Substratum as a structuring influence on assemblages of Arctic bryozoans

The nature of the substratum is a fundamental factor determining the types of organisms and communities found in many terrestrial and benthic habitats. The extent to which this is true in extreme environments was investigated using bryozoan assemblages as model organisms in an Arctic fjord (Kongsfjorden 79°N, 12°E) in summer 2001 using SCUBA. Twenty-seven substrate samples of 0.25 m² were taken at 10 m depth from the inner glacial basin to the mouth of the fjord. Multivariate analyses revealed four different bryozoan assemblages. The sea floor of the inner basin of Kongsfjorden near the glacial fronts was characterized by low diversity and dominance of the ctenostome species Alcyonidium disciforme Smitt. Highest richness and diversity occurred on rock substratum with mean size >10 cm², on which the most common species was the pioneer Harmeria scutulata Busk (abundance: 15%). On smaller rocks with mean size <2 cm², the runner-like pioneer species Electra arctica Borg comprised most individuals of the assemblage (98%). Yet another pioneer, Celleporella hyalina Linnaeus, was the most abundant species (49%) on substratum dominated by algae. Thus, in each habitat type, pioneers dominated but different species and to different extents. There was much variation in species composition and abundance within assemblages of heterogeneous habitats, and this study emphasizes the importance of microhabitats and physical conditions. Heterogeneity was evident at scales of <1 m.