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Modern Shelf Anoxia |
1 Alfred Wegener Institute for Polar and Marine Research (AWI), Columbusstrasse, D-2850 Bremerhaven, Germany
2 San Marcos University, Lima, Peru
3 University of Concepción, Chile
4 Instituto del Mar del Perú, Callao, Peru
Besides a well oxygenated shallow water area characterized by an extremely rich benthic life, most of the Peruvian and Chilean continental shelf and upper slope lies within an oxygen minimum zone (omz), the poor macrobenthic colonization of which is in striking contrast to the abundant pelagic life in the overlying waters. However, the eukaryotic benthos in this zone shares the seafloor with an important prokaryotic component of filamentous (‘sphagetti’) bacteria belonging mainly to the genus Thioploca. Under ‘normal’, non-Niño conditions, near-seafloor oxygen saturation values in this area rarely exceed 0.5 ml 1–1, and in some cases the bottoms are even anoxic. During strong El Niños, saturation periodically increases to >3.5 ml l–1, and higher oxygenation may persist for more than a year after the end of the event proper (which is characterized by increased water temperatures). In the shallow parts of the omz, the changes induced by El Niño cause drastic increases of macrobenthic density, biomass and diversity; immigration of benthos and fish species from (sub)tropical equatorial and oceanic areas; changes in trophic structure; and reduction of ‘spaghetti’ bacteria. In the case of the particularly strong 1982–83 El Niño, the communities of the shallower part of the omz had shifted back into their pre-Niño state about 13 months after the end of the warming event. In the deeper part of the omz, despite a similarly strong increase of oxygen saturation, a clearly positive development of macrobenthos during El Niño could not be observed. The reasons that may be responsible for this difference are discussed.
