The illumination of the shore varies widely with rise and fall of the tide. When the tide recedes, the shore is directly exposed to light. When the shore is covered, the illumination is much reduced, especially where the water is very turbid.
Inadequate light is probably one of the factors limiting the downward spread of some of the algae on the shore. Direct exposure to light may favour the growth of some plants but high intensities of light are probably lethal to many red algae. Strong sunlight is detrimental to many organisms due to the combined effects of radiation, heating and drying, but it is difficult to dissociate their separate influences.
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