A plant growing at sea level under clear skies will receive only about 20% less light than a plant growing at the same latitude at an altitude of 4000 m. The same percentage reduction of visible radiation occurs within about 2 m of the clearest oceanic water, and within less than 20 cm of turbid coastal water. Thus, although both air and water are commonly regarded as transparent and colourless media, light penetration through water is at least 2000-fold less than that through air. Light will, therefore, limit plant growth far more often under water than on land, and so it is necessary to examine the behaviour of light in the sea in some detail. This discussion will lead to a definition of the ‘photic zone’ — the region of the sea to which photosynthetic plant growth is necessarily restricted — and the subsequent treatment of other aspects of the marine environment can largely be limited to this surface layer of the sea.

LIGHT IN THE SEA

Measurement

Light measurement in botanical studies has been bedevilled for many years by the existence of the ‘light meter’ and a system of ‘photometric’ standards and units (lumen, lux, foot-candle, etc.). These instruments and units were specifically designed for the spectral response of the human eye, and may give very misleading results if applied in a general biological or ecological context.