SOFT-BOTTOM VERSUS HARD-ROCK - COMMUNITY ECOLOGY OF MACROALGAE ON INTERTIDAL MUSSEL BEDS IN THE WADDEN-SEA

On the extensive mud and sand flats of the Wadden Sea, intertidal muss el beds (Mytilus edulis L.) represent one of few secondary hard substr ata and thus potentially serve as attachment surfaces for macroalgae i n an environment which is largely dominated by sediments. However, the only abundant colonizing seaweed on mussel beds is a special form of rockweed, Fucus vesiculosus forms mytili (Nienburg), which lacks a hol dfast and reproduces only vegetatively. Ephemeral macroalgae are absen t from mussel beds, and large mussel patches remain completely uncolon ized by seaweeds. I tested the effect of grazing by periwinkles, Litto rina littorea L., on algal cover through experimental exclusion of sna ils. Under reduced grazing pressure, ephemeral green and red algae (En teromorpha, Ulva and Porphyra) developed and either partially or compl etely covered the surface on mussel bed plots, but only in the absence of Fucus. Additional experiments showed that Fucus dispersal from fer tile populations to mussel beds is possible within a range of at least 25 m. Between 5 and 25 m from a fertile population on rocky substratu m F. vesiculosus recruited successfully on artificial settlement surfa ces, in the absence and to a lesser extent also in the presence, of li ttorinid grazing. if F. vesiculosus zygotes settle on mussel surfaces, they are subject to very high post-settlement mortality. Field experi ments showed that survival of Fucus zygotes inoculated onto live musse ls was close to zero, under high and low grazing intensities. Survival was elevated on mortar-filled mussel valves when snail grazing pressu re was reduced, and especially if barnacles were present. As demonstra ted in laboratory experiments, the recruitment failure on live mussels was due to the accumulation of faeces and pseudofaeces which shade, b ury and eventually kill Fucus zygotes underneath. Adult thalli are eit her not susceptible to these adverse effects of mussel biodeposition o r can counterbalance them by growth. Since propagation can occur throu gh fragmentation of adults, sexual reproduction is unnecessary for the survival of the Fucus population. Just as on rocky shores, the abunda nce of ephemeral seaweeds on mussel beds of the Wadden Sea tidal flats is controlled by grazing. With opportunistic life strategies ephemera ls are characterized by high growth rates and reproductive capabilitie s which allow them to respond rapidly to spatial and temporal reductio ns in grazing pressure. In contrast, adults of the long-lived perennia l seaweed, Fucus, are largely unaffected by grazing, and juvenile stag es, which would be subject to herbivory as well as strong biodepositio n on Wadden Sea mussel beds, are omitted from the life cycle of Fucus in this environment. (C) 1998 Elsevier Science B.V. All nights reserve d.