Upwelling and Lagoonal Ecosystems of the Dry Pacific Coast of Baja California
321
clupeids (E. mordax and S. caeruleus) in the nekton niche off Baja California (Walsh et al. 1977).
Furthermore, changes in frequency and intensity of upwelling, which
must be strong enough to infuse nutrients and enrich the trophic pyramid
but avoid turbulent mixing of the water column to maintain concentrations of food organisms (Bakun 1990), are likely to influence the reproductive success of pelagic fish and the replacement of species. For instance, the
Pacific sardine (Sardinops caeruleus) was the dominant species in the California Current ecosystem but was replaced by the northern anchovy
(Engraulis mordax) in the 1950s. Both anchovy and sardine populations
follow 60-year cycles in the Santa Barbara Basin off Southern California
and over the last 1,700 years, nine major recoveries and subsequent collapses of the sardine have occurred (Baumgartner et al. 1992).
21.3.2 El Nino Effect on the California Current Ecosystem
In the North Pacific basin, Southern Oscillation (ENSO) episodes are associated with a cyclonic circulation and a strong (40-90cm s- 1 ) and broad
inshore countercurrent that is responsible for the displacement of
southern, typically warm-water species towards higher latitudes along the
west coast of North America. Synchronous with the development of El
Nino in the tropics, large-scale anomalies of a winter barometric pressure
field over the North Pacific cause anomalous atmospheric circulation in
the region (Mysak 1986). Their impact on the California Current system off
Baja California includes enhanced countercurrent intensity, reduced
upwelling or increased downwelling, widespread and intensive warming of
upper mixed layer, depressed salinity and thermo- and nutriclines, higher
dissolved oxygen, lower inshore nutrients, and higher sea levels (Lynn et al.
1995).
Biological changes during ENSO events are related to reduced primary
productivity (Torres-Moye and Alvarez-Borrego 1987), delayed phytoplankton blooms (Lenarz et al. 1995), changes in kelp forest carrying
capacity (Dayton et al. 1998), arrival of early life stages of some warm water
species, and low reproductive success of some species of rockfish. Zooplankton abundance is primarily influenced by large-scale variations of
the California Current flow. While increased southward transport leads to
biomass enhancement, reduced transport causes abnormally low zooplankton biomass, probably reflecting primary productivity oscillations in
response to nutrients. Since large-scale environmental forcing of the California Current ecosystem has disruptive effects on the ecology of the
region, processes like competition and predation might not play a domi-
321
clupeids (E. mordax and S. caeruleus) in the nekton niche off Baja California (Walsh et al. 1977).
Furthermore, changes in frequency and intensity of upwelling, which
must be strong enough to infuse nutrients and enrich the trophic pyramid
but avoid turbulent mixing of the water column to maintain concentrations of food organisms (Bakun 1990), are likely to influence the reproductive success of pelagic fish and the replacement of species. For instance, the
Pacific sardine (Sardinops caeruleus) was the dominant species in the California Current ecosystem but was replaced by the northern anchovy
(Engraulis mordax) in the 1950s. Both anchovy and sardine populations
follow 60-year cycles in the Santa Barbara Basin off Southern California
and over the last 1,700 years, nine major recoveries and subsequent collapses of the sardine have occurred (Baumgartner et al. 1992).
21.3.2 El Nino Effect on the California Current Ecosystem
In the North Pacific basin, Southern Oscillation (ENSO) episodes are associated with a cyclonic circulation and a strong (40-90cm s- 1 ) and broad
inshore countercurrent that is responsible for the displacement of
southern, typically warm-water species towards higher latitudes along the
west coast of North America. Synchronous with the development of El
Nino in the tropics, large-scale anomalies of a winter barometric pressure
field over the North Pacific cause anomalous atmospheric circulation in
the region (Mysak 1986). Their impact on the California Current system off
Baja California includes enhanced countercurrent intensity, reduced
upwelling or increased downwelling, widespread and intensive warming of
upper mixed layer, depressed salinity and thermo- and nutriclines, higher
dissolved oxygen, lower inshore nutrients, and higher sea levels (Lynn et al.
1995).
Biological changes during ENSO events are related to reduced primary
productivity (Torres-Moye and Alvarez-Borrego 1987), delayed phytoplankton blooms (Lenarz et al. 1995), changes in kelp forest carrying
capacity (Dayton et al. 1998), arrival of early life stages of some warm water
species, and low reproductive success of some species of rockfish. Zooplankton abundance is primarily influenced by large-scale variations of
the California Current flow. While increased southward transport leads to
biomass enhancement, reduced transport causes abnormally low zooplankton biomass, probably reflecting primary productivity oscillations in
response to nutrients. Since large-scale environmental forcing of the California Current ecosystem has disruptive effects on the ecology of the
region, processes like competition and predation might not play a domi-