A ten-year time series from Monterey Bay, California: Seasonal, interannual and long-term patterns

F. P. Chavez, R. P. Michisaki, G. E. Friederich, J. T. Pennington, B. Schlining, C. Fayos, P. Walz, C. Sakamoto, R. Hopcroft, R. Kudela, C. Castro, E. Mauri, and K. R. Buck

Figure 1.  An AVHRR image of sea surface temperature (SST) overlaid with the Monterey Bay coastline and  locations of the major stations.  This view is from April 1995 and shows a plume of cold   upwelled water (blue and pink regions) orignating north of Santa Cruz and  travelling south across the mouth of Monterey Bay.   Upwelling usually occurs in the spring.  Click on the image for an enlarged view (54k).

Introduction

The coastal ocean off western North America has received considerable oceanographic study because of its fisheries and proximity to large human populations. The oceanography of this region is strongly influenced by the process of coastal upwelling. In general, coastal upwelling occurs along eastern ocean margins when equatorward winds act in combination with the Coriolis force to move surface waters offshore, drawing deeper water to the surface. This ‘upwelled’ water occurs as a cool band along the coast, typically 10’s of km broad, separated from warmer offshore waters by a variable series of fronts, plumes and eddies which can extend >100 km offshore. Upwelled water is nutrient-rich, and supports high levels of phytoplankton and higher trophic level. In the northeast Pacific, coastal upwelling occurs seasonally.  

Monterey Bay, the site of the present study (herein ‘MB’), is a deep (>1000 m), non-estuarine embayment in central California broadly open to the coastal ocean (Fig. 1). Its oceanography has received considerable study (physical oceanography reviewed by Breaker and Broenkow, 1994). Skogsberg (1936) and Skogsberg and Phelps (1946) defined three oceanographic periods still in common use: (1) a spring/summer ‘upwelling season’; (2) a summer/fall ‘oceanic season’; and (3) a winter ‘Davidson Current season’, characterized by a low thermal gradient water column to 100 m. In 1989 the Monterey Bay Aquarium Research Institute (MBARI) began an intensive study of the coastal upwelling system of central California (Chavez 1996) incorporating biweekly to monthly ship expeditions together with continuous observations from strategically placed moored platforms (Chavez et al., 1997). The purpose of this presentation is to describe the seasonal, interannual and long-term trends in environmental (winds, currents, temperature, salinity), chemical (nutrients) and biological (chlorophyll, primary production, phytoplankton taxonomic groups) for MB. In addition AVHRR and SeaWiFS images are included to provide spatial context.

Aboard our research vessel the R/V Point Lobos, time series measurements have been collected on a continuing series of twice monthly day cruises since 1989, switching to monthly cruises in 1994. From 1989 to 1998 a total of 206 cruises have been completed, collecting samples from over 600 stations.   Three major station are routinely sampled: inshore station C1, mid-bay station H3 (a California Cooperative of Fisheries, CalCOFI, station) later changed to the Mooring 1 site, and an outer bay station at Mooring 2 (see Figure 1).  Several other stations were sampled including minor coastal station 'H1', outer bay station 'C7', and sections of CalCOFI Line 67. Augmenting this time series are data from the "Shift-Up", "Coastal Ocean Processes", "Pegasus", and "Studies of Ecological and Chemical RespoSses to Environmental Trends" (SECRET) projects, see Table 1 below.

Biological, chemical, and physical measurements were collected using a conductivity, temperature, and depth rosette (CTD), see Table 2.  The CTD utilized was either a Sea-Bird 19 or 911 Plus CTD mounted in a General Oceanics 12 bottle rosette with 5 liter Niskin bottles fitted with silicon O-rings.  Profiling data was collected on the downcast where conductivity, temperature and depth data were collected at 24 Hz and processed with modified Sea-Bird software that bins the data to 1 meter intervals.   Additionally the CTD has been outfitted with auxiliary sensors, a downwelling cosine Photosynthetically Active Radiance (PAR) (Licor QCP 200) and a SeaTech fluorometer and transmissometer.  Water samples were collected on the upcast at the nominal depths of 0m, 5m, 10m, 20m, 40m, 60m, 80m, 100m, 150m and 200m. 

Table 2. Samples collected from shipboard CTD for the time series study.

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 MBARI provides these data "as is", with no warranty, express or implied, of the data quality or consistency. Data are provided without support and without obligation on the part of the Monterey Bay Aquarium Research Institute to assist in its use, correction, modification, or enhancement.