Biological interactions and their role in community structure in the rocky intertidal of Helgoland (German Bight, North Sea)

Abstract

Over 3 successive seasonal cycles (April 1986 to October 1988, field experiments were established within 3 intertidal levels in the sheltered rocky intertidal of Helgoland (North Sea, German Bight). Competitors for space (A4ytilus edulis, macroalgae), herbivores (iittorina spp.) and predators (Carcinus maenas) were either excluded from areas (0.25 m 2) covered by undisturbed communities or enclosed at natural densities on areas that were cleared before of animals and plants. All the experimental fields (each 0.25 m 2) were covered by cages with 4 mm gauze at the sides and a plexiglas top. The results of the experiments in the u p p e r i n t e r t i d a 1 (occupied by Littorina spp. and Enteromorpha) showed that a natural density of herbivores could not prevent algal settlement and had only little influence on algal growth. Instead, abiotic factors (storms, algae washed ashore) decreased the stock of the green algae. Experiments in the m i d i n t e r t i d a 1, dominated by Myfilus (50 % cover), Fucus spp. (20 %) and grazing L. littorea (100 ind. m -2) showed that community structure was directly changed both by grazing periwinkles and by competition for space between mussels and macroalgae. Whenever Littorina was excluded, the canopy of Fucus spp. increased continuously and reached total cover within two years. In addition to the increase of Fucus spp., the rock surface and the mussel shells were overgrown by Ulva pseudocurvata, which covered the experimental fields during parts of the summer in the absence of herbivores. As soon as perennial species (fucoids) covered most of the experimental areas, the seasonal growth of Ulva decreased drastically. Presence and growth of macroalgae were also controlled by serious competition for space with mussels. Established Nfytilus prevented the growth of all perennial and ephemeral algae on the rocks. However, the shells of the mussels provided free space for a new settlement of Fucus and Ulva. In the 1 o w e r i n t e r t i d a I (dominated by total algal cover of it:. serratus, herbivores such as L. littorea and L. mariae, and increasing number of predators such as Carcinus), the feeding activity of herbivores can neither prevent the settlement of the fucoid sporelings nor reduce the growth of macroalgae. F. serratus achieved a total canopy on the rock within one year. Doubled density of herbivores prevented the settlement of Pucus and most of the undercover algae. Predation by Carcinus on Littorina spp. had little influence on the herbivore community patterns. However, the crabs supported the establishment of macroalgae by excluding the mussels from the lower intertidal. In summary, the community organization and maintenance in the mid and lower intertidal is influenced to a high degree by biological interactions. Whereas both the relatively important herbivory by L. littorea and competition for space between mussels and macroalgae dominate in the mid intertidal, predation reaches it s highest relative degree of importance for community structure in the lowe...

How an introduced seaweed can affect epibiota diversity in different coastal systems

“…Mean water temperature is 17°C in summer and 4°C in winter. Detailed information on abiotic factors, the ecology and geology was provided by Hagmeier (1930), Wurster (1962) and Janke (1986Janke ( , 1990. The shore is populated by a rich macroalgal community including many habitat-providing fucoids such as Ascophyllum nodosum (L.) Le Jol., Fucus sp.…”

Effects ofFucus vesiculosus covering intertidal mussel beds in the Wadden Sea

“…and Jaera albifrons, were significantly more abundant on F. vesiculosus -covered mussel beds and were all at least partly found as epifauna directly on the algal thalli. Feeding on adult fucoids by herbivores was described for Littorina obtusata (van Dongen, 1956;Williams, 1990b), for the isopod Idothea balUca (Haahtela, 1984) or -under circumstances of temporal starvation -for Carcinus maenas (Janke, 1990). However, filamentous greens are generally preferred as food by grazers, when given the alternative to fucoids, because the latter succeed in deterring grazers by mechanical as well as chemical defense mechanisms (Nicotri, 1980;Watson & Norton, 1985;Hay & Fenical, 1988).…”

“…However, filamentous greens are generally preferred as food by grazers, when given the alternative to fucoids, because the latter succeed in deterring grazers by mechanical as well as chemical defense mechanisms (Nicotri, 1980;Watson & Norton, 1985;Hay & Fenical, 1988). Herbivore grazing on juvenile fucoids, which has been described for rocky shores (Lubchenko, 1983;Chapman, 1990;Janke, 1990), finds no parallel on intertidal sediments because there are no young fucoids present, as reproduction occurs solely vegetatively. The F. vesiculosus thalli in K6nigshafen showed grazing marks which have been observed to originate from Gammarus spp., but field-and laboratory experiments of Reise and Asmus & Asmus (unpubl.…”

Habitat-specific size structure variations in periwinkle populations (Littorina littorea) caused by biotic factors

“…Hawkins and Hartnoll 1983;Anderson and Underwood 1997). SpeciWcally, periwinkles aVect benthic community structure and recruitment success of structurally important seaweeds (Lubchenco 1978;Janke 1990;Wilhelmsen and Reise 1994;Albrecht 1998;Buschbaum 2000;Trussell et al 2002Trussell et al , 2003. The ecological eVect of periwinkles may vary with body size.…”

“…We chose the exposed rocky shore of the island of Helgoland and a sheltered soft sediment environment near the island of Sylt as study areas. At both sites, genetically identical populations of the periwinkle L. littorea (Wilhelmsen 1998) reach high densities of hundreds of individuals per square metre (Janke 1990;Wilhelmsen and Reise 1994;Buschbaum 2002;Reichert and Buchholz 2006). Our Wrst step was to establish the shell morphology and the size and density distribution of L. littorea on the exposed rocky and sheltered sedimentary shores.…”