Experimental evidence that Fucus vesiculosus (Phaeophyta) controls filamentous algae by means of the whiplash effect

Fucus vesiculosus is the only large perennial algal species in the tideless and brackish northern Baltic Sea. It is often covered by a thick layer of filamentous algae which may have a negative effect on Fucus. Most of these epiphytic species also grow on rock surfaces and are thus facultative epiphytes. A field experiment demonstrated that Fucus controls the amount of these fast-growing and invasive filamentous species by means of the so-called whiplash effect, i.e. removing them through wave-induced thallus movements. Each experimental unit consisted of two permanent sample plots, one in which Fucus had been completely removed and the other as control. The amount of filamentous algae was monitored for 5 months, May–October, using a modified coverage estimation method. In addition to the estimated area covered by a species, the average height of its canopy was measured. A new unit of measurement, species volume (= area × height), is introduced together with biomass comparisons. On exposed shores, Fucus does control the volume of filamentous algae, but on sheltered shores its effect is negligible. The whiplash effect offers a plausible explanation for differences in the decline of Fucus in the northern Baltic observed between exposed and sheltered sites.