Benthic foraminifera along an offshore-fjord gradient: a comparison with amphipods and molluscs

Faunal pattern of 'total' (live + dead) foraminifera is compared with environmental factors at ten stations along an offshore-fjord gradient in Norway. TWINSPAN, a numerical classification method divides the fauna into three main groups: 1) species having their main occurrence offshore; 2) generalist species; and 3) species with their main occurrence in fjords. The relationship between environmental variables and faunal pattern is analysed using Canonical Correspondence Analysis (CCA). The environmental variables that explain most of the variance in species data are percentage sand,'distance to pool', loss on ignition and sill depth. Together these variables account for 39% of the variance. The foraminiferal fauna is compared with the amphipod and mollusc fauna along the same transect. These groups respond differently to the environment. Foraminifera have their highest abundance at outer fjord localities, amphipods offshore and molluscs at inner fjord localities. A unimodal response of foraminiferal abundance to organic matter is indicated. Foraminifera show no clear decrease in diversity from offshore to inner fjord, in contrast to the other two groups, and diversity is negatively correlated to abundance. Both abundance and diversity pattern of foraminifera is probably regulated by the influx of organic matter and a combination of competition and predation. The faunal pattern for all three groups is correlated to sill depth. The most likely explanation is that the effect of the sill depth on the organic flux to fjord basins and on bottom currents, regulates the sediment environment in a way that is decisive for the basin fauna.