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

The fjords of northern Norway are generally weakly stratified and subject t o frequent large-scale advective water mass exchanges. In Balsfjord, studie s of production and trophic relationships have shown that most of the biolo gical production is mineralised in the pelagic food webs, or is exported. T his study examines the soft-bottom macrofauna (> 1 mm) in the deep basin of Balsfjord (185 m). Yearly monitoring surveys were conducted from 1977 to 1 987, supplemented by samples taken in 1994, to assess long-term changes. Da ta to assess seasonal changes were obtained in 1979, 1982 and 1994. The fau na was strongly dominated by small annelids comprising 78 species and repre senting 97% of the specimens. The numerically dominant species were the pol ychaetes Myriochele oculata, Levinsenia gracilis and Lumbrineris mixochaeta , and the oligochaete tubificoides cuspisetosus. Species numbers ranged fro m 20 to 59 per survey, densities from 1700 to 9800 ind. m(-2) and diversiti es (Shannon-Wiener H-log2') from 1.9 to 3.6. Deposit feeders constituted ab out 90% of the specimens, carnivores about 10% and suspension feeders a sma ll fraction only. During the study period, species numbers and densities in creased, while the structure of the community remained largely the same. Se asonal variations were insignificant. It may seem that the species assembla ge 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 exch anges of plankton into and out of the fjord.