Seasonal variations in faunal distribution and activity across the continental slope of the goban spur area (NE Atlantic)

Abstract

Density, biomass and community structure of macrofauna were estimated together with several sediment characteristics at seven stations ranging from 208 m to 4460 m water depth along the OMEX transect in the Goban Spur area (NE Atlantic) during three seasons (October 1993, May 1994, and August 1995). Median grain size decreased with increasing water depth and showed no differences between the seasons. The percentages of organic carbon and total nitrogen were highest at mid-slope depths (∼1000 to 1500 m), and were significantly higher in August at the upper part of the slope to a depth of ∼1500 m. The C:N ratio in the surface layer amounted to ∼7 to 8 in May, ∼10 to 12 in August and ∼14 to 17 in October at all stations (except the deepest at 4460 m, where it was ∼11 in May and August), indicating arrival of fresh phytodetritus in May, and therefore seasonality in food input to the benthos. Densities of macrofauna decreased exponentially with increasing water depth. Significantly higher densities of macrofauna were found in May at the upper part of the slope to a depth of ∼1500 m. These differences were mainly due to high numbers of postlarvae of echinoids at the shallowest station and ophiuroids at the deeper stations. Biomass values also decreased with increasing water depth, but biomass was relatively high at the ∼1000 m station and low at ∼1500 m, due to relatively high and low mean weights of the individual macrofaunal specimens. No significant differences in biomass were found between the seasons. Respiration was high (15 to 20 mgC·m −2·d −1) in May at the upper part of the slope to a depth of ∼1000 m and low (1–3 mg C·m −2·d −1) at the deeper part. At the shallowest stations to a depth of ∼1000 m respiration was highest in May, at the mid-slope stations (∼1400–2200 m) it was highest in August, whereas the deepest stations (∼3600 to 4500 m) did not show any differences in respiration rates. In conclusion; seasonal variation in organic input is reflected in denstiy, community structure and activity of the macrofauna along the continental slope in the NE Atlantic.