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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

Map of cruise transect. (89612 bytes)
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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Last Updated: 07 June, 2002