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 K6nigshafen, 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~ of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with < I/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 (IAttorina 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 tittle difference between covered and uncovered mussel beds. On the other hand, grazing herbivoresthe flat periwinkle LittoHna mariae, the isopod Jaera albi[rons 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.

Recruitment of seaweeds through small reproductive stages is limited on sediment inundated rocky shores and largely unsuccessful in soft sediment environments. Burial in sediment has several potentially negative effects for seaweed propagules, and these effects were differentiated in a laboratory experiment. We investigated how light deprivation, sediment type (grain size, organic content, and origin), and sediment chemistry (oxygen presence and toxicity through hydrogen sulfide) affected survivorship and growth of Fucus serratus L. embryos. Presence of hydrogen sulfide had overriding negative impacts on both survivorship and growth of Fucus embryos, independently of sediment type and light availability. In contrast, simple anaerobiosis generally did not impair survival or growth of the embryos. Fine sediments, 3 mm thick, significantly reduced embryo survivorship, presumably through accumulation of metabolic waste products in the immediate vicinity of the embryos as a consequence of constrained diffusion. This effect was equally pronounced in the presence of a 1‐mm layer of organically rich biodeposits. Irradiance levels did not affect survival of embryos but influenced growth. Decreasing thickness and increasing coarseness of sediments together represented a gradient of enhanced light penetration and diffusion. Growth of embryos increased along this gradient. In nature, soft sediment environments with organically enriched muds (e.g. tidal flats and salt marshes) represent habitats least favorable for colonization through small reproductive stages of seaweeds.

H i s t o r i c a l c h a n g e s a n d i n v e n t o r y of m a c r o a l g a e f r o m K 6 n i g s h a f e n B a y in t h e n o r t h e r n W a d d e n S e a

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