Revista Chilena de Historia Natural 75 (1): 217-231, 2002
RESEARCH ARTICLE
Multiscale upwelling forcing cycles and biological response off north-central
Chile
The physical forcing of the upwelling along the subtropical west coasts of the continents
encompasses a broad range of time scales which shape both phytoplankton biomass (Chl- a) and primary productivity (carbon
fixation) changes over any given time interval. The narrow continental shelf and the steep alongshore orography off north-central Chile
provide for a unique combination of year-round, upwelling-favorable winds with quasi weekly upwelling pulses associated with
atmospheric coastal-trapped disturbances (coastal lows). This variability is modulated by intraseasonal oscillations in the depth of the
thermo/nutricline, produced by coastal-trapped waves in the ocean, upon which annual (seasons) and interannual (ENSO) cycles are
superimposed. During coastal field experiments off Cruz Grande bay (29º S), carried on in November 1987 and 1988 (opposite
extremes of the ENSO cycle), mean changes of the phytoplankton-integrated Chl-a (B) and carbon fixation rate (PP) from the active to
the relaxed phases of the local upwelling forcing cycle (phyto-pattern) were characterized. Those data were contrasted against similar
ones reported off Punta Lengua de Vaca (Coquimbo, 30º S) and off Mejillones peninsula (Antofagasta, 23º S), encompassing different
seasons and phases of intraseasonal and interannual (ENSO) cycles from 1992 to 1997. A “warm” phyto-pattern was schematically
characterized by a significant increase in B and a quasi-steady evolution of PP from the active to the relaxed phases of one complete
upwelling event. Conversely, relative small changes in B and a significant increase in PP characterized a “cold” phyto-pattern. It is
proposed here that the ENSO “cold/warm” signal may be offset by more than one opposite “thermal” condition (seasonal and/or
intraseasonal) in defining a “warm” or “cold” phyto-pattern associated with a particular cycle of the local upwelling
forcing.
Key words:
Chile, coastal
upwelling, phytoplankton, local and remote upwelling forcing cycles, coastal trapped waves, ENSO cycle
