters it was possible to obtain an approximately constant quantum sensitivity throughout the photosynthetic range. Commercial instruments using filter combinations with a silicon photodiode, and measuring the total quantum flux in the range 400 to 700 nm, are readily available.
The photosynthetically available radiation at any depth can be measured with a submersible quanta meter. Smith (1968) suggested that the vertical attenuation coefficient for downwards irradiance of PAR is the best single parameter by means of which different water bodies may be characterized with respect to the availability of photosynthetically useful radiant energy within them. As we shall see in the next chapter, Kd for PAR is not exactly constant with depth even in a homogeneous water body. Nevertheless the variation is not great and knowledge of, say, the average value of Kd (PAR) over a certain depth range such as the euphotic depth, is very useful. It is determined by measuring downwelling PAR at a series of depths within the depth interval of interest and making use of the approximate relation (see §§ 6.1, 6.3)
where Ed (z) and Ed (0) are the values of downward irradiance at z m and just below the surface, respectively. The linear regression coefficient of ln Ed (z) with respect to depth gives the value of Kd Or, less accurately, Kd may be determined from measurements at just two depths, z1 and z2
Equations 5.1 and 5.2 can also be used to obtain Kd(1), the vertical attenuation for downward irradiance in a narrow waveband centred on 1, when vertical profiles of irradiance have been measured with a spectroradiometer (see below), or a radiometer with a narrow-band interference filter.
To determine the irradiance-weighted vertical attenuation coefficient, wK(av), (§1.3) we use the irradiance data (downward, upward, net or scalar, as appropriate) for a series of depths at equal intervals down to a depth below which the remaining radiation is a trivial proportion of the total in the water column. For N depth intervals
To achieve adequate smoothing of the data, each K(z) value is calculated from
i.e. from the diminution in irradiance over two depth intervals. Each K(zi, zi+2) is then multiplied by E(zi+1), the irradiance in the depth interval in between, in the numerator of eqn 5.3.
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