Possible ecological effects from maldanid (annelida, polychaeta) “super-dominance” in a small North Norwegian sill system

The sediment communities — with particular reference to polychaete functions—were studied at four stations at depths between 7 and 26 m in a slightly sewage-polluted North Norwegian sill fjord basin with recorded sub-zero temperatures six months a year. The sampled macro-fauna (> 1 mm sieve) included ca. 53300 individuals and 152 taxa, of which the polychaetes comprised ca. 49000 individuals and 69 taxa. The total abundance increased from 5721 individuals m-2 at the 7 m station to 20653 individuals m-2 at the 26 m station. Similarly, the maldanid polychaete abundance — comprising deep subsurface head-down feeding detrivores (mainly Maldane sarsi) — increased with depth from 483 individuals m-2 to 12510 individuals m-2. Based on literature data, a maldanid-driven sediment turnover rate of up to ca. 4500 1 m-2 year-1 was calculated. It is suggested that the maldanids, through feeding at some depth in the sediment and faecal deposition at the sediment surface, may greatly contribute to maintain a year-around organic carbon supply to the found quite rich and diverse sediment surface detrivorous fauna in the Vesterbotn basin. The subsurface detrivorous fauna, which probably feed at shallower sediment depth than the Maldanidae, was found to be impoverished at the maldanid-dominated stations.