The Peruvian Coastal Upwelling System
239
plankton feeders while pilchards and anchovies feed on neritic and epipelagic resources such as diatoms and zooplankton. Although zooplankton
(i.e., large copepods and euphausiids) is the principal diet of anchovy,
during phytoplankton dominance direct consumption of microalgae may
also occur (Pauly et al. 1989). In the absence of anchovies and pilchards,
most of the plankton production is likely to be used further offshore by
euphausiids and mesopelagic fishes, including hake (Merluccius gayi).
Both jack and Spanish mackerels prey on anchovy and after the collapse of
anchovy stocks {1964-1971), the trophic flow via pilchard increased sevenfold (J arre-Teichmann 1998). Anchovy and, to a lesser degree, pilchard,
mackerels, and hake sustain dense populations of guano birds, like cormorant (Phalacrocorax bougainvillii), booby (Sula variegata), and pelican
(Pelecanus thagus), as well as sea lions ( Otaria byronia) and fur seals (Arctocephalus australis; Muck 1989).
16.4 El Nifio Impact on the Ecosystem
The oceanographic conditions along the Peruvian coast are influenced by
an interannual variability known as El Nino-Southern Oscillation
(ENSO). The ENSO cycle consists of a cold period (La Nina) and a warm
period (El Nino) with anomalous warming of the eastern Pacific during
about 3 months. El Nino occurs every 2-7 years (mean, 4 years), causing
a deepening of the pycnocline and thus rendering the upwelling process
inefficient (Barber et al. 1985). Other profound ecological alterations of
the Peruvian coastal upwelling system during El Nino events are due to
the arrival of Kelvin waves, temperature rise and deeper thermocline,
altered currents, reduced nutrient transport, dissolved oxygen increase
and increase in H 2 S reduction in bottom sediments, etc. (Wyrtki 1982;
Arntz 1986; Arntz and Tarazona 1990; Arntz and Fahrbach 1991; Jaimes
1999).
The severity of El Nino depends on the month of occurrence, relative
strength and duration, and distance from the major (i.e., equatorial) zone
of impact. Among the principal manifestations of El Nino is the "tropicalization" of the Oceanic Province off central and southern Peru. Under
these circumstances the distribution of many species in the upwelling
system changes due to immigration of species from the Panamanian Province and emigration of species to northern Chile. Some of the long-term
changes, however, appear to be associated with global climatic change
since simultaneous oscillations of pelagic shoaling fish occur in different
upwelling systems.
239
plankton feeders while pilchards and anchovies feed on neritic and epipelagic resources such as diatoms and zooplankton. Although zooplankton
(i.e., large copepods and euphausiids) is the principal diet of anchovy,
during phytoplankton dominance direct consumption of microalgae may
also occur (Pauly et al. 1989). In the absence of anchovies and pilchards,
most of the plankton production is likely to be used further offshore by
euphausiids and mesopelagic fishes, including hake (Merluccius gayi).
Both jack and Spanish mackerels prey on anchovy and after the collapse of
anchovy stocks {1964-1971), the trophic flow via pilchard increased sevenfold (J arre-Teichmann 1998). Anchovy and, to a lesser degree, pilchard,
mackerels, and hake sustain dense populations of guano birds, like cormorant (Phalacrocorax bougainvillii), booby (Sula variegata), and pelican
(Pelecanus thagus), as well as sea lions ( Otaria byronia) and fur seals (Arctocephalus australis; Muck 1989).
16.4 El Nifio Impact on the Ecosystem
The oceanographic conditions along the Peruvian coast are influenced by
an interannual variability known as El Nino-Southern Oscillation
(ENSO). The ENSO cycle consists of a cold period (La Nina) and a warm
period (El Nino) with anomalous warming of the eastern Pacific during
about 3 months. El Nino occurs every 2-7 years (mean, 4 years), causing
a deepening of the pycnocline and thus rendering the upwelling process
inefficient (Barber et al. 1985). Other profound ecological alterations of
the Peruvian coastal upwelling system during El Nino events are due to
the arrival of Kelvin waves, temperature rise and deeper thermocline,
altered currents, reduced nutrient transport, dissolved oxygen increase
and increase in H 2 S reduction in bottom sediments, etc. (Wyrtki 1982;
Arntz 1986; Arntz and Tarazona 1990; Arntz and Fahrbach 1991; Jaimes
1999).
The severity of El Nino depends on the month of occurrence, relative
strength and duration, and distance from the major (i.e., equatorial) zone
of impact. Among the principal manifestations of El Nino is the "tropicalization" of the Oceanic Province off central and southern Peru. Under
these circumstances the distribution of many species in the upwelling
system changes due to immigration of species from the Panamanian Province and emigration of species to northern Chile. Some of the long-term
changes, however, appear to be associated with global climatic change
since simultaneous oscillations of pelagic shoaling fish occur in different
upwelling systems.