Progress in Oceanography 53 (2002) 1–27 www.elsevier.com/locate/pocean Benthic processes on the Peru margin: a transect across the oxygen minimum zone during the 1997–98 El Nin ˜o L. Levin a,* , D. Gutie ´rrez b , A. Rathburn a1 , C. Neira c2 , J. Sellanes b , P. Mun ˜oz b , V. Gallardo b , M. Salamanca b a Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0218, USA b University of Concepcio ´n, Project FONDAP-Humboldt, Casilla 160-C, Concepcio ´n, Chile c Institute for Chemistry and Biology of the Marine Environment, University of Oldenburg, Postfach 2503, D-26111, Oldenburg, Germany Abstract Oxygen minimum zones (OMZs) are widespread features in the most productive regions of the world ocean. A holistic view of benthic responses to OMZ conditions will improve our ability to predict ecosystem-level consequences of climatic trends that influence oxygen availability, such as global warming or ENSO-related events. Four stations off Callao, Peru (~12°S, Station A, 305 m; Station B, 562 m; Station C, 830 m and Station D, 1210 m) were sampled to examine the influence of the low bottom-water oxygen concentration and high organic-matter availability within the OMZ (O 2  0.5 ml L -1 ) on sediments, benthic communities, and bioturbation. Sampling took place during early January 1998, an intense El Nin ˜o period associated with higher-than-normal levels of O 2 on the shelf and upper slope. Peru slope sediments were highly heterogeneous. Sediment total organic carbon content exceeded 16%, lamination was present below 6 cm depth, and filamentous sulfur bacteria (Thioploca spp.) were present at Station A, (305 m, O 2  0.02 ml L –1 ). Deeper sites contained phosphorite crusts or pellets and exhibited greater bottom-water oxygenation and lower content and quality of organic matter. X-radiographs and 210 Pb and 234 Th profiles suggested the dominance of lateral transport and bioturbation over pelagic sedimentation at the mid- and lower slope sites. Macrofauna, metazoan meiofauna and foraminifera exhibited coherence of density patterns across stations, with maximal densities (and for macrofauna, reduced diversity) at Station A, where bottom-water oxygen concentration was lowest and sediment labile organic matter content (LOC: sum of protein, carbohydrate and lipid carbon) was greatest. Metazoan and protozoan meiofaunal densities were positively correlated with sediment LOC. The taxa most tolerant of nearly anoxic, organic- rich conditions within the Peru OMZ were calcareous foraminifera, nematodes and gutless phallodrilinid (symbiont- bearing) oligochaetes. Agglutinated foraminifera, harpacticoid copepods, polychaetes and many other macrofaunal taxa increased in relative abundance below the OMZ. During the study (midpoint of the 1997–98 El Nin ˜o), the upper OMZ boundary exhibited a significant deepening (to 190 m) relative to ‘normal’, non-El Nin ˜o conditions ( 100 m), possibly causing a mild, transient oxygenation over the upper slope (200–300 m) and reduction of the organic particle flux to * *Corresponding author. Tel.: +1-858-534-3579; fax: +1-858-822-0562. E-mail address: llevin@ucsd.edu (L. Levin). 1 Present address: Indiana State University, Dept. of Geography, Geology, and Anthropology, Terre Haute, IN 47809 2 Present address: Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, California 92093-0218, USA 0079-6611/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII:S0079-6611(02)00022-8