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Productivity Cycles in the Monterey Bay Pelagic Ecosystem and La Niña - Boom or Bust?

Within the past three years, Monterey Bay has experienced two acute and distinctly different climatic events that have had far-reaching impacts on seasonal productivity within the offshore ecosystem. The 1997-98 El Niño event was the strongest recorded this century and caused profound declines in nutrient upwelling, phytoplankton growth, zooplankton productivity, and seabird and marine mammal abundance and reproduction (see Ecosystem Observations 1998, page 17).

This year La Niña conditions have prevailed in the Northeast Pacific. La Niña events have, in many ways, the opposite effects of El Niños. Typically, La Niñas are characterized by colder than normal ocean temperatures occurring along most of the West Coast of North America. This cold water is the result of intense wind-driven upwelling along the coast (the northwest winds during the spring of 1999 were some of the strongest and most persistent on record!). The La Niña event has provided us once again with a unique opportunity to gain insights into the ecology of the pelagic ecosystems during acute climatic events.

Upwelling and Phytoplankton Productivity
Strong northwest winds during the spring of 1999 resulted in extremely high levels of upwelling. Upwelling indices were up to two standard deviations above normal for the central California region. AVHRR (Advanced Very High Resolution Radiometers) satellite imagery from this period revealed numerous, long filaments of cold nutrient-rich water extending far offshore from headlands associated with coastal upwelling centers including Points Arena, Reyes, Año Nuevo, Sur, and Conception. Nutrient input associated with this intense upwelling resulted in a broad band of high phytoplankton productivity that extended up to 100 km offshore of Monterey Bay. Upwelling-favorable winds dramatically diminished in July and were largely absent throughout late summer and fall. In response, phytoplankton levels in Monterey Bay decreased markedly, and by late summer values were around the long-term mean for this time of year.

Monterey Bay Zooplankton Abundance 1997-99

Figure 1: Mean seasonal monthly zooplankton abundance (integrated backscatter, 200kHz) within Monterey Bay from 1997-1999.

Patterns of primary productivity within the central California region during 1998 were distinctly different to those described above. Upwelling and phytoplankton levels were anomalously low during the spring and early summer of 1998 when the pelagic ecosystem was still heavily under the influence of El Niño conditions. By July 1998 upwelling-favorable winds returned to central California and upwelling indices remained well above the long-term mean late into the fall.

Zooplankton Abundance
High levels of primary productivity in the spring of 1999 translated to high zooplankton abundance in early- and mid-summer. This followed the typical seasonal pattern of summer peaks in zooplankton biomass following spring peaks in phytoplankton productivity. Zooplankton levels during the summer of 1999 were the highest recorded during the past three years, and they remained high throughout the fall (Figure 1). In comparison, zooplankton levels were distinctly lower in the summer of 1998 but increased following the resurgence of upwelling in the late summer and fall.

Krill Abundance and Species Composition
High primary productivity levels in the spring of 1999 resulted in high levels of larval production and recruitment to krill populations within the central California region. In addition, krill -- particularly larvae and juveniles—were broadly distributed offshore, due in all likelihood to offshore water flow caused by strong levels of upwelling. Numerous surface swarms of large reproductive adults were observed within Monterey Bay on several occasions in April and May 1999, indicating ongoing spawning activity. These swarms were targeted by a number of predators including fish, seabirds, and even opportunistic gray whales en route to their feeding grounds in Alaska! This was in stark contrast to conditions in the spring of 1998, when krill abundance was conspicuously lower. Krill levels declined noticeably with the cessation of coastal upwelling in July 1999, however it is unclear whether this was due to declines in total abundance, dispersal to deeper water, or a combination of both.

Figure 2: Thysanoessa spinifera, a relatively large krill found in cooler coastal waters from Alaska to California. (©Baldo Marinovic)

A major shift in the species composition of the krill community in Monterey Bay also occurred between 1998 and 1999. Euphausia pacifica typically dominates the krill community year round in Monterey Bay; however, in the early part of 1998 the southern species Nyctiphanes simplex comprised a substantial portion of the community and the cool temperate Thysanoessa spinifera (Figure 2) was virtually absent. By late 1998, with the onset of coastal upwelling, the species composition gradually shifted back to a more typical mix of E. pacifica and T. spinifera. This pattern continued into 1999, with high levels of recruitment for T. spinifera observed both in the spring and fall.

