Benthos communities in oxygen deficient shelf and upper slope areas of the Peruvian and Chilean Pacific coast, and changes caused by El Niño

Abstract

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.

Subtidal macrozoobenthos communities from northern Chile during and post El Niño 1997–1998

“…The polychaete Paraprionospio pinnata and the mollusc Nassarius gayi were the most dominant species. Our results are in accordance with previous studies for the upwelling systems of the Peruvian (Tarazona 1984;Tarazona et al 1985Tarazona et al , 1988aTarazona et al , 1996Arntz et al 1991;Yupanqui et al 2007) and Chilean coast in the HCS affected by EN (Gallardo 1985;Gutiérrez et al 2000;Laudien et al 2007).…”

“…Positive effects of EN events on macrozoobenthos communities have been previously reported for the upwelling systems along the Peruvian coast (Tarazona 1984;Tarazona et al 1985Tarazona et al , 1988aTarazona et al , 1996Arntz et al 1991;Arntz and Fahrbach 1996; but see Yupanqui et al 2007), north (Laudien et al 2007) and central Chile (Gallardo 1985;Gutiérrez et al 2000), and were generally suggested to be related with increased water temperature and dissolved oxygen during EN. However, as far as we are aware, no previous studies had reported the potential of this oceanographic anomaly to affect species assembling processes in marine benthic communities.…”

“…Despite the large amount of oceanographic climate information existing on this phenomenon (Philander 1990;Escribano et al 2004;Dijkstra 2006;Nyenzi and Lefale 2006;Santos 2006, Sokolikhina et al 2006, and its known impacts on the diversity of marine soft-bottom benthic communities of shallow zones in the HCS (Peru-Chile, e.g., Tarazona et al 1985Tarazona et al , 1988aTarazona et al , 1996Gallardo 1985;Salzwedel et al 1988;Arntz et al 1991;Gutiérrez et al 2000;Laudien et al 2007), our knowledge of the mechanism and processes driving community structure during EN is still scarce.…”

“…Several studies on EN effects on subtidal macrozoobenthos communities inhabiting oxygen minimum zone (OMZ) off Chile and Peru (see Levin 2003;Helly and Levin 2004) suggest that increases in bottom temperature and oxygenation caused by EN lead to higher macrozoobenthic species richness, but also cause decreases in the numerical dominance of formerly highly abundant taxa (Arntz et al , 1991Tarazona et al 1988bTarazona et al , 1996Arntz and Fahrbach 1996;Levin et al 2002;Arntz et al 2006). Given that species diversity (understood as an integrative measure of the number of species in a given site regarding to their relative abundances) is positively affected by increases in species richness, but negatively affected by increased species dominance (Stirling and Wilsey 2001), it could be hypothesized that species diversity of sublittoral coastal communities should raise during EN because of an increased number of species and/or a decreased species dominance within communities.…”

“…However, for terrestrial ecosystems, it has been shown that disturbances could move communities away from this interactive steady state, restarting the assembly processes and causing changes in the species co-occurrence patterns within communities (Badano et al 2005). For macrozoobenthos communities of shallow and deep waters off Chile and Peru, changes in bottom temperatures and oxygen concentration associated to EN represent a strong disturbance (Arntz et al , 1991Arntz and Fahrbach 1996;Tarazona et al 1985Tarazona et al , 1988bTarazona et al , 1996Gallardo 1985;Gutiérrez et al 2000;Gallardo et al 2004). Thus, it could be proposed that EN may restart successional processes of sublittoral communities, which should be reflected in a change in the species co-occurrence patterns.…”