El Niño and similar perturbation effects on the benthos of the Humboldt, California, and Benguela Current upwelling ecosystems

Abstract

Abstract. 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 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 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-

Recurrent, thermally-induced shifts in species distribution range in the Humboldt current upwelling system

“…One striking aspect of the results is that although the two clams showed opposite responses to ENTC, they both seem to be sensitive to LNTC, showing slower growth in comparison with NTC. This is probably the reason why (i) D. obesulus has been unable to establish viable populations within southern areas colonized during EN (Tomicic, 1985), and (ii) remnants of M. donacium populations are not able to rapidly recolonize northern beaches during LN (Arntz et al, 1987(Arntz et al, , 2006, as would be expected under a contrasting response pattern. Despite the fact that D. obesulus and M. donacium are sympatrically distributed along large parts of the coastal HCS, their biogeography reflects different environmental affinities, which may explain their contrasting responses to the extremes of ENSO cycle.…”

“…Arntz and Fahrbach, 1991;Arntz et al, 2006;Thiel et al, 2007). Table 3 shows changes in abundance reported for several shallow-water molluscs distributed along the HCS (Fig.…”

“…A whole array of abiotic factors is modified during ENSO, notably salinity, wave action, sediment load, upwelling strength and hence nutrient and food availability, which act simultaneously to produce an integrated effect on these species (e.g. Arntz et al, 2006) that may be difficult to address in experimental studies.…”

“…Soto, 1985;Tomicic, 1985;Arntz and Fahrbach 1991;Díaz and Ortlieb, 1993;Urban, 1994;Laudien et al, 2007). Unfortunately, cause-and-effect explanations of the described changes during extremes of ENSO are hampered by the lack of experimental and physiological evidence (Arntz et al, 2006;Thatje et al, 2008). Donax obesulus Reeve, 1854 and Mesodesma donacium Lamarck, 1818 are two common surf clams of reflective and dissipative sandy beaches.…”

“…First, it is not clear whether sympatric D. obesulus and M. donacium develop competitive interactions (Arntz et al, 1987). Second, M. donacium reveals a remarkably poor ability to recover its former distribution following EN events in spite of several cycles of warm and cold (Arntz et al, 2006). Third, parasitic interactions and, interspecific interactions generally have been shown to play a role in mortality events and species distribution (Dugan et al, 2004;Riascos et al, 2008).…”