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.
Soft-bottom macrozoobenthos was sampled at monthly intervals between September 1981 and September 1984 at a normally hypoxic site (34 m depth) in Ancón Bay (Peru). Temperature, salinity, and dissolved oxygen were measured and related to changes in community structure. Large increases in the number of species, density, biomass, and diversity of macrozoobenthos were observed during the 1982-1983 El Niño (EN) thermal anomaly and for 1 year afterwards. These favourable changes were mainly associated with increased oxygen concentrations found in water masses near the bottom. Results are consistent with the hypothesis that the processes of post-catastrophic recovery of macrozoobenthos on marine soft bottoms are largely predictable, although certain particular mechanisms seem to exist for post-hypoxic recovery during El Niño in areas of the Peruvian coastal upwelling.
Data on rainfall, water surface temperature and salinity from the Bay of Santa Marta, Colombian Caribbean, are presented for the period November 1978 tó May 1982. Including data already published (Müller, 1979) generalizations of the anual patterns are obtained. Additional measurements of the variation of water temperature and salinity in the course of one day, preliminary values of light quanta, together with available data on oxygen content and saturation, pH, alcalinity, stability, wind direction and velocity, hours of sunshine, air temperature, and water discharge of the river Manzanares permitted a quantitative characterization of the dry and rainy season which can be observed in the area of investigation.
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