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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


The 1997-98 El Niņo depressed chlorophyll and primary production values across the central California upwelling system from the late upwelling season of 1997 until the oceanic season of 1998 (Figure 2; Table 2). This depression probably has different causes in different seasons. In spring and summer this is likely due to reduced nutrient supply either through reduced upwelling, or upwelling of low nutrient waters (see Castro, 1999 EPOC Poster). In fall and winter it may again be due to reduced upwelling of nutrients, which does occur intermittently in these seasons, or to advection of low nutrient and biomass waters from the south and/or offshore. The 100-200 km offshore spatial domain appears to be most affected (Table 2), as chlorophyll or productivity values were 80-90% below climatology this far offshore during El Niņo summers. The nearshore (0-50 km) and intermediate (50-100 km) domains were not as strongly depressed (Table 2). This spatial pattern may develop because the 100-200 km domain is on the offshore margin of the normal upwelling zone, and processes such as El Niņo, which reduce nutrient supply to the nearshore domain, may essentially eliminate supply of new nutrients to such ‘downstream’ areas.

A similar pattern develops during both the 1992-93 and 1997-98 El Niņo's in the 0-55 km cruise data. In these data chlorophyll values at station M1 are less depressed during El Niņo than at the nearshore station C1 (Figures 4 and 5). Station M1 is located in the path of a persistent upwelling plume, where both strong and no upwelling depress chlorophyll values (Pennington and Chavez, in press). At this site weak upwelling, as is typical of El Nino years, favors bloom formation. Station C1 is downstream or on the margins of the M1 upwelling plume during normal years, even though C1 is nearshore inside Monterey Bay. During El Niņo years nutrient supply on these margins is reduced, and chlorophyll and productivity levels are accordingly depressed. This effect is similar but on a smaller spatial scale than that observed for the 0-275 km cruises. If El Niņo-caused chlorophyll and productivity reductions are spatially predictable, such patterns may have important consequences for the spatial distribution of higher trophic levels and fisheries stocks during El Niņo years.


We gratefully acknowledge the David and Lucile Packard Foundation for their funding of this time series work. We thank the crews of the R/Vs Point Lobos, Point Sur, and New Horizon as well as the many MBARI science personnel who participated in the work.


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