What have we learned?
Both the 1997-98 El Niño and the current La Niña events have provided us with unique insights into ecological processes within the Monterey Bay pelagic ecosystem. Productivity at multiple levels demonstrated predictable responses to both these events, declining dramatically in 1997-98 and reaching high levels in late 1998 and 1999. There were, however, unexpected patterns observed during both events as well. In 1998 the return of upwelling late in the season resulted in dense krill aggregations within Monterey Bay that attracted record numbers of whales. In contrast, the fall of 1999 was characterized by relatively low levels of coastal upwelling. As a result, krill aggregations were more diffuse and foraging whales more scarce. One implication of these findings is that it appears that episodic periods of upwelling that persist into the fall are necessary to maintain dense aggregations of krill within Monterey Bay, even during years when overall (and especially springtime) productivity is high. These findings highlight the value of acute climatic events in providing insights into ecological processes as well as the importance of maintaining ongoing ecosystem monitoring within the pelagic waters of the Sanctuary.

Baldo Marinovic1, Donald A. Croll1, Francisco Chavez2,
and Scott R. Benson3

1 University of California Santa Cruz
2 Monterey Bay Aquarium Research Institute
3 Moss Landing Marine Laboratories


Contrasting Effects of La Niña and El Niño on Recruitment of Juvenile Rockfishes

Understanding how and why populations and communities change over time is one of the greatest challenges to marine ecologists and fisheries biologists alike. Such knowledge is critical to distinguishing population responses to natural environmental fluctuations from those caused by human impacts. Critical to this goal is recognizing how life history traits influence how populations respond to environmental variation. This is made very clear by observing how populations responded to the recent dramatic environmental changes experienced between the El Niño of 1998 followed abruptly by the La Niña of 1999.

Figure 1: Changes in recruitment of young-of-year rockfish between El Niño and La Niña years. "Kelp complex" rockfish include kelp, gopher, and black and yellow rockfish. "Mid-water aggregating" rockfish include olive, yellowtail, and black rockfish. Error bars represent one standard error.

One of the most important life history traits of most marine organisms is the great distance that young can be dispersed from the adults that produce them. Because the eggs and larvae of most reef fishes and invertebrates (barnacles, mussels, crabs, sea urchins, and the like) are carried great distances by water currents, reproduction by a population of adults has little direct bearing on how many young will replenish that population that year. Instead, local populations are reliant on the delivery of young that are produced by adults elsewhere. This delivery of young to a population can vary markedly from year to year, greatly influencing the number of juveniles and, eventually, adults in a population. Ecologists refer to this replenishment of populations by young from the plankton as "recruitment" and know now that it is critical to understanding year-to-year variation in population size.

Figure 2: Changes in recruitment of young-of-year kelp, gopher, and black and yellow rockfish to experimental kelp plots between La Niña and El Niño years.

Surveys of rockfish recruitment during the El Niño conditions of 1998 and the La Niña of 1999 demonstrate the dramatic effects of such large-scale punctuated events on reef fish populations and communities. We surveyed rockfish recruitment visually by counting young (< 8 cm long) rockfish in kelp beds along the coast of Monterey during the summers of 1998 and 1999. We also created plots of giant kelp plants and monitored recruitment to these controlled habitats from one year to the next. During El Niño conditions, shallow dwelling rockfishes such as gopher, kelp, and black and yellow rockfish recruited in great numbers (Figure 1). At the same time, recruitment of juvenile blue, black, olive, yellowtail, and boccacio rockfish was poor. However, the pattern was completely reversed the following year under La Niña conditions (Figure 1). The marked change in recruitment of the kelp-associated species to kelp plots indicates that such changes are not based on changes in the amount of kelp (Figure 2). In fact, there was much more kelp during La Niña conditions. These observations indicate not only how these climatic events contribute to year-to-year changes in replenishment of reef fish populations, but also how species respond very differently to such events.

