Long-term variation in algal intertidal floras

A long-term study (1973-9) was made of the intertidal floras of two headlands, one exposed to the ocean, the other in a bay. Many algal species varied erratically in frequency or occurrence from year to year, apparently due to local chance causes, but in some years many species showed reductions in frequency. In these years the changes in the flora appear to be associated with sand movements, which are possibly due to locally damaging storms. Recovery of the floras follows these depressions, but it is stressed that this recovery depends on the existence of a reservoir of spore-producing plants which in turn depends on the maintenance of good water quality in the sea. The naturally occurring variations in floras are greater than are any induced by a low volume well-treated sewerage outfall, except in the immediate area of the outfall.