Soft-bottom macrofauna and responses to organic enrichment in the subarctic wates of Tromsّ, northern Norway

The soft-bottom macrofauna (> 1 mm) of Tromsøysund and Sandnessund near Tromsø, northern Norway (69°40′N, 19°0′E) was sampled quantitatively in 1983, 1990 and 1992 in order to assess enrichment effects from discharges of municipal sewage and fish factory effluents. The studies comprised 30 stations between 7 and 40 m depth at varying distances from effluent outfalls. In total 395 species/taxa were recorded. The polychaetes were the most important group both with regard to the numbers of species (146) and specimens (80% of total), but bivalves and amphipods were also common. Most stations had species numbers between 40 and 100 and densities between 2000 and 8000 ind. per m2. The species assemblages could be related to organic enrichment, depth and sampling year. Evidence suggests that the organic discharges did not seriously affect the waters surrounding Tromsø, but the generally high abundances, the prevalence of polychaetes and a numerical importance of ‘opportunistic’ species (Polydora, Chaetozone, Heteromastus) might suggest that the whole area was stimulated by the organic inputs. Sharply delimited strongly enriched zones characterized by dense populations of Capitella capitata were found on shallow sites close to large outfalls in Tromsøysund. These local and restricted effects of the discharges may be related to strong tidal currents and wave action that disperse effluent components. The faunal composition remained largely stable from 1983 to 1992 at 9 revisited stations, but the number of small bivalve and crustacean taxa increased and the abundance of the dominant polychaete Polydora socialis strongly decreased. The effluent discharges were supposed to have increased from 1983 to 1992, but the faunal changes did not provide evidence of an increased enrichment of the area during the period.