The macrofauna and main functional interactions in the sill basin sediments of the pristine Holandsfjord, Northern Norway, with autecological reviews for some key-species

In order to study the sediment macrofauna (> 1 mm) in a pristine sill fjord system, baseline investigations were carried out in 1991, 1992 and 1994 in the 30 km long triple-basined Holandsfjord, Northern Norway. Three sampling stations were located in the deepest part of each basin and one in the outer margin of the fjord just outside the outer basin. The innermost basin has relatively high inorganic sedimentation rates caused by glacier-derived melt water run-off. The bottom sediments were generally clay-silt dominated with an organic carbon content between 7 and 11 mg g(-1) in the sill basins and between 21 and 22 mg g(-1) in the outer margin of the fjord. The Shannon-Wiener diversity index ranged between 3.0 and 4.2, the number of individuals between 373 and 4910 ind. m(-2) and the number of taxa per station between 23 and 45. The highest diversity was found at the outer margin of the fjord while the highest number of taxa, in two of three sampling pears, occurred at the fjord head. The fauna at the fjord head was numerically dominated by the subsurface feeding detrivorous polychaete Scoloplos armiger, the surface feeding detrivorous polychaetes Levinsenia gracilis, Chaetozone spp. and Myriochele oculata and the surface feeding detrivorous cumacean Endorella emarginata. In the middle part of the fjord the fauna was specifically dominated by the, respectively, surface and subsurface feeding detrivorous polychaetes Aricidea spp. and Praxillella affinis and the subsurface feeding detrivorous bivalve Thyasira minuta. In the more shelf-water influenced outer margin of the fjord a specific dominance of the surface feeding detrivorous polychaete Pseudopolydora paucibranchiata, the suspensivorous polychaete Jasmineira candela, the suspensivorous epifaunal bivalve Kelliella miliaris and an unidentified species of the genus Thyasira were recorded. The proportion of deep subsurface head-down feeding polychaetes (Heteromastus filiformis and the Maldanidae) ranged between 12 and 56% of the total abundance per station. It is suggested that such polychaete taxa, by depositing of faeces on the uppermost sediment layer and also by feeding selectively on relatively organic-rich particles, may have an important energy-exploiting role in communities occupying fine-particulated fjord sediments with relatively low concentrations of organic carbon.