1 General Introduction
1.1 Description of the study site ................................................................................ 2 2
The distribution of Helgoland's marine fauna and flora has been surveyed in various ways ( e.g. Lüning 1970; Kornmann & Sahling 1977; Karez 1991;
Jacobs 1993; summarized by Harms 1993). The NE rocky intertidal zone of Helgoland ("Nordost-Felswatt", hereafter "NE-Intertidal", Figure 3) is the only part of the island's shoreline where the sea reaches up to the natural red sandstone cliff without protection by seawalls, except from a few shelves left unsheltered for rock-breeding seabirds. The special physiognomy of the NE-Intertidal and its species distribution have been described by Janke (1986), who also, for this site, performed the most thorough and important experi-ments in respect to the present study (Janke 1989, 1990).
From the cliff seaward, a flat rocky terrace follows with a horizontal extension of >300 m from MHW to ML W, with a difference in vertical Ievel of 2.4 m (Janke 1990). Thus, the mean angle of gradient is less than 0.5°.
Along the whole stretch, the NE-Intertidal is sheltered from western storms by a seawall ("N-Mole", see Figure 3). Spring low tides occur in early morning and evening hours. Hot midday conditions affect the lower intertidal organisms only when low tides coincide with southeastern storms. Genuine tidepools, surveyed in studies of many other coasts CLubeheneo 1982;
Chapman 1990b; Metaxas et al. 1994; Benedetti-Cecchi & Cinelli 1996; van Tarnelen 1996), are absent from the rocky flat. Parallel to the N-Mole, grooves cut through the rock. These originate from the broken up layers of sandstone that eroded differentially. The bias of the layers runs NE-wards, right-angled to the grooves, reaching a difference in height of a few
deci-22 1 general introduction
North Sea
German Bight
X
Helgoland
?
Helgeland Bay
N-Mole
/ NE-Intertidal
500 m
Figure 3: Map of the island of Helgoland and its location in the German Bight (North Sea). Sites of experiments were the rocky flats of the NE-Intertidal, the N-Mole and an intertidal field of concrete rubble (indicated with an arrow).
meters on horizontal distances of ca. 0.5 - 5 m. This pattern of grooves distorts the normal zonation, leading to two tidal gradients: a large scale gradient along the land-sea axis and a small scale gradient, right-angled to the former. Small scale distributions of macroalgae have been described else-where (J anke 1986), here only zonation patterns of the most conspicuous macroalgae along the large scale gradient will be considered.
Some of the grooves retain enough water during most low tides to allow, near the lower end of the large scale intertidal gradient, growth and survival of Corallina officinalis. On the layers' surfaces in the lower intertidal zone, ad-jacent to subtidal stands of Laminaria digitata, a dense canopy of Fucus serratus begins, extending landwards over ca. half of the regularly emergent
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,...
area. It is accompanied by an understorey population of Chondrus crispus, that does not form an own belt in the lowermost intertidal zone as on North-American Atlantic shores (see below), and Cladophora rupestris patches. The main canopy of Fucus serratus is rather dense with a leaf area index (LAI) of
>> 1. Landwards, the F. serratus canopy becomes more scattered with a lower LAI. Dense patches of Mytilus edulis populate the tops of the sandstone slabs.
F. vesiculosus alone or together with F. serratus builds patches of sparse canopies. In the uppermost zone of the main NE-Intertidal, plateaus of rocks more recently fallen from the cliff are often populated by a dense mixed canopy, mostly of F. vesiculosus and F. spiralis and, in a few cases, F. ser-ratus. The area surrounding these boulders is populated from ca. March to September by dense stands of green ephemeral algae, mainly Enteromorpha spp. (Janke 1986). On big concrete blocks of the eastward extents of the NE-Intertidal, F. spiralis (and still higher Porphyra spp.) forms a distinct zone.
The typical zonation pattem of Fucus spp. (landwards F. serratus - F. vesicu-losus- F. spiralis), conspicuous on most of Helgoland's seawalls and the main subject of this study, is only with difficulties discernible in the main NE-Intertidal.
Ascophyllum nodosum is absent from the NE-Intertidal, presumably due to the semi-exposed character of the site (Janke 1986) or due to the brittle substratum. In the more sheltered southern harbour, it forms a dense canopy.
The overwhelming dominance of F. serratus in the lower intertidal zone is in contrast to other coasts. E.g. on N-Atlantic coasts of America, this zone is often dominated by Chondrus crispus that is competitively dominant and less susceptible to grazing there (Lubchenco 1980; Worm & Chapman in prep.).
Lubchenco ( 1980) suggested that in Europe grazing by limpets and ice scouring remove Chondrus crusts and, thus, facilitate the establishment of F.
serratus. However, several findings contradict this contention for Helgoland:
Janke (1990; or see review of Chapman 1995, p. 26) found no evidence for a competitive superiority of Chondrus over Fucus. Helgolands coasts are nor-mally not affected by ice and limpet grazing. Fucus itself is affected by ice scouring (McCook & Chapman 1993; Kiirikki 1996), although is has high regenerative abilities (McCook & Chapman 1992). However, in the experi-ments of the present study, Chondrus was not included.
The grazer guild consists mainly of Littorina spp. and small crustaceans, while limpets, a major structuring force on e.g. many British coasts (Southward &
Southward 1978; Hawkins 1981), are absent from the Helgoland intertidal.
24 1 general introduction
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Another contrast to many other coasts (e.g. Hartnoll & Hawkins 1985) is the absence of dense bamacle populations from the main NE-Intertidal. This is presumably due to the brittle consistence of the red sandstone, since they settle on other substrata in Helgoland's intertidal zone, e.g. on concrete and PVC frames of Janke's (1990) enclosure cages (pers. observation). Lubchenco (1983) showed that barnacles may be important for the establishment of fucoids.
More comprehensive descriptions of the distribution of Helgoland's intertidal species are given by Markharn & Munda (1980), Janke (1986) and some of the studies summarized by Harms (1993).
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