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2006
Arntz, WE, Gallardo VA, Gutierrez D, Isla E, Levin LA, Mendo J, Neira C, Rowe GT, Tarazona J, Wolff M.  2006.  El NiƱo and similar perturbation effects on the benthos of the Humboldt, California, and Benguela Current upwelling ecosystems. Advances in Geosciences. 6:243-265.: European Geosciences Union, c/o E.O.S.T. 5, rue Rene Descartes Strasbourg Cedex 67084 France, [mailto:egu.production@copernicus.org], [URL:http://www.copernicus.org/EGU] AbstractWebsite

To a certain degree, Eastern Boundary Current (EBC) ecosystems are similar: Cold bottom water from moderate depths, rich in nutrients, is transported to the euphotic zone by a combination of trade winds, Coriolis force and Ekman transport. The resultant high primary production fuels a rich secondary production in the upper pelagic and nearshore zones, but where O sub(2) exchange is restricted, it creates oxygen minimum zones (OMZs) at shelf and upper slope (Humboldt and Benguela Current) or slope depths (California Current). These hypoxic zones host a specifically adapted, small macro- and meiofauna together with giant sulphur bacteria that use nitrate to oxydise H sub(2)S. In all EBC, small polychaetes, large nematodes and other opportunistic benthic species have adapted to the hypoxic conditions and co-exist with sulphur bacteria, which seem to be particularly dominant off Peru and Chile. However, a massive reduction of macrobenthos occurs in the core of the OMZ. In the Humboldt Current area the OMZ ranges between <100 and about 600 m, with decreasing thickness in a poleward direction. The OMZ merges into better oxygenated zones towards the deep sea, where large cold-water mega- and macrofauna occupy a dominant role as in the nearshore strip. The Benguela Current OMZ has a similar upper limit but remains shallower. It also hosts giant sulphur bacteria but little is known about the benthic fauna. However, sulphur eruptions and intense hypoxia might preclude the coexistence of significant mega- und macrobenthos. Conversely, off North America the upper limit of the OMZ is considerably deeper (e.g., 500-600 m off California and Oregon), and the lower boundary may exceed 1000m. The properties described are valid for very cold and cold (La Nina and "normal") ENSO conditions with effective upwelling of nutrient-rich bottom water. During warm (El Nino) episodes, warm water masses of low oxygen concentration from oceanic and equatorial regions enter the upwelling zones, bringing a variety of (sub)tropical immigrants. The autochthonous benthic fauna emigrates to deeper water or poleward, or suffers mortality. However, some local macrofaunal species experience important population proliferations, presumably due to improved oxygenation (in the southern hemisphere), higher temperature tolerance, reduced competition or the capability to use different food. Both these negative and positive effects of el Nino influence local artisanal fisheries and the livelihood of coastal populations. In the Humboldt Current system the hypoxic seafloor at outer shelf depths receives important flushing from the equatorial zone, causing havoc on the sulphur bacteria mats and immediate recolonisation of the sediments by mega- and macrofauna. Conversely, off California, the intruding equatorial water masses appear to have lower oxygen than ambient waters, and may cause oxygen deficiency at upper slope depths. Effects of this change have not been studied in detail, although shrimp and other taxa appear to alter their distribution on the continental margin. Other properties and reactions of the two Pacific EBC benthic ecosystems to el Nino seem to differ, too, as does the overall impact of major episodes (e.g., 1982/1983(1984) vs. 1997/1998). The relation of the "Benguela Nino" to ENSO seems unclear although many Pacific- Atlantic ocean and atmosphere teleconnections have been described. Warm, low- oxygen equatorial water seems to be transported into the upwelling area by similar mechanisms as in the Pacific, but most major impacts on the eukaryotic biota obviously come from other, independent perturbations such as an extreme eutrophication of the sediments ensuing in sulphidic eruptions and toxic algal blooms. Similarities and differences of the Humboldt and California Current benthic ecosystems are discussed with particular reference to ENSO impacts since 1972/73. Where there are data available, the authors include the Benguela Current ecosystem as another important, non-Pacific EBC, which also suffers from the effects of hypoxia.

2004
Munoz, P, Salamanca MA, Neira C, Sellanes J.  2004.  Nitrogen sediment fluxes in an upwelling system off central Chile (Concepcion Bay and adjacent shelf) during the 1997-1998 El Nino. Revista Chilena De Historia Natural. 77:305-318.   10.4067S0716-078X2004000200009   AbstractWebsite

Two stations representing coastal and shelf environments (bay of Concepcion and adjacent shelf, respectively) were sampled in order to assess benthic nitrogen fluxes. During the study period anomalously high oxygen bottom waters conditions were observed, coinciding with the 1997-1998 El Nino event. From both sites, intact sediment cores were collected for incubation experiments to directly estimate total ammonium fluxes. Pore-water chemistry was also assessed to estimate diffusive fluxes of nitrogen compounds (NH(4)(+), NO(3)(-) and NO(2)(-)), The NH(4)(+) flux from incubation experiments ranged from 0.55 to 2.18 mmol m(-2) day(-1) in the inner bay and from 1.84 to 2.14 mmol m(-2) day(-1) on the shelf. NO(3)(-) diffusive fluxes to the sediments were observed in the bay in March (-17 x 10(-2) mmol m(-2) day(-1)) and on the shelf in November (-2.7 x 10(-2) mmol m(-2) day(-1)), associated with the lowest NH(4)(+) diffusive fluxes and coincident with a stronger gradient of Eh in the first centimeters of the sediments. Besides, total NH(4)(+) fluxes lower than in normal periods were observed, suggesting the influence of less reducing conditions, consequence in turn of the lower fluxes of particulated organic material and higher dissolved oxygen in the bottom waters. These conditions were probably imposed by the El Nino event. On the other hand, and relative to previous reports, an increase of irrigation coefficients (DI) was estimated. Thus, the flux extension of other nitrogen compounds could have acquired relevance in relation to NH(4)(+) fluxes during the study period.