A ten-year time series from Monterey Bay,
California: Seasonal, interannual and long-term patterns
C O N C L U S I O N S
Fig. 6. Monthly SeaWiFS composites
for six months of 1998. Compare the El Niņo winter of 1997-1998 (January, top panel) with
the La Niņa (?) winter of 1998-1999 (November, bottom panel).
There are clear examples of physical-biological coupling on seasonal
and decadal scales. Surprisingly the interannual connections are not as clear. Seasonally
the spring and early summer are periods of upwelling. The upwelling signal is evident in
winds, currents, temperature, nitrate, chlorophyll, primary production and centric
diatoms. During the late summer and early fall (the oceanic period) there are increases in
temperature, a shoaling and intensification of the poleward undercurrent, decreases in
nitrate concentration, chlorophyll and primary production, and a shift in species
assemblages. A deeper mixed layer and minima in biological and chemical properties
characterize the winter.
The long-term trend is for the ecosystem to change from the upwelling
condition to a more oceanic condition. Temperatures are increasing, the poleward
undercurrent is intensifying, subsurface nitrate concentration, chlorophyll and primary
production are decreasing, and a shift in species assemblages like that observed in the
seasonal cycle is occurring. The increase in temperature has been well documented but the
other changes have only been hypothesized. Time series like the ones presented here are
extremely rare yet they are required before an accurate assessment of changes associated
with global change can be made. Based on ten years of data we have constructed a coherent
story of how the ecosystem may be changing, however, much longer series are required to
confirm this story.
We gratefully acknowledge the David and Lucile Packard Foundation for their generous
funding and continued support. We thank the crews of the R/V Point Lobos, Point Sur,
and New Horizon as well as the science support staff. The Pacific Fisheries
Environmental Lab (PFEL) and the Navy Fleet Numerical Meteorological and Oceanographic
Center (FNMOC) provided the upwelling indices and the USGS provided the wind data from
Breaker, L. C., W. W. Broenkow (1994) The circulation of Monterey Bay
and related processes. Oceanography and Marine Biology Annual Review 32,
Chavez, F. P. (1996) Forcing and biological impact of onset of the 1992 El Nino in
central California. Geophysical Research Letters, 23, 265-268.
Chavez, F. P., J. T. Pennington, R. Herlien, H. Jannasch, G. Thurmond and G. E.
Friederich (1997) Moorings and drifters for real-time interdisciplinary oceanography. Journal
of Atmospheric and Oceanic Technology, 14, 1199-1211.
Skogsberg, T. (1936) Hydrography of Monterey Bay, California. Thermal Conditions,
1929-1933. Transactions of the American Philosophical Society held at Philadelphia for
Promoting Useful Knowledge, 29, 1-152.
Skogsberg, T. and A. Phelps (1946) Hydrography of the Monterey Bay, California Thermal
Conditions, Part 2 (1934-1937). Proceedings of the American Philosophical Society, 90,