Soft-bottom macrofauna in the high-latitude ecosystem of Balsfjord, northern Norway: Species composition, community structure and temporal variability

Abstract

Abstract The fjords of northern Norway are generally weakly stratified and subject to frequent large-scale advective water mass exchanges. In Balsfjord, studies of production and trophic relationships have shown that most of the biological production is mineralised in the pelagic food webs, or is exported. This study examines the soft-bottom macrofauna ( > 1 mm) in the deep basin of Balsfjord (185 m). Yearly monitoring surveys were conducted from 1977 to 1987, supplemented by samples taken in 1994, to assess long-term changes. Data to assess seasonal changes were obtained in 1979, 1982 and 1994. The fauna was strongly dominated by small annelids comprising 78 species and representing 97 % of the specimens. The numerically dominant species were the polychaetes Myriochele oculata, Levinsenia gracilis and Lumbrineris mixochaeta, and the oligochaete Tubificoides cuspisetosus. Species numbers ranged from 20 to 59 per survey, densities from 1700 to 9800 ind. m 2 and diversities (Shannon-Wiener H'log2) from 1.9 to 3.6. Deposit feeders constituted about 90 % of the specimens, carnivores about 10 % and suspension feeders a small fraction only. During the study period, species numbers and densities increased, while the structure of the community remained largely the same. Seasonal variations were insignificant. It may seem that the species assemblage was dependent on a low, irregular supply of largely low-quality nutrient material. The increasing densities may probably be related to the nutrient dynamics in the fjord, which is significantly influenced by advective exchanges of plankton into and out of the fjord.