Download Long-term change and stability in the California Current

ARTICLE IN PRESS
Deep-Sea Research II 50 (2003) 2583–2594
Long-term change and stability in the California Current
System: lessons from CalCOFI and other long-term data sets
Ginger A. Rebstock*
Department of Biology, University of Washington, Box 351800, Seattle, WA 98195-1800, USA
Received 8 March 2002; received in revised form 15 August 2002; accepted 1 November 2002
Abstract
The California Current System (CCS) is a highly variable system, both spatially and temporally, that is strongly
affected by low-frequency climatic fluctuations. This paper reviews evidence for long-term (decadal-scale) change in the
physics and biology of the CCS over the last 50–100 years, as well as evidence for stability in planktonic community
structure and long-term persistence of populations. Increases in water temperature, thermocline depth and stratification
in the CCS have been accompanied by changes in populations of kelp, diatoms, foraminifera, radiolarians, intertidal
invertebrates, zooplankton, fish and seabirds. However, there is also evidence for stability in assemblages of larval fish,
calanoid copepods and radiolarians. Statistical averaging (the portfolio effect) may explain some aspects of stability in
assemblages. Advection of planktonic populations may account for rapid recovery of biomass and dominance structure
following perturbations such as strong El Niño events. Planktonic populations in the CCS may be adapted to largescale biotic and abiotic variability, through a combination of advection of populations and life history traits.
Several lessons may be learned from the California Cooperative Oceanic Fisheries Investigations and other long-term
data sets: (1) long time series are needed to understand the dynamics of the ecosystem; (2) life histories are important
determinants of species responses to environmental forcing, even in the plankton; and (3) the CCS is simultaneously
variable and stable, and these properties are not necessarily in conflict.
r 2003 Elsevier Ltd. All rights reserved.
1. Introduction
The California Current System (CCS) forms the
eastern boundary of the North Pacific subtropical
gyre. As in other eastern boundary current
systems, the euphotic zone is enriched by winddriven upwelling as well as isopycnal shoaling due
to geostrophic adjustment (Chelton et al., 1982).
This enrichment of surface waters makes the CCS
*Tel.: +1-206-616-4054; fax: +1-206-221-7839.
E-mail address: [email protected] (G.A. Rebstock).
a highly productive system, supporting diverse
marine life and major fisheries.
The CCS is highly variable both spatial and
temporally. Mesoscale features such as fronts,
eddies and jets produce steep gradients in physical
and biological properties. Time scales of variability include events lasting days to weeks, such as
passage of storms or wind-driven upwelling, as
well as seasonal, interannual, interdecadal and
longer periods.
The CCS is one of the best-studied oceanic areas
in the world. The California Cooperative Oceanic
Fisheries Investigations (CalCOFI) program
0967-0645/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0967-0645(03)00124-3
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G.A. Rebstock / Deep-Sea Research II 50 (2003) 2583–2594
began monitoring physical and biological properties of the CCS in 1949, in response to the collapse
of the sardine fishery in California (Hewitt, 1988).
The program continues and has expanded the
number of properties measured over the years.
Marine research institutions along the west coast
of North America have also advanced our knowledge of the CCS.
Several climatic phenomena influence the CCS
on interannual scales. The El Niño–Southern
Oscillation (ENSO) has profound, if generally
short-lived effects on the CCS (McGowan and
Walker, 1993; but see Rebstock, 2002). During the
El Niño or warm phase of the ENSO, equatorward
transport is diminished, poleward transport in the
counter-current increases, the thermocline deepens, primary production is reduced, the timing
of the spring bloom may be altered, zooplankton
biomass declines, and range boundaries of both
planktonic and nektonic species shift (Lenarz et al.,
1995; Mullin, 1998; Murphree and Reynolds,
1995; Roesler and Chelton, 1987; Smith, 1985).
The La Niña or cold phase of the ENSO generally
has opposite effects on the CCS, although La Niña
has not been as well studied as El Niño (Murphree
and Reynolds, 1995).
