Soft-bottom fauna and oxygen minima in sub-arctic north Norwegian marine sill basins

Abstract

Marine sill basins are generally characterized by vertically stratified water masses. During periods of water stagnation, the bottom fauna may be severely affected by the depletion of oxygen. In northern Norway, stratification is less and water exchange processes stronger than in topographically similar boreal basins, which would generally imply better oxygen conditions. In this study the soft-bottom macrofauna (>1 mm) of three northern Norwegian sill basins was examined with respect to species diversity, abundance and functional relationships. Simultaneous data for hydrographical parameters (salinity, temperature, dissolved oxygen) and sediments (total organic carbon, fine fraction) were recorded. All basins were subject to discharges of municipal wastewater. In the basins, the water masses were stratified from spring to late autumn, but apparently freely circulating in winter. During stratification the oxygen content fell to 22–55% saturation in the bottom water. The fauna was generally species rich and diverse, but the number of species was lower than outside the basins. Polychaetes feeding at or just above the sediment surface were numerically dominant. The proportion of detrivorous polychaetes feeding below the sediment surface decreased in relation to oxygen minimum levels. The study suggests that, due to relatively strong water exchange dynamics, northern sill fjords are less vulnerable to organic discharges than more southerly located sill fjords.

Keywords:

• Feeding functions
• northern Norway
• oxygen concentrations
• sill fjords
• soft-bottom fauna

Acknowledgments

We are indebted to the following colleagues for their help with species identifications, statistical analyses and planning of the oceanographic sampling: Rosie Evans (Polychaeta), Lars Golmen (hydrography), Hans-Petter Mannvik (Ophiuroidea), Lena Ringstad Olsen (statistical analyses), Brage Rygg (Crustacea), Wim Vader (Crustacea) and Per Bie Wikander (Mollusca). We are also grateful to Rob Barret for linguistic corrections. The Norwegian Institute for Water Research (NIVA), Akvaplan-niva Ltd and the University of Tromsø took part in the field surveys and provided technical assistance during the processing of the benthic and seawater samples. The work was financed by the municipal authorities of the cities of Harstad and Tromsø.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Additional information

Notes on contributors

Børge Holte

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark