MOLLUSK FAUNA ALONG AN OFFSHORE FJORD GRADIENT

To explore the relationship between molluscan species diversity and selected environmental variables along a fjord gradient, we took 2 replicate samples with an epibenthic sledge at 10 localities along a transect from the Norwegian Trough to the inner parts of a fjord system. The Shannon-Wiener diversity (H') of 65 species decreased markedly from offshore to the innermost stations; there were fewer but more abundant species in the fjords than offshore. Diversity was positively correlated to sill depth, and negatively correlated to distance to 'species pool' (= recruitment area), C/N, and % carbon in the surface sediment. TWINSPAN, a numerical classification method, divided the mollusc fauna into 3 groups: (I) offshore, (II) outer fjord, and (III) inner fjord fauna. The relation between the faunal pattern and 15 environmental variables was analysed with Canonical Correspondence Analysis (CCA). The environmental variables that explained most of the variance between species in the CCA were sill depth, distance to 'species pool', C/N, % C, % loss on ignition, and median phi-size (phi), and together these variables accounted for 74 % of the variance in the species data. Four hypothetical explanations are offered as to why the diversity of the deep fjord fauna of molluscs and amphipods is low compared to adjacent offshore areas. The 2 most likely explanations are that the sediment habitat (i.e. food resources, measured as C/N, and grain size) may be more diverse offshore than in fjords, and that the higher influx of organic matter in fjords, measured as % carbon in the sediment, may favour the dominance of a few opportunistic species.