The ENSO is primarily a tropical phenomenon,
and not all tropical El Niño events affect the CCS.
Conversely, not all warm or cold events in the CCS
are associated with a tropical ENSO event
(Simpson, 1984). The strength and position of
the Aleutian Low atmospheric pressure system
also affects the CCS by altering the frequency,
intensity and tracks of cyclonic storms in the
North Pacific. Effects of a strong Aleutian Low are
similar to those of a tropical El Niño (Norton and
McLain, 1994).
In addition, local heating can affect water
temperature and thermocline depth (McLain
et al., 1985). Climatic phenomena that influence
the CCS, including ENSO, the Aleutian Low and
local heating, are not independent of each other,
and their effects can be difficult to differentiate.
Decadal-scale climatic variability also strongly
influences the CCS. Periods of a decade or longer
during which climatic and oceanographic variables
fluctuate around some mean value have been
termed climatic regimes, while more or less abrupt
changes from one mean value to another are
known as regime shifts (Isaacs, 1976; MacCall,
1996). A well-documented regime shift occurred in
the North Pacific in the winter of 1976–77
(Ebbesmeyer et al., 1991; Graham, 1994). Other
North Pacific regime shifts have been hypothesized
around 1910 (Lluch-Belda et al., 2001), 1925, 1947
and 1989 (Beamish et al., 1999; Overland et al.,
1999). Lluch-Belda et al. (2001) suggested that
climatic regimes are not as prevalent or important
to the ecosystem as warming or cooling periods,
which may last decades.
Coherent, decadal-scale fluctuations have been
identified in the dominant patterns (leading
principal components) of winter (November—
March) North Pacific sea-surface temperature
and sea-level pressure. The variations in SST are
known as the Pacific Decadal Oscillation or PDO
(Mantua et al., 1997). Since the 1920s, the state of
the PDO has reversed every two to three decades.
A positive PDO is associated with an intense
Aleutian Low, cool waters in the central North
Pacific and warm waters in the CCS.
Interannual and decadal-scale variability are
illustrated in Fig. 1, a time series of water
temperature measured at the Scripps pier in La
Jolla, California, USA. Interannual variations are
obvious, including strong El Niño signals in the
Fig. 1. Surface temperature at Scripps Institution of Oceanography pier, La Jolla, California, USA, 1916–96. Anomalies
( C) from the long-term mean for each month. Data provided
by the Shore Station Program, Marine Life Research Group,
SIO.
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late 1950s and the early 1980s. However, the later
El Niño events do not stand out as clearly as the
1958–59 event, because they occurred during a
warm-water regime (positive PDO), while the
1958–59 event occurred during a period of
relatively cold water (negative PDO). Three
decadal-scale regimes are apparent in the time
series (see MacCall, 1996). Prior to the mid-1940s,
the temperature was highly variable, but centered
near the long-term mean. The period from the late
1940s through the early 1970s was characterized
by cool water, interrupted by the 1958–59 El Niño.
The 1976–77 regime shift is evidenced by a sudden
increase in water temperature, beginning a warm
regime.
In spite of physical and biological variability on
many time scales, populations and some assemblages persist in the CCS for long periods,
suggesting some kind of continuity or stability. A
stable population, assemblage or system is one
that remains at or returns to equilibrium when a
disturbing force is applied. This equilibrium does
not necessarily imply a constant value, but often
involves fluctuations within limits. A persistent
population is one that does not go extinct during
some period (Connell and Sousa, 1983). Disturbance is often equated with aperiodic or episodic
environmental variability (Floder and Sommer,
1999; McGowan, 1990). A perturbation is defined
as a response of a population or community to a
disturbance (Connell and Sousa, 1983). As seen in
Fig. 1, large potential disturbances are frequent in
the CCS and sometimes last for several years.
Stability can be achieved either by resistance to
potentially disturbing forces or by recovery
following responses to disturbances (adjustment
stability). Stochastic boundedness is another concept that may be applicable to the CCS. Population levels fluctuate within bounds; they neither go
extinct nor return to the same values (Connell and
Sousa, 1983).
