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ABSTRACT-Macrofauna density and biomass were estimated in 4 sediment layers (0-1, 1-5, 5-10 and 10-15 cm) from 12 stations ranging from 185 to 4460 m water depth in the Goban Spur area (NE Atlantic). At the same statlons profiles of median grain slze, % organic carbon and ' K total n~trogen were measured to a sediment depth of 15 cm. Macrofauna densltles decreased exponentially from -8000 m' at 208 m to -550 m ' at the deepest stations (3673 and 4460 m). Most of the macrofauna was concentrated In the upper 1 cm of the sediment. The hlghest proportion (70 to 8OUi1) was in the upper 1 cm found at lntermedlate depths (670 to 1425 m), whereas at the deeper stations the fauna was more evenly distributed Polychaeta were the most abundant taxon, with a high proportion of subsurface deposit-fccders at the deep statlons and a high proportion of f~lter-feeders plus surface deposit-feeders at ~ntermediate depth. Biomass values showed a very variable pattern, mainly due to a few extremely large individuals at some of the stations. Excluding these large individuals, a maximum biomass value was observed around 1000 m water depth. At most stations the biomass was concentrated below the upper 1 cm, which was also reflected in the vertical distribution pattern of the mean individual weight, showing small individuals in the upper 1 cm and larger individuals In the deeper sediment layers. Both density and biomass fell within the range of values usually observed at these depths. Only the biomass value at 1034 m was extremely high and concentrated in the upper 1 cm of the sediment. This may be due to pulsed inputs of fresh material at this station, whereas at the deepest stations (3673 and 4460 m) a constant low input of refractory material supports the fauna of mainly deep-living subsurface depositfeeders. Median gram size decreased with increasing water depth from -90 pm at 208 m to -8 pm below 2000 m. The vert~cal profiles showed very homogeneous sedlments to a depth of 15 cm at all stations, probably due to bioturbation by the relat~vely abundant depos~t-feeders. In general, the % of total N and organlc C in the upper 1 cm of the sediment Increased with Increasing water depth, but showed a peak at -1000 m The C% C and especially N were Ioxver In the deeper sediment layers at the deeper stations, resulting In C -N ratlos of -12 to 15 in contrast to -7 to 8 In the upper 1 cm of the sediment The C:N ratio at the midslope stations (-1000 to 1500 m) d~d not decrease with depth in the sediment, ~n d icating a rapid burial of organic matter by the hlgh numbers of surface deposit-feeders at these stations.

Abstract. The cockle Cerastoderma edule is one of the large and important species of the tidal flats of the Wadden Sea. C. edule disturbs the upper sediment layer due to its crawling and “shaking” behaviour. About 7% of a cockle population move within one week. The mean distance of movement is ‐4 cm per week, which, in the case of a 2‐year‐old cockle with a length of ˜3 cm, means that ˜20 cm2 of sediment is disturbed to a depth of ˜3 cm. Besides these measurable movements, cockles also regularly “shake” themselves, thereby disturbing the sediment around them to at least ˜0.5 cm. At a density of 500 cockles per m2 of a mean length of 3 cm, ˜16% of the sediment surface is occupied by cockles and another ˜29% is disturbed by cockles within one week. To study the effects of cockles on recruitment of other benthic species, densities of 0, 125, 250, 375 and 500 cockles (mean length 3 or 3.5 cm) were added to 1‐m2 plots within large depopulated squares in April. Subsequently, sediment samples were sieved once a month (from June to September) and the densities of all macrozoobenthic species were estimated. The presence of cockles significantly reduced the densities of juveniles of the bivalve species C. edule, Macoma balthica, Mya arenaria, Tellina (Angulus) tenuis, and Ensis directus and of the worm species Pygospio elegans, Lanice conchilega, Eteone longa, Anaitides spec., Nephtys hombergii, Heteromastus filiformis, Scoloplos armiger, and Tharyx marioni and of the amphipods Corophium volutator and C. arenarium. Non‐significant effects were found only in Capitella capitata and Nereis diversicolor. For all species mentioned above a negative effect was already observed at the lower cockle densities of 125 ‐ 250 per m2. Reductions of about 50% or more were found at densities of 500 per m2, which was more than could be expected based on the area occupied by the cockles (˜16%), but close to the area occupied plus disturbed (˜45%). It is concluded that (dense) assemblages of adult cockles can have a strong influence on the macrozoobenthic community.

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