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 tid eless 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 th us facultative epiphytes. A field experiment demonstrated that Fucus c ontrols the amount of these fast-growing and invasive filamentous spec ies by means of the so-called whiplash effect, i.e. removing them thro ugh wave-induced thallus movements. Each experimental unit consisted o f two permanent sample plots, one in which Fucus had been completely r emoved and the other as control. The amount of filamentous algae was m onitored for 5 months, May-October, using a modified coverage estimati on 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 x height), is introduced together with biomass comparisons. On exposed shores, Fucus does control the volume of fila mentous algae, but on sheltered shores its effect is negligible The wh iplash effect offers a plausible explanation for differences in the de cline of Fucus in the northern Baltic observed between exposed and she ltered sites.