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. 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, 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 panicle flux to the seabed. Future sampling may determine whether the Peru margin system exhibits dynamic responses to changing ENSO-related conditions.