Similar patterns have been reported for other areas along the central coast by biologists with the California Department of Fish and Game and the National Marine Fisheries Service. Such observations, along with data on distribution of rockfish larvae, suggest that differences in patterns of larval dispersal and timing of spawning, coupled with differences in the timing and strength of coastal upwelling, may be responsible for the strong differences in how species respond to such climatic events. Such implications highlight the importance of relationships between life histories and oceanographic features in understanding the dynamics of reef fish populations and communities.

--Mark Carr and Arnold Ammann
Department of Ecology and Evolutionary Biology,
University of California Santa Cruz

Long-Term Variations of the Northeast Pacific

The Monterey Bay National Marine Sanctuary is located on the eastern edge of the northeast Pacific (NEP), a region that experiences large variations of the atmosphere and ocean that can strongly affect environmental conditions in the Sanctuary. These NEP variations are part of atmosphere-ocean fluctuations that occur throughout the entire north Pacific. Often, these fluctuations can be traced back to disturbances that occurred in southern and eastern Asia, the tropical Pacific, or even more remote regions. The best known of these long-term variations are the normal seasonal changes in winds, sea surface temperatures (SSTs), precipitation, runoff, and other physical factors. Seasonal variations in the NEP are part of the worldwide seasonal cycle, a cycle that is especially pronounced in southern and eastern Asia and in the western tropical Pacific.

Figure 1: Sea surface temperature (SST) anomalies for the Pacific Ocean, 1991-1997. Positive (shaded) values indicate SSTs that are warmer than average; negative values indicate the opposite. The average is for the 29-year period, 1968-1996. Contour interval is 0.2 degrees C.

These seasonal variations can vary themselves, leading to what are called climate anomalies, the best known of which are El Niño and La Niña events. The most recent of these are the 1997–1998 El Niño and the 1999 La Niña. Such events occur primarily in the tropical Pacific but have global impacts. During these events, a number of environmental factors in the NEP deviate from their seasonal norms. For example, during El Niño events, SSTs tend to be anomalously warm in a broad swath extending along the coast of western North America, from the Aleutians to Baja California, while SSTs in the central North Pacific are unusually cool. A reverse pattern of SST anomalies tends to occur during La Niña events.

El Niño and La Niña events tend to last about a year and recur about every two to seven years. Ecologically important climate variations also occur on much longer time scales. In particular, the east Asian—North Pacific region undergoes variations that last on the order of ten to twenty years. These variations, often referred to as Pacific decadal oscillations, can have major impacts on oceanic and atmospheric conditions and, thereby, large impacts on marine and terrestrial organisms. Figure 1 shows the average SST anomalies in 1991 -- 1997, during the phase of one of these decadal oscillations in which SSTs in the NEP and much of the tropical Pacific are unusually warm, while the central North Pacific is unusually cool. This is similar to the SST anomaly patterns seen during El Niño events. During the opposite phase, the SST anomalies tend to be the reverse of those in Figure 1, and similar to those during La Niña events. There are strong links between decadal variations and El Niño and La Niña events, with a large number of El Niño events occurring during the phase of decadal events shown in Figure 1, and a large number of La Niña events during the opposite phase. Scientists are studying these links and the mechanisms by which a variety of climate anomalies affect the NEP, especially coastal regions such as the Sanctuary.

There are similarities in the response of coastal marine ecosystems to El Niño and longer-term warming because the anomalous physical conditions associated with these phenomena are similar (e.g., warmer SSTs, deeper mixed layer, shifts in ocean currents). While many of the unusual biological events in 1998 may be due to the 1997—1998 El Niño, many others are related to longer-term climate change trends, compounded by El Niño's effects. An improved ability to understand and predict physical conditions and local environmental responses to climate change will increase our ability to anticipate and mitigate changes to marine populations in the Sanctuary.

--Tom Murphree
Naval Postgraduate School

--Frank Schwing
Pacific Fisheries Environmental Laboratory

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Last modified on: March 31, 2000