In stratified coastal marine waters hypoxia is a growing problem affecting bottom-dwelling animals Earlier studies suggest oxygen concentrations of about 2 m l l l (Rosenberg 1980) as the lower tolerance limit for many benthic species in coastal areas. We exposed several conspicuous infaunal species on the NE Atlantic continental shelf, contained within their sediment habitat, to gradually reduced oxygen concentrations. Tolerance to hypoxia for the 8 species examined was in the range of 0.5 to 1.0 m l l l (8 to 15 % saturation), which they could tolerate for several days to weeks. The ophiuroid Amphiura fihformis left its protected position in the sediment at an oxygen concentration of 0.85 ml1-I (13 % sat.), whereas A. chiajei emerged from the sediment at 0.54 m l l l (8 % sat.).

The Gullmarsfjord on the Swedish west coast has a sill at 40 m and a maximum depth of 118 m. The stagnant bottom water is usually renewed with oxygen-rich water each spring. In 1997 this did not occur, and the fauna was eliminated at depths below about 100 m and severely reduced between 80 and 100 m depth. In spring 1998, the whole fjord was re-oxygenated and the succession of the benthic fauna was studied at 5 stations over a 2 yr period. Simultaneously, the benthic habitat quality (BHQ) was assessed by analysing sediment redox conditions and faunal burrow structures in sediment profile images. Succession of the benthic fauna is described as increases in number of species, abundance and biomass. Conspicuous colonisers below 100 m depth were the polychaete Capitella capitata and the heart urchins Echinocardium cordatum and E. flavescens. Return to preoxygen stressed conditions was slowest at the deepest stations. By using a multivariate technique (multi-dimensional scaling) we show that the benthic communities at all depths more or less returned to the same faunal composition as during pre-disturbed conditions. The pioneering and mature ben-

Abstract-Sediment total organic carbon (TOC) content is considered to be a primary food source for benthic invertebrates and a major factor influencing the partitioning and bioavailability of sediment-associated organic contaminants. Most studies report that both toxicity and uptake of sediment-associated contaminants by benthic organisms are inversely proportional to sediment TOC content. The aim of this study was to determine the importance of the TOC quality for the bioavailability of sediment-associated organic contaminants and the growth of benthic macrofauna. The common infaunal brittle star Amphiura filiformis was exposed to a base sediment covered by a 14 C-polychlorobiphenyl (3,3Ј4,4ЈϪ 14 C-tetrachlorobiphenyl (TCB)) contaminated top layer (0-2 cm), enriched to the same TOC content with 31 g TOC/m 2 of different quality and origin. The following carbon sources, ranging from labile to refractory, were used: (1) green macroalga (Ulva lactuca), (2) brown macroalga (Ascophyllum nodosum), (3) eelgrass (Zostera marina), (4) phytoplankton (Ceratium spp.), and (5) lignins of terrestrial origin. Characterization of the organic matter quality was accomplished by measuring the content of amino acids, lipids, C, N, and polyphenolic compounds. The reactivity of the sedimentary organic matter was assessed by means of respiration and dissolved inorganic nitrogen flux measurements. The experiment was carried out in 1-L glass jars, each containing four brittle stars and the contaminated and enriched sediment. The jars were circulated in a flow-through mode with filtered seawater. Somatic growth (regeneration of a precut arm) and bioaccumulation of 14 C-TCB were measured at 10 sampling occasions during 48 d of exposure. Growth rates, TCB uptake rates, and steady-state concentrations differed significantly between treatments and were correlated to the qualities of the organic substrates. The greatest TCB accumulation and growth were observed in treatments with the most labile organic substrates and the lowest accumulation and growth were seen with the most refractory substrates. Results from this experiment suggest that the bioaccumulation of organic contaminants in benthic infauna is dependent on the nutritional quality of the sedimentary organic matter.

The hypothesis that the semi-mobile brittle star Amphiura filiformis may have densitydependent migratory behaviour and that their dispersion may be dependent on food availability was examined. Sediment with intact fauna dominated by A. filiformis (-2250 ind. m-2) was brought to the laboratory in 0.22 m2 plexiglass boxes. The experiment was a 2-factorial design with density (high or moderate) and food (fed in excess or starved) as factors. To investigate migration in the high and moderate densities, the abundance of A. filiformis was experimentally manipulated in the boxes at the start of the experiment to 'low' (100 ind. 0.11 m-') numbers in one-half of all boxes and to 'high' (350 ind. 0.11 m-') or 'moderate' (220 ind. 0.1 1 m-') numbers in the other half. The experiment was run for 63 d . At termination of the experiment, a mean number of 90 and 36 ind. box-' had migrated into the low density side of the box from the high density and moderate density side, respectively. Significantly higher migration rates per ~ndividual were observed from the high density side compared to the moderate density side. The migration rate was estimated from a diffusion coefficient based on the 'Random Walk' theory. No significant differences in migration rates were observed between fed and starved boxes, indicating that dispersal was primarily density-dependent rather than food-dependent. Based on the diffus~on coefficient, a total dispersal of all A. filiformls from the high density side was estimated at 47 m h-'. Fed A. fillformis had a higher weight and larger gonads than starved brittle stars. A separate experiment was set up to study migratory behaviour. It appeared that A. filiformis can move both on the sediment surface and within the sediment. In conclusion, migration in A. filiformis may be a common feature, which may cause a more or less continuous displacement of sediment with significant ecological effects.

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