Effect Of The 1997-98 El Niņo On Chlorophyll And Primary
Production Across The Central California Upwelling Zone: Temporal Evolution, Spatial
Pattern, And Comparison To Climatology
Francisco P. Chavez, J. Timothy Pennington, and Reiko P. Michisaki
Figure 1. CalCOFI Line 67 and MBARI major stations from which data used in this
presentation are taken. Stations range from 5km to 275km offshore.
Effects of the 1997-98 El Niņo on chlorophyll and primary production
levels across the central California upwelling zone were observed with quarterly cruises
from 0-275 km of the coast over 1997-99, and with semi-monthly cruises from 0-55 km of the
coast over the decade 1989-99. The 0-275 km cruises show (1) normal early upwelling season
(February, March, April) phytoplankton biomass and growth in 1997, (2) progressive
restriction of high chlorophyll and productivity waters inshore during the late upwelling
(May, June, July) and oceanic (August, September, October) seasons of 1997, (3) depression
of biomass and growth during the 1997-98 Davidson and 1998 early and late upwelling
seasons, and (4) recovery to normal and above-normal values during and following the 1998
oceanic season. El Niņo chlorophyll and productivity levels were as much as 80-90% below
climatology, but with chlorophyll and productivity values less strongly depressed over a
shorter period nearshore relative to offshore (0-50 km vs. 50-100 km and 100-200 km
spatial domains). Phytoplankton biomass and growth appears compressed towards shore (~50
km) during El Niņo.
Data from the 0-55 km cruises show that at a Monterey Bay station (C1),
typically downstream from a recurring upwelling plume, the 1997-98 El Niņo
produced a modest reduction in chlorophyll and primary production in 1997 but strongly
depressed values during the early and late upwelling seasons in 1998. At an outer bay
station (M1), which typically lies in the path of an upwelling plume, chlorophyll and
productivity values were similarly depressed in both 1997 and 1998.
Results of both cruise series show that biomass and production are
reduced during El Niņo. The 0-275 km cruises indicate offshore areas experience the
strongest negative anomalies; this spatial pattern is not clear in the 0-55 km data. Such
spatial pattern could have important consequences for distribution of higher trophic level
organisms during El Niņo.
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