In this paper, I review evidence for long-term
change in the CCS, in both physical properties and
biological populations, over the last 50–100 years.
These changes may be manifestations of decadal
or longer-scale fluctuations or of anthropogenic
global change. Some population fluctuations
caused by direct human persecution of species,
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such as pinnipeds and sea otters (MacCall, 1986),
are not discussed. In spite of large fluctuations in
the environment and in many populations, there is
evidence for stability at the community or
assemblage level in some cases. I also discuss this
evidence and attempt to reconcile this observed
stability and variability.
2. Long-term change in the California Current
System
Decadal-scale oscillations or trends in climate
are well known in the North Pacific (Latif and
Barnett, 1994; Mantua et al., 1997; McGowan
et al., 1998; Ware and Thomson, 2000). Climatic
regime shifts occurred in the winter of 1976–77
(Ebbesmeyer et al., 1991), 1989 (Hare and
Mantua, 2000) and around 1925 and 1947
(Beamish et al., 1999; Overland et al., 1999).
ENSO events, which strongly influence the CCS
(McGowan, 1985; Murphree and Reynolds, 1995)
have increased in frequency since the mid-1970s
(Trenberth and Hoar, 1997). Seasonal upwellingfavorable winds along the California coast have
intensified (Bakun, 1990; Schwing and Mendelssohn, 1997). The frequency and intensity of winter
cyclones, as well as extreme surface winds and
wave heights have increased in the North Pacific
over the last 50 years (Graham and Diaz, 2001).
Hydrographic conditions in the CCS have
changed, apparently in response to climatic regime
shifts. Upper ocean temperature increased off
southern California between the 1950s and the
1990s (Roemmich, 1992; Roemmich and McGowan, 1995a). This surface-intensified warming has
been accompanied by an increase in thermocline
depth and water-column stratification (McGowan
et al., 2003; Miller, 1996). Weinheimer et al. (1999)
presented evidence from microfossils of increasing
thermocline depth and stratification in the 1940s
and in 1960. Increased stratification and thermocline depth tend to counteract the enriching effect
of the increase in upwelling that might be expected
due to changes in wind intensity. Pennington and
Chavez (2000) also found higher temperatures and
stronger stratification in the upper 100 m in fall
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and winter in Monterey Bay, in addition to a delay
in the onset of upwelling in spring.
Biological populations spanning several trophic
levels and representing many different taxa have
also undergone long-term change in the CCS.
Evidence exists for change in populations of kelp,
diatoms, foraminifera, radiolarians, intertidal invertebrates, zooplankton, fish and seabirds.
Much has been published on the Pacific sardine
(Sardinops sagax) since the catastrophic fishery
collapse of the 1940s that spawned the CalCOFI
program. Sediment cores from an anoxic basin off
southern California contain evidence that the
Pacific sardine as well as the northern anchovy
(Engraulis mordax) has undergone interdecadal
fluctuations in abundance for at least the last 1700
years (Baumgartner et al., 1992; Soutar and Isaacs,
1969). Sardine and anchovy stocks in the CCS and
other parts of the world have undergone similar
fluctuations in historical times (e.g., Lluch-Belda
et al., 1992).
In addition to sardine and anchovy, many other
fish stocks show decadal-scale population fluctuations in the CCS. Other pelagic fishes such as hake
(Merluccius productus), mackerel (Scomber japonicus) and jack mackerel (Trachurus symmetricus) as
well as large predatory fishes such as tuna
(Thunnus spp.) and marlin (Tetrapturus audax)
have undergone long-term change in abundance or
seasonal timing of migration (MacCall, 1996).
Salmon (Onchorhynchus spp.) catches in the northeastern Pacific appear to fluctuate with the PDO.
