Int. Revueges. Hydrobiol. zyxwvu I zyxwvu 80 I 1995 I 2 I 327-331 P. JOHN D. LAMBSHEAD', TIMOTHY J. FERRERO~ and GEORGE A. WOLLF~ 'Nematode and Polychaete Research Group, The Natural History Museum, London, SW7 5BD, UK *Environmental Organic Chemistry and Geochemistry Group, Oceanography Laboratories, Department of Earth Sciences, The University, PO Box 147, Liverpool, L69 3BX, UK Comparison of the Vertical Distribution of Nematodes from Two Contrasting Abyssal Sites in the Northeast Atlantic Subject to Different Seasonal Inputs of Phytodetritus zy key words: nematodes, meiofauna, vertical distribution, phytodetritus, bioturbation, deep sea Abstract The vertical distribution of nematodes in the sediments of three cores from the phytodetritus- influenced Porcupine Abyssal Plain station is compared with three cores from the Madeira Abyssal Plain station in the DEEPSEAS programme. Nematode vertical distributions are compared with sediment organic chemistry data sampled at the same time from the same sites. The results support the two hypotheses erected by THIEL (1983) that the penetration of meiofauna into deep-sea sediments might be correlated with the input of organic matter and that a greater biological activity of larger organisms would increase the penetration of nematodes into the sediment through increase downward transport of food material. 1. Introduction The MAST supported DEEPSEAS Programme is an ongoing, multidisciplinary, investigation into the effects of the seasonal flux of phytodetritus on to the sea floor of the deep northeast Atlantic. Two sites have been selected, a station on the Porcupine Abyssal Plain (PAP), and a station on the Madeira Abyssal Plain (MAP), differing as little as possible except in the depth of winter mixing in the overlying water column. The locations are described in detail by RICE et zyxwvut al. (1994). At PAP the winter-mixed depth is in excess z of 500m and the seafloor receives a strong seasonal input of aggregated phytodetritus. In contrast, the winter-mixed depth at MAP is less than 150m so the seafloor does not receive a significant phytodetrital seasonal signal. Phytodetritus flux has been shown to influence deep-sea meiofauna, e.g. it produces a detectable change in the horizontal spatial distribu- tion of both single-celled and metazoan meiofauna (LAMBSHEAD and GOODAY, 1990; RICE and LAMBSHEAD, 1994; LAMBSHEAD and HODDA, 1994). It has been suggested that the penetration of meiofauna into deep-sea sediment might be correlated with the input of organic matter (see THIEL, 1983; VINCX et al., 1994). THIEL (1983) further suggested that in high food concentration zones in the deep sea, bioturba- tion due to large animal activity would be high, mixing incoming food material deeper into the sediment than in more oligitrophic areas. He postulated that meiofauna distribu- tions should reflect this. The DEEPSEAS chemistry and meiofauna programmes allow these hypotheses to be tested. 2. Methods Three cores were collected from Station 11908 (PAP) in August-September 1989 and from Station 12174 (MAP) in August-September 1990 using an SMBA multiple corer (BARNETT et zyx id., 1984) with a