EFFECTS OF FUCUS-VESICULOSUS COVERING INTERTIDAL MUSSEL BEDS IN THE WADDEN SEA

The brown alga Fucus vesiculosus forma mytili (Nienburg) Nienhuis covered about 70% of mussel bed (Mytilus edulis) surface area in the lower intertidal zone of Konigshafen, a sheltered sandy bay near the island of Sylt in the North Sea, Mean biomass in dense patches was 584 g ash-free dry weight m(-2) in summer. On experimental mussel beds, fucoid cover enhanced mud accumulation and decreased mussel density. The position of mussels underneath algal canopy was mainly endobenthic (87% of mussels with > 1/3 of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with < 1/3 of shell buried in mud). Mussel density underneath fucoid cover was 40 to 73% of mussel density without algae. On natural beds, barnacles (Balanidae), periwinkles (Littorina littorea) and crabs (particularly juveniles of Carcinus maenas) were significantly less abundant in the presence of fucoids, presumably because most of the mussels were covered with sediment, whereas in the absence of fucoids, epibenthic mussel clumps provided substratum as well as interstitial hiding places. The endobenthic macrofauna showed Little difference between covered and uncovered mussel beds. On the other hand, grazing herbivores the flat periwinkle Littorina mariae, the isopod Jaera albifrons and the amphipods Gammarus spp. were more abundant at equivalent sites with fucoid cover. The patchy growth of Fucus vesiculosus on mussel beds in the intertidal Wadden Sea affects mussels and their epibionts negatively, but supports various herbivores and increases overall benthic diversity.