Positive states of the PDO are associated with low
catches of salmon in the CCS, but high catches in
Alaska (Mantua et al., 1997). Declines in abundance of nearshore reef fishes in the Southern
California Bight have been accompanied by shifts
in dominance from northern to southern species
(Holbrook et al., 1997). Off southern California,
the distribution of larvae of a number of species
shifted between 1951–76 and 1977–98. In particular, several species of myctophids, midwater fish
that are not commercially exploited, expanded
their larval distributions to the north and/or
inshore after the mid-1970s (Moser et al., 2001).
The fluxes of planktonic diatoms (Lange et al.,
1990), foraminifera (Lange et al., 1990; Weinheimer et al., 1999), and radiolarians (Weinheimer
and Cayan, 1997) to the sediments of the Santa
Barbara Basin, off southern California, have
varied over the last 50–100 years. The diatom flux
declined sharply in the early 1970s, while the
foraminiferan flux has declined more gradually
since the 1950s, accompanied by a shift in species
composition around 1970 (Lange et al., 1990). The
percentage of deep-dwelling radiolarians decreased
over the last century, with a sharp decline in the
early 1940s (Weinheimer et al., 1999). Total
radiolarian flux was high in the late 1800s, low in
the early 1900s and high since the 1950s. Fluctuations were coherent among several biogeographic
groups, suggesting changes in the carrying capacity of the system. Although radiolarian assemblages appear to be generally stable, decreases in
rank correlation among years and increases in
diversity occurred around 1955 and 1976–77
(Weinheimer and Cayan, 1997).
Barry et al. (1995) presented evidence for a shift
in community composition in a rocky intertidal
area in Monterey Bay between the early 1930s and
the early 1990s. Invertebrate species with a southern biogeographic affinity tended to increase in
abundance, while northern species tended to
decrease. Cosmopolitan species did not show a
clear pattern, with some species increasing and
others decreasing. Water temperature at the site,
particularly the average summer maximum, increased over the same period.
Veit et al. (1996) reported a decline in overall
seabird numbers over 8 years off southern
California, although some species increased in
abundance. The decline was led by a 90% decrease
in numbers of sooty shearwaters (Puffinus griseus),
the numerically dominant species in the CCS.
Sydeman et al. (2001) reported species-specific
changes in reproductive performance, a more
sensitive indicator of environmental change than
population size, of seabirds breeding on islands off
California between 1969 and 1997. They also
found changes in diet composition of several
species, especial a decline of juvenile rockfish
(Sebastes spp.) around 1989.
Kelp (Macrocystis pyrifera) forests off San
Diego have undergone large interannual to interdecadal change in canopy extent and plant size,
although it is more difficult to separate effects of
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climate change from those of direct human
disturbance such as pollution and dredging in this
nearshore environment (Tegner et al., 1996).
Median and maximum plant size, but not plant
density declined between the 1950s and the 1990s.
Macrozooplankton biomass undergoes large
changes on interannual to interdecadal scales
(e.g., Bernal, 1981; Chelton et al., 1982). These
low frequencies (periods greater than 1 year)
account for a large portion of the variability in
zooplankton biomass in the CCS (Bernal, 1981).
Off southern California, zooplankton displacement volume, an index of biomass, has declined by
70% since the 1950s (Roemmich and McGowan,
1995a, b).
Off Baja California, the zooplankton volume
has also undergone large interannual and interdecadal fluctuations (Lavaniegos et al., 1998).
Although the large, long-term decline seen off
southern California was not obvious, perhaps
because of the paucity of samples since about
1970, zooplankton volumes in the 1990s were the
lowest on record. Upwelling-favorable winds
increased in the region, but without an increase
in zooplankton standing stocks.
Species-level analyses using CalCOFI samples
have been completed for some groups of zooplankton and are underway for others. Hyperiid
amphipods and presumably their gelatinous hosts
have declined in abundance and species richness
off southern California since the mid-1970s
(Lavaniegos and Ohman, 1999).
