Benthic processes on the Peru margin: a transect across the oxygen minimum zone during the 1997–98 El Niño

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 (O2 < 0.5 ml L−1) on sediments, benthic communities, and bioturbation. Sampling took place during early January 1998, an intense El Niño period associated with higher-than-normal levels of O2 on the shelf and upper slope. lope 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, O2 < 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 210Pb and 234Th 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 Niño), the upper OMZ boundary exhibited a significant deepening (to 190 m) relative to ‘normal’, non-El Niño conditions (< 100 m), possibly causing a mild, transient oxygenation over the upper slope (200–300 m) and reduction of the organic particle flux to the seabed. Future sampling may determine whether the Peru margin system exhibits dynamic responses to changing ENSO-related conditions.