SOFT-BOTTOM MACROFAUNA AND RESPONSES TO ORGANIC ENRICHMENT IN THE SUB-ARCTIC WATERS OF TROMSO, NORTHERN NORWAY

The soft-bottom macrofauna (> 1 mm) of Tromsoysund and Sandnessund nea r Tromso, northern Norway (69 degrees 40'N, 19 degrees 0'E) was sample d 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 varyin g distances from effluent outfalls. In total 395 species/taxa were rec orded. The polychaetes were the most important group both with regard to the numbers of species (146) and specimens (80% of total), but biva lves and amphipods were also common. Most stations had species numbers between 40 and 100 and densities between 2000 and 8000 ind. per m(2). 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 Tromso, but the generally high abundances, the prevalence of polychaetes and a numerical importa nce of 'opportunistic' species (Polydora, Chaetozone, Heteromastus) mi ght suggest that the whole area was stimulated by the organic inputs. Sharply delimited strongly enriched zones characterized by dense popul ations of Capitella capitata were found on shallow sites close to larg e outfalls in Tromsoysund. These local and restricted effects of the d ischarges 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 s mall bivalve and crustacean taxa increased and the abundance of the do minant polychaete Polydora socialis strongly decreased. The effluent d ischarges were supposed to have increased from 1983 to 1992, but the f aunal changes did not provide evidence of an increased enrichment of t he area during the period.