I found evidence for species-specific responses to
climatic conditions in calanoid copepods off
southern California (Rebstock, 2002). Between
1951 and 1999, 19 out of 24 species analyzed
showed abrupt step changes or more gradual
monotonic trends in density. Most of the step
changes occurred around 1977 and 1990, coincident with climatic regime shifts. Rare warmwater species as a group increased in frequency of
occurrence in samples after 1977. Other step
changes occurred in the late 1950s and the early
1980s, probably triggered by major El Niño
events. The changes are not easily explained by
simple advection of copepod assemblages. For
example, the seven species that abruptly increased
in density in the mid-1970s (no species declined at
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this time) include warm-water species as well as
cool-water Transition Zone species (Rebstock,
2002). Differences in life history traits in the two
dominant species of calanoid copepods off southern California may explain their different responses to climatic forcing.
Several perturbations occurred in the species
composition of the calanoid copepods between
1951 and 1999, in which dominance structure was
temporarily altered. Most of these perturbations
were associated with warm-water events in the
CCS. Five of the six perturbations have occurred
since the mid-1970s (Rebstock, 2001), perhaps a
response to an increased frequency in El Niño
events.
3. Stability in the California Current System
Little has been published on biological stability
in the CCS and other eastern boundary currents,
either because few populations or communities
appear stable, or because few researchers have
looked for stability. Although individual species
undergo large fluctuations in abundance in the
CCS, there is evidence for stability at the
assemblage or community level. Recurrent groups
of larval fish were found to persist over decades
(Moser and Smith, 1993). In spite of large
fluctuations in abundance, patterns of association
of species with environmental conditions have
remained constant.
The dominance structure of calanoid copepods
in spring off southern California has also remained
stable over the last 50 years (Rebstock, 2001). The
rare species have remained rare and the dominant
species have remained dominant. Perturbations to
this dominance structure occurred in 6 years
between 1951 and 1999, but in each case, the
usual structure recovered within 1 or 2 years.
In spite of step changes and monotonic trends in
density of many species of calanoid copepods, the
density of the total calanoid assemblage showed
no long-term trends between 1951 and 1999
(Rebstock, 2002). For total calanoid copepods,
interannual fluctuations seem to be more important than decadal-scale fluctuations.
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Weinheimer and Cayan (1997) noted that the
radiolarian community structure in the Southern
California Bight has remained stable over the last
100 years. Although decreases in rank correlation
between years were seen in two periods, most rank
correlations were significant. Weinheimer and
Cayan concluded that no changes in source water
of the radiolarian assemblage had occurred during
the period of record.
Many populations in the CCS, in spite of
undergoing large fluctuations in abundance on
interannual and interdecadal scales, show longterm persistence. These populations respond to
changes in their physical and biotic environment
with large departures from their mean abundances, but do not go extinct. Small pelagic fish
generally show this persistence, in spite of the
additional disturbance of overfishing (Beverton,
1990). Biomass of the Pacific sardine off California, for example, was reduced between 2 and 3
orders of magnitude and remained at virtually
undetectable levels for over 20 years (Beverton,
1990), but the stock is now considered fully
recovered (California Department of Fish and
Game, 1999). These populations may best be
described as stochastically bounded (Connell and
Sousa, 1983).
Much of the discussion of any kind of stability
in eastern boundary currents comes from the
literature on small pelagic fishes. Walsh (1978)
suggested that pelagic populations or communities
shift location in response to climatic change and
our perception of variability is at least partly due
to our Eulerian rather than Lagrangian perspective. However, he noted that some populations,
such as guano birds in Peru, suffer declines during
El Niños because they are unable to migrate with
their prey. Mathisen (1989) hypothesized that
variations in timing of spawning of small pelagic
fish help buffer the populations against environmental perturbations.
