Light Penetration through Shallow Flowing Water, Comparing the Effects of Surface Lenses in Laminar Flow with Bubbles Added in Turbulent Flow

Abstract

Swatland HJ

Penetration of light into water is important for aquatic photosynthesis. In shallow water, refraction at the water surface may result in bright and dark areas underwater, while turbulence increases both this surface lensing and shadowing by bubbles. Microscope measurements of static bubbles showed that, relative to their size, small bubbles cast stronger shadows than large bubbles. With moving water (depth ≈ 1 cm), optical fibers linked to photomultipliers showed turbulence changed the pattern of transmitted light, causing more, shorter and less intense peaks of light than in laminar flow. In laminar flow, light penetration was higher at 700 nm than at 470 nm. A wavelength difference was also found beyond the visible spectrum (850 > 365 nm).

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