Fouling studies on natural-stone substrates in the tidal zone of the North Sylt wadden sea: Sessile and hemisessile animals

Abstract

Six different types of test substrates, arranged in order of texture from smooth (Solnhofen limestone, Bunter sandstone, and basalt) to rough surfaces (Middle Triassic limestone, granite, and basaltic lava), were exposed in tidal zone of the wadden sea near the harbour of List (Island of Sylt, North Sea). The test substrates were fixed to panels at the midtide to high-water level, the midtide to low-water level, and 75 cm below the latter (sublittoral level). Animal settlement was primarily influenced by abiotic factors at the two higher levels. In the sublittoral zone, however, influences of biotic factors (competitors and predators) predominated. Therefore, the physical quality of the chosen substrates more decisively affected the growth of settling animals above the midtide to low-water mark than in the deeper zone. At the midtide to high-water levelBalanus balanoides only settled on the rough surface of the Middle Triassic limestone and in the troughs of the Bunter sandstone; barnacles attached to the even surface of the latter, were destroyed by wave beating. They died on the rough surface of the dark-coloured granite, when this stone was warmed during low-water. At the midtide to low-water level, the barnacles survived best on the rough surface of the Middle Triassic limestone and the granite. Only the larvae ofB. crenatus andB. improvisus preferred to settle on the even surface of the Bunter sandstone and the basalt; but there they died from wave beating or desiccation. The surface of the basaltic lava, formed by sharp-edged pores, was unsuitable for settling of barnacles. Colonies of Hydrozoa covered each substrate with a somewhat rough surface structure, even the basaltic lava. At the sublittoral level, differences in settling between the single substrates disappeared more and more. Nevertheless, the three species of barnacles showed the same preferences in settling, as they did at the higher levels. In August,Asterias rubens destroyed all barnacles and thus restricted the lower limit of barnacle settlement at the low-water mark. The distribution of epibiotic organisms is dependent upon the density of their “living substrates”, directly attached to the stone surfaces. During the short time of their growth, hemisessile young,Mytilus edulis settled on thready forms like algae, or on raised areas like the top of barnacle shells. Therefore, young mussels could be found on stones, that already carried a compact cover ofEnteromorpha sp. or a dense settlement of adult barnacles. The polychaetePolydora ciliata rarely burrowed directly into stony substrates (Middle Triassic limestone, Solnhofen limestone). It settled primarily between barnacle shells where it was sheltered from wave beating and at the same time profited from the current produced by the filter-feeding organs of the barnacles. The density of this polychaete was directly proportional to the density of the barnacles. AfterA. rubens had destroyed the barnacles in the sublittoral zone,P. ciliata disappeared too. At the low-water mark, however, theP. ciliata population between living and active barnacles increased. Hence, the lower limit ofP. ciliata — as those of barnacles and mussels — was fixed by this predator.