Ecology of bathyal polychaete fauna at an Arctic-Atlantic boundary (Faroe-Shetland Channel, North-east Atlantic)

By reference to a series of 15 sampling stations spanning the West Shetland Slope (150-1000 m; Faroe-Shetland Channel, North-east Atlantic) we examined the potential environmental controls on the standing stock, diversity and composition of the polychaete fauna. In contrast to the majority of studied bathyal environments, the Faroe-Shetland Channel has a highly complex and dynamic hydrographic regime, particularly notable for extreme thermal variability at mid-slope depths (i.e. 7°C range at ca. 500 m). Contrary to general expectation, polychaete biomass increased (rather than decreased) with depth. Species diversity exhibited a parabolic pattern with depth, maximum diversity occurring at depths of 350-550 m, rather shallower than observed in other bathyal studies, and possibly linked with a maximum in habitat temperature range. Multivariate analyses of faunal composition suggested a separation of the sampling stations into a shallower and a deeper group, with temperature exerting a major control on polychaete species distributions. The decline in diversity below 600 m (i.e. the descending limb of the parabolic relationship) may be a result of historically limited immigration/recolonization of the thermally isolated Arctic deep-water basins that feed the cold-water flow through the Faroe-Shetland Channel. The bathymetric distribution of polychaetes and other benthos in this region appears to be intimately linked with the thermal regime, having a long-term impact (geological timescales) on the deep-water species pool and leading to local enhancement of diversity where cold- and warm-water masses meet and mix.