Bakun (2001) noted that physical variability in
the oceans is red-shifted, that is, variability is
higher at lower frequencies. Marine populations
must be able to track these low-frequency variations or go extinct. He argued that serial spawning
in different environments by small pelagic fish
results in different generations being adapted to
different conditions. Some of these generations are
likely to be adapted to new conditions in the event
of long-term environmental change. Populations
need to track long-term changes but not interannual variability such as ENSO events. Adapting
to short-term events would leave the population
poorly adapted to more average conditions. This
implies that interannual fluctuations in populations may be part of an evolutionary strategy to
cope with large long-term environmental change.
Bakun suggested that the presence of several
reproductive age classes in a population and the
greater fecundity of older classes act as a low-pass
filter for environmental variability. Cury et al.
(2000) also contended that multiple reproductive
age classes buffer populations of higher trophic
level predators from high-frequency variability. A
weak cohort has little effect on the population if
many other age classes are also breeding.
4. Reconciling stability and change
It has long been known that communities may
be more stable than the individual species making
up those communities. On theoretical grounds,
May (1974) noted ‘‘These models illustrate very
clearly how instability in the populations of
individual species may be found alongside stability
in the total population of the overall community’’.
Statistical averaging (the portfolio effect) may
account for the higher stability of assemblages
compared to individual species, if the species are
not concordant in their fluctuations (Lehman and
Tilman, 2000). This portfolio effect may explain
the lack of a long-term trend in total calanoid
copepods, but it cannot explain the stability of
species composition seen in copepods and radiolarians. In addition, the macrozooplankton
assemblage off southern California has undergone
a long-term decrease in abundance, in spite of the
lack of decline in the calanoid copepods, a major
component of the zooplankton.
MacCall (1986) suggested that the ecosystem of
the CCS may be adapted to large fluctuations in
abundance of component species such as the
anchovy. These variations may have little effect
on other components of the system. Anchovy and
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Calanus pacificus
Rank Correlation (ρ)
(a)
1.0
0.8
0.6
0.4
0.2
0
-0.2
-0.4
Metridia pacifica
Rank Correlation (ρ)
(b)
1.0
0.8
0.6
0.4
0.2
0
-0.2
-0.4
Total Female Calanoids
Rank Correlation (ρ)
(c)
1.0
0.8
0.6
0.4
0.2
0
-0.2
-0.4
rin
g
n
er
r
m
m
r
n
er
g
te
m
tu
in
Sp
W
Au
Su
rin
m
m
g
te
tu
m
rin
in
Sp
W
Au
Su
Sp
sardine have undergone large population changes
in the CCS for at least 1700 years (Baumgartner
et al., 1992; Soutar and Isaacs, 1969). These
fluctuations have continued in recent times, with
only very weak correlations with calanoid copepod
populations off southern California (Fig. 2).
However, higher trophic level components of the
system, such as fishes, marine mammals and
seabirds, have been impacted by fisheries and
direct persecution by humans in historical times.
We lack the data on most component species or
assemblages to determine if populations fluctuated
coherently before large-scale human disturbances.
In contrast to MacCall’s (1986) hypothesis,
Cury et al. (2000) presented evidence that small
pelagic fish are capable of exerting both top-down
control of their prey and bottom-up control of
their predators. However, they did not find a
significant relationship between pelagic fish catch
and zooplankton biomass in the CCS, even with
an overall decline in zooplankton biomass and a
generally increasing trend in catches of pelagic fish.
