Effects of the 1997–1999 El Niño and La Niña events on zooplankton abundance and euphausiid community composition within the Monterey Bay coastal upwelling system
Introduction
There is wide consensus that marine ecosystems in the northeast Pacific respond strongly to acute climatic events such as El Niño Southern Oscillations (ENSO) (Barber and Chavez, 1983, Chelton, Bernal and McGowan, 1982, Lenarz, Ven Tresca, Graham, Schwing and Chavez, 1995, McGowan, Cayan and Dorman, 1998, Wooster and Fluharty, 1985). Within the nearshore pelagic ecosystem off Central California, El Niño events are characterized by increases in ocean temperature and sea level (Lenarz, Ven Tresca, Graham, Schwing and Chavez, 1995, Ramp, McClean, Collins, Semtner and Hays, 1997), enhanced onshore and poleward flow (Emery, Royer, & Reynolds, 1985), a deeper mixed layer, deeper nutricline, negative anomalies in coastal upwelling (Chavez, 1996, Hayward, Mantyla, Lynn, Zmith and Chereskin, 1994), decreases in primary productivity (Chavez, 1996, Lenarz, Ven Tresca, Graham, Schwing and Chavez, 1995), and decreased zooplankton abundance (Lenarz, Ven Tresca, Graham, Schwing and Chavez, 1995, Smith, 1985).
Considerable debate exists as to whether El Niño-related impacts off the west coast of North America result as a consequence of remotely forced oceanographic mechanisms linked to the eastern tropical Pacific or from locally forced mechanisms associated with atmospheric phenomena. Several studies suggest sea level and ocean temperature anomalies can be linked to coastally trapped Kelvin waves generated in the equatorial Pacific (Chavez, 1996, Huyer and Smith, 1994, Norton and McLain, 1994). Alternatively, there is evidence that warming in the northeast Pacific associated with El Niño events can result from either changes in local atmospheric pressure systems (Simpson, 1985) or from a combination of remotely and locally forced mechanisms (Huyer and Smith, 1994, Norton and McLain, 1994). McGowan (1984) suggested that the community structure of zooplankton could be used to investigate whether El Niño impacts off California resulted as a consequence of poleward flow, onshore flow, or in situ mechanisms.
Monterey Bay, located on the central California coast, is a large, non-estuarine embayment with unrestricted access to the open ocean. It is further distinguished by the presence of a large, submarine canyon that runs along the central axis of the bay (Shepard, 1973). Strong northwest winds drive seasonal upwelling in the spring and early summer, which results in a cool, nutrient rich band of water that supports high levels of primary productivity, zooplankton, and higher trophic levels (Pennington & Chavez, 2000). This highly productive zone is typically tens of kilometers wide during the peak of the upwelling season and collapses coastward in the late summer and falls as a consequence of weakening winds and reduced upwelling (Abbot and Barksdale, 1991, Chavez, 1996, Olivieri and Chavez, 2000, Pennington and Chavez, 2000). Southwesterly winds associated with winter storms cause upwelling to cease in the winter and the water column becomes well mixed. Three distinct marine climates have been described for the Monterey Bay region based on these physical/chemical oceanographic characteristics: a spring/summer upwelling period, a fall oceanic or ‘relaxation’ period, and a winter or ‘Davidson’ period (Bolin and Abbott, 1963, Pennington and Chavez, 2000, Rosenfeld, Schwing, Garfield and Tracy, 1994).
Long-term observations within the Monterey Bay (Chavez, Pennington, Herlein, Jannasch, Thurmond and Friedrich, 1997, Olivieri and Chavez, 2000, Pennington and Chavez, 2000) have allowed researchers to assess the impacts of El Niño events on physical oceanography and primary productivity within this region. However, there is a lack of comparable studies for zooplankton and higher trophic levels. Limited data suggest that seasonal fluctuations in zooplankton abundance lag several months behind productivity cycles associated with the distinct marine climates within Monterey Bay. In typical years, maximum abundances of zooplankton occur in the summer and early fall (Barham, 1957, Bauduini, 1997, Silver and Davoll, 1975, Silver and Davoll, 1976, Silver and Davoll, 1977). Lenarz et al. (1995) and Bauduini (1997) reported lower than normal zooplankton abundance off central California during the 1992–1993 El Niño, but they were unable to attribute these as El Niño-related effects because of a lack of suitable climatologies. Even less is known about euphausiid population dynamics within the Monterey Bay region. To date there has been only one seasonal investigation on this subject (Barham, 1957) and no information is available during El Niño events.
In this article, we present seasonal data on zooplankton abundance and krill population dynamics from the nearshore pelagic ecosystem off Monterey Bay, California, for 1997–1999. This time frame is of particular interest as it includes two acute yet distinctively different climatic events: the 1997/1998 El Niño and the 1999 La Niña. We use these data to examine whether changes in zooplankton community structure are consistent with remote or locally forced physical oceanographic mechanisms.
Section snippets
Study area
The Monterey Bay study area comprised a grid of seven transect lines spaced 3 nautical miles apart and covering an area of approximately 909 km2 (Fig. 1). Transect lines began inshore at the 30 fathom curve (55 m) and extended WNW to 122.08°W. Transects ranged in length from 10 km (5.4 nmi) to 22 km (11.9 nmi). Monthly oceanographic surveys were conducted within this grid monthly from May to November in 1997–1999. Two supplementary surveys were conducted in March and April of 1998 in order to
Physical oceanography
Mean sea surface temperature (SST) values were near the long-term average for the May–July 1997 surveys. Temperatures increased from 13.8 to 17.0 °C between July and August 1997 and remained anomalously warm through November 1997 (Fig. 2). Temperatures gradually declined during the winter months, but remained warmer than normal during the spring of 1998. SST decreased dramatically in July 1998 in conjunction with an increase in coastal upwelling but increased again in the late summer/early
El Niño effects on zooplankton abundance
Summertime peaks in zooplankton abundance have been reported in Monterey Bay by a number of researchers, who have examined zooplankton abundance over limited spatial and temporal scales (Barham, 1957, Bauduini, 1997, Silver and Davoll, 1975, Silver and Davoll, 1976, Silver and Davoll, 1977). Our seasonal measurements of zooplankton abundance within the Monterey Bay between 1997 and 1999 (Fig. 3) confirm these observations of peak abundances occurring in the summer and early fall, i.e. several
Summary
El Niño effects on the zooplankton community within the Monterey Bay, California, were first manifested in a dramatic decline in zooplankton abundance and shift in the euphausiid species composition during the summer of 1997. The rapid appearance of southern fauna, particularly the neritic euphausiid N. simplex, suggests that these biological effects were caused, at least in part, by remotely forced oceanographic processes. Zooplankton abundance and community composition gradually returned to a
Acknowledgements
We thank the crews of the R/V John Martin, R/V David Johnston, personnel at Moss Landing Marine Laboratory’s Ship Operations, and the numerous field and laboratory volunteers who assisted in the collection and analysis of data. Two anonymous reviewers provided valuable comments during the preparation of this manuscript. Research was supported by Office of Naval Research Grants N00014-95-10646 and N00014-99-10192 to D.C., National Oceanic and Atmospheric Administration contract 40ABNF600916 to
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