The rate of decrease of illumination with depth is expressed as the extinction coefficient, kA.
where IAd1 and IAd2 are the intensities of light of wavelength A at depths di and d2 metres. For accurate measurements of illumination beneath the sea surface, photomultipliers and photoelectric photometers are used (Kampa, 1970). But for rough measurements, the secchi disc (Figure 3.8) provides a simple method which has often been used by biologists.
This is a white disc, 30 cm in diameter, which is lowered into the water to the depth at which it just disappears from sight. The extinction coefficient can then be roughly determined from the empirical relationship k = 1.45/d where d is the maximum depth in metres at which the disc is visible (Walker, 1980).
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.