Changes in the macrozoobenthos of the intertidal zone at Helgoland (German Bight, North Sea): a survey of 1984 repeated in 2002

Abstract

Changes in the presence and absence of invertebrates as well as in species conspicuousness were documented in a rocky intertidal community based on surveys in 1984 and 2002. In 2002 six vertically and/or morphologically different stations of an intertidal platform were sampled. Five of these six habitats had already been surveyed in 1984. Replicating precisely the method of the first assessment, presence/absence changes as well as changes in species conspicuousness of 83 invertebrate species were documented, indicating that this intertidal community changed considerably during the 18-year interval. Compared with the study in 1984, 27 species newly appeared, whereas 32 species disappeared. Furthermore, 16 species increased in conspicuousness, whereas eight invertebrates decreased. The total number of species in 2002 was 154 versus 158 in 1984. Although algal species were not recorded as thoroughly as invertebrates, a massive decline in cover of Halidrys siliquosa was noted. Conversely, two invasive algal species became established after 1984, Sargassum muticum (since 1988), a cosmopolitan fucoid alga that prefers shallow subtidal areas for colonization, and Mastocarpus stellatus (introduction in the 1980s) that particularly colonized areas in the mid intertidal. In 1984 the mid intertidal zone was dominated by the brown alga Fucus serratus, whereas in 2002 the blue mussel Mytilus edulis and the periwinkle Littorina littorea were the most conspicuous organisms. Annual mean sea surface temperature (BAH measurements) warmed by 1.1°C over the past four decades. Range-related community shifts, introductions of non-indigenous species and the input of pollutants, are considered to explain long-term ecological changes in the invertebrate community at Helgoland.

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

“…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.…”

“…Blue mussels and the PaciWc oyster occur at a low density on the macroalgae-dominated intertidal at Helgoland. More detailed descriptions of the ecology, geology and of abiotic factors are given by Hagmeier (1930), Wurster (1962), Janke (1986Janke ( , 1990), Bartsch and Tittley (2004) and Reichert and Buchholz (2006).…”

Community composition of the rocky intertidal at Helgoland (German Bight, North Sea)

“…Species of which the taxonomic position is tentative were combined to a complex. Furthermore, some species were combined into a complex if the morphological distinctions between two species were diYcult to quantify or too timeconsuming to investigate (see Reichert and Buchholz 2006 for some examples).…”

“…Franke and Gutow 2004;Hiscock et al 2004;Reichert and Buchholz 2006;Sánchez et al 2005). The general objective of ecological monitoring is to collect and analyse information about the state of a system and its change in time and space related to natural Xuctuations or man-induced impacts.…”

“…However, all intertidal studies deal with particular taxonomic groups only or are merely semi-quantitative. For instance, a comprehensive study of invertebrate communities in the intertidal of Helgoland was carried out by Janke (1986) and repeated by Reichert and Buchholz (2006), whereas the composition of algae over the past decades was described by Bartsch and Tittley (2004). However, there is no quantitative assessment of communities as a whole, including both algae and invertebrates.…”