Soft bottom versus hard rock:: Community ecology of macroalgae on intertidal mussel beds in the Wadden Sea

Abstract

On the extensive mud and sand flats of the Wadden Sea, intertidal mussel beds ( Mytilus edulis L.) represent one of few secondary hard substrata and thus potentially serve as attachment surfaces for macroalgae in 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 forma mytili (Nienburg), which lacks a holdfast and reproduces only vegetatively. Ephemeral macroalgae are absent from mussel beds, and large mussel patches remain completely uncolonized by seaweeds. I tested the effect of grazing by periwinkles, Littorina littorea L., on algal cover through experimental exclusion of snails. Under reduced grazing pressure, ephemeral green and red algae ( Enteromorpha, Ulva and Porphyra) developed and either partially or completely covered the surface on mussel bed plots, but only in the absence of Fucus. Additional experiments showed that Fucus dispersal from fertile 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 substratum F. vesiculosus recruited successfully on artificial settlement surfaces, in the absence and to a lesser extent also in the presence, of littorinid grazing. If F. vesiculosus zygotes settle on mussel surfaces, they are subject to very high post-settlement mortality. Field experiments showed that survival of Fucus zygotes inoculated onto live mussels was close to zero, under high and low grazing intensities. Survival was elevated on mortar-filled mussel valves when snail grazing pressure was reduced, and especially if barnacles were present. As demonstrated in laboratory experiments, the recruitment failure on live mussels was due to the accumulation of faeces and pseudofaeces which shade, bury and eventually kill Fucus zygotes underneath. Adult thalli are either not susceptible to these adverse effects of mussel biodeposition or can counterbalance them by growth. Since propagation can occur through fragmentation of adults, sexual reproduction is unnecessary for the survival of the Fucus population. Just as on rocky shores, the abundance of ephemeral seaweeds on mussel beds of the Wadden Sea tidal flats is controlled by grazing. With opportunistic life strategies ephemerals are characterized by high growth rates and reproductive capabilities which allow them to respond rapidly to spatial and temporal reductions in grazing pressure. In contrast, adults of the long-lived perennial seaweed, Fucus, are largely unaffected by grazing, and juvenile stages, which would be subject to herbivory as well as strong biodeposition on Wadden Sea mussel beds, are omitted from the life cycle of Fucus in this environment.