These have always occurred naturally, but they have increased in frequency, number and size in recent years, notably since man-made nitrogen fertilisers came into widespread use in the 1950s.

赤潮现象过去是自然现象，但近年来，它的频率、次数、规模都有所上升，尤其是自从上世纪50年代，人造氮肥的广泛使用以来更甚。