MacCall’s (1986) hypothesis of ecosystem adaptations to large fluctuations in biomass of component species implies some compensating
mechanisms that he did not comment on. McGowan and Walker (1993) suggested one possible
mechanism. They noted the recovery of zooplankton biomass in the CCS following perturbations
such as strong El Niño events, and suggested that
advection of populations into the CCS is more
important than biological processes within it in
maintaining zooplankton communities. Weinheimer and Cayan (1997) also noted the importance
of source water in maintaining planktonic assemblages. They concluded that no major changes in
source water for radiolarians have occurred off
southern California during the last 100 years. The
correlation between zooplankton biomass and
volume transport in the CCS (Chelton et al.,
1982) supports the idea that advection is an
important process in structuring planktonic communities, although it is not clear how much direct
advection of planktonic biomass occurs. The
correlation also could be the result of increased
local productivity associated with higher volume
transport (e.g., Roesler and Chelton, 1987). The
occurrence of subarctic and equatorial species of
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Fig. 2. Rank correlations (Spearman’s r) between copepod
population densities in spring and anchovy larvae densities in
spring, summer, autumn and winter. Time lags range from
spring of the previous year (anchovy leads copepods) to spring
of the following year (copepods lead anchovy). Densities are
number per 10 m2 for anchovy, and number per m2 for
copepods. All densities are averaged over CalCOFI lines 80–
93.3, offshore to station 70 on each line. Winter is defined as
December–February, spring as March–May, Summer as June–
August, and Autumn as September–November. Only samples
collected at night were used. Dashed lines indicate approximate
critical values for r; based on the Fisher z transformation (Zar,
1984). The critical values are low, due to uncorrected
autocorrelation in the time series. (a) Rank correlations
between C. pacificus females and northern anchovy larvae. (b)
Rank correlations between M. pacifica females and northern
anchovy larvae. (c) Rank correlations between total female
calanoid copepods and northern anchovy larvae.
copepods off California (Fleminger, 1964, 1967;
Rebstock, 2002) and the shifting of range boundaries north and south with changes in transport in
the CCS (Brinton and Reid, 1986; Roesler and
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Chelton, 1987) provide some evidence that longdistance advection occurs.
In situ biological processes also may play a role
in regulating planktonic populations in the CCS.
Calanus pacificus and several other calanoid
species may take a decade to recover high
population levels following strong El Niño events,
while species such as Metridia pacifica may recover
within a year (Rebstock, 2002). C. pacificus, one of
the dominant copepod species off southern California (Rebstock, 2001), overwinters in deep
basins in the region (Alldredge et al., 1984), and
the overwintering stock may reseed the regional
population each spring (Osgood and Checkley,
1997). The overwintering strategy of some species,
including C. pacificus, may buffer populations
against moderate El Niño conditions, which
generally affect the CCS most strongly in fall
and winter (Lynn et al., 1995). Note that part of
the C. pacificus population off southern California
remains active during winter (Mullin and Brooks,
1967; Ohman et al., 1998), possibly poised to take
advantage of a shift to more favorable winter
conditions.
Several mechanisms for buffering populations
of higher trophic level species against highfrequency variability in eastern boundary currents
have been proposed. They include spawning at
different times (Mathisen, 1989) and locations
(Bakun, 2001), presence of more than one reproductive year class (Bakun, 2001; Cury et al., 2000),
ability to migrate with prey populations or shifting
hydrographic conditions (Cury et al., 2000; Walsh,
1978), prey switching (Cury et al., 2000), and
genetic variation within species (Cury et al., 2000).
Some of these mechanisms may be valid for
zooplankton as well. Zooplankton biomass in the
southern parts of the CCS does not have a strong
seasonal cycle (Roesler and Chelton, 1987). The
breeding season is extended or year-round in many
calanoid copepods off southern California (e.g.,
Mullin and Brooks, 1967). As long as adverse
conditions do not last more than 1 year, part of the
population should be reproducing during more
favorable conditions. Planktonic populations are
advected with their water masses to some degree,
thereby remaining with favorable hydrographic
conditions. Many calanoid copepods are fairly
generalist grazers, feeding on particles within a size
range regardless of composition (Mullin, 1966).
They presumably switch prey readily. Little is
known about genetic diversity or life history traits
of most planktonic species but differences in
overwintering strategies among calanoid copepod
species (Ohman et al., 1998) suggest that speciesspecific life history traits are varied and may
represent different strategies for coping with
environmental variability in the CCS.
The hypothesis that advection of populations
maintains planktonic community composition and
biomass in the CCS assumes some stability in the
source region. The CCS is dominated by equatorward flow and, at least in terms of the calanoid
copepods, by Transition Zone species (Fleminger,
1964). Although the CCS may be considered an
ecotone rather than an ecosystem, it has a
characteristic planktonic fauna (McGowan,
1974), which even large low-frequency environmental changes have not substantially altered over
the last 100 years.
5. Summary and lessons
In summary, the CCS has undergone long-term
change in hydrographic properties over the last
century, apparently in response to low-frequency
fluctuations in climate. There is also ample
evidence for long-term change in populations in
the CCS, including kelp, diatoms, foraminifera,
radiolarians, intertidal invertebrates, zooplankton,
fish and seabirds. At the same time, there is
evidence for stability in assemblages of larval fish,
calanoid copepods and radiolarians. Few attempts
have been made to reconcile these apparently
conflicting observations, but the answer probably
lies in a combination of statistical, physical and
biological factors. Finally, even populations that
undergo large fluctuations in abundance persist
over long periods in the CCS. Responses of
populations to climatic change are interesting
and informative, but stability of assemblages and
persistence of populations can also inform us on
the dynamics of the California Current ecosystem.
Several lessons may be learned from the longterm data sets discussed here. First, long time
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G.A. Rebstock / Deep-Sea Research II 50 (2003) 2583–2594
series are necessary for a full understanding of the
system dynamics, especially with respect to decadal-scale climate variability. Studies lasting only a
few years could give very different pictures
depending on timing in relation to ENSO events
and climatic regime shifts. Only over longer
periods, spanning several disturbances, recoveries
and regime shifts, do we begin to sense the
dynamic stability of the CCS.
Second, life histories and biological interactions
are important, even in the plankton. In spite of the
presumed importance of physical advection, fluctuations in density of calanoid copepods are not
always concordant among species with similar
biogeographic affinities (Rebstock, 2002). The two
dominant species of calanoid copepods off southern California, C. pacificus and M. pacifica, are
both Transition Zone species, yet their population
fluctuations are apparently driven by different
environmental stimuli. Differences in their life
history traits may explain these different responses.
Finally, the CCS is simultaneously a highly
variable and stable system. Populations in the CCS
undergo large fluctuations in abundance, species
composition, and range boundaries on interannual
to interdecadal scales, in response to climatic
variations. However, they are apparently adapted
to low-frequency variability in both biotic and
abiotic conditions. In addition to stability in total
abundance of assemblages of some planktonic
organisms resulting from statistical averaging,
some compensating mechanisms, possibly including advection of populations and life history traits,
provide stability in species composition. Even
species that undergo large fluctuations in abundance persist in the CCS. Variability and stability
are both properties of the CCS and are not
necessarily in conflict.
Acknowledgements
First and foremost, I acknowledge the CalCOFI
program. The program has been kept going by
many dedicated people for over 50 years and their
hard work has yielded much of our knowledge of
the long-term dynamics of the CCS. I also thank
2591
the Pelagic Invertebrates Collection at Scripps
Institution of Oceanography (SIO) for granting
access to the plankton samples, and Annie Townsend for her cheerful assistance in obtaining the
samples. Melissa Carter and Kirk Ireson provided
invaluable assistance with sample preparation. The
SIO pier temperature data were provided by the
Shore Station Program at SIO. The anchovy data
were provided by Paul Smith, National Marine
Fisheries Service, La Jolla. Comments by two
anonymous reviewers greatly improved the manuscript. My work on the CalCOFI samples was
supported by graduate fellowships from the US
Office of Naval Research and Environmental
Protection Agency (STAR Graduate Fellowship),
as well as a grant from the US GLOBEC program,
jointly funded by the National Science Foundation
and the National Oceanic and Atmospheric
Administration. This is GLOBEC contribution
#395. This research was supported in part
under NSF grant OCE-9711369. This paper
was also supported in part by the National
Sea Grant College Program of the US Department of Commerce’s National Oceanic and
Atmospheric Administration under NOAA Grant
#NA06RG0142, project number R/F-72PD,
through the California Sea Grant College Program. The views expressed herein do not necessarily reflect the views of any of those organizations.
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