Participants: V. Holliday, A. Huyer, P. Kremer, A. MacCall, D. Mackas, K. Parker, W. Pearcy, J. Schumacher, T. Strub, R. Tipper, and D. Ware
Among the most striking biological contrasts is the paucity of surface spawning by epipelagic fishes in the central region (II; see Parrish et al. 1981). This region is characterized by strong seasonal upwelling near coastal promontories and by a variety of mesoscale features such as coastal jets, eddies, and filaments that tend to transport organisms offshore. The fish species that do reproduce in this region tend to brood eggs or larvae, or to spawn demersally in protected waters. Equally striking is the concentration of spawning activity in Region III: about 90% of the epipelagic fish biomass (hake, sardine, anchovy) in the southern part of the California Current system spawns in the Southern California Bight and waters offshore. Primary and secondary production in Region III is therefore important to a significant fraction of California Current fishes.
A number of critical physical processes are likely to govern much of the variability in marine populations over time in the CCS and other eastern boundary currents:
We strongly recommend conducting comparative studies in all three regions of the CCS. Forcing functions differ spatially, but some marine species depend on processes in all three regions, so a successful GLOBEC program will have to incorporate the significant processes operating on a broad latitudinal scale. One component of this program should include intensive field studies that focus on processes thought to be important in each region. As a second component, we recommend time series sampling in each of the three regions for at least a decade. The time series might include a combination of moored arrays, satellite observations, and ship surveys. Suitable localities within each region might include the west coast of Vancouver Island, or the coasts of Washington or northern Oregon (Region I); Point Arena to Point Sur (Region II); and the Southern California Bight (Region III). In each region there are oceanographic institutions and shipboard resources that could be applied to the task. It is likely that existing field programs in each of the three regions could be expanded.
Figure 5. Generalized regional variations in physical and biological processes within the California Current System. The boundaries between Regions I, II, and III are only approximate and vary over time. The generalizations regarding Region III apply primarily to the Southern California Bight.
In Region I, coastal wind stress is relatively strong, and wind direction reverses seasonally as well as on shorter time scales. Winter storms are particularly strong and frequent, leading to intense mixing and alongshore northward advection (Huyer et al. 1978; Thomson 1981; Hickey 1989; Thomson et al. 1989). Except for the region near the Strait of Juan de Fuca, the coastline is relatively straight, and the shelf is continuous, though narrow, over large alongshore scales (hundreds of km). Significant freshwater input is provided throughout most of the year by the Strait of Juan de Fuca and by the Columbia River. Large estuaries are relatively common and are thought to provide nurseries for several important species (e.g., Dungeness crab, McConnaughey et al., in press; Pacific herring, Haegele and Schweigert 1985). Primary production rates (Perry et al. 1989) and zooplankton biomass (Mackas 1992) have strong seasonal variations in this region. Some of the major copepod species overwinter at depth, then reappear in the surface layer for relatively short periods of growth in spring and summer. Several species (e.g., Neocalanus plumchrus, N. cristatus, Eucalanus bungii, Calanus pacