| 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 O2 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 H2S. 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
Niña and "normal") ENSO conditions with effective upwelling of
nutrient-rich bottom water. During warm (El Niño) 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 Niño 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 Niño 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 Niño" 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. |