On a ∼3- to 6-year cycle, weakening of the trade winds reduces equatorial
upwelling and the warm pool shifts eastward, deepening the thermocline and
nutricline in the eastern tropical Pacific. These events, termed El Niños, reduce
nutrient supply to the surface from the upwelling that does occur despite the reduced
trade-wind activity. Strutton and Chavez (2000) compared nutrient levels,
phytoplankton stocks, and productivity between the 1997–1998 El Niño period and
normal conditions in late 1998. At the peak of the El Niño, nitrate was less than or
equal to 1 μM all along the eastern equatorial zone, a sharp reduction from the 3–6
μM usually present for 3–6° of latitude to either side of the equator out to 155°W. The
phytoplankton size distribution shifted sharply to very small cells; chlorophyll
concentration was about half of usual, non-El Niño conditions; and daily primary
production was equally reduced. This tendency to strongly reduced production
extends north and south along the coast of the Americas to the same distance as
warming extends in any given El Niño. Thus, coastal plankton and fish populations
are exposed to warming, which raises metabolism and, thus, food demand, at the same
time that food production drops. Impacts of El Niños on equatorial marine biota are
also strong. Those involving plankton, which have short life spans and high
population growth potential, come and go rapidly with the events. Effects on larger,
longer-lived animals, particularly forage fish and seabirds, involve substantial
mortality and long population-recovery times.
(^) Mesozooplankton abundance along the tropical belt drops sharply as well. Dessier
and Donguy (1987) developed a 1979–1984 time-series of copepod abundance
estimates in the equatorial zone by filtering the nightly filling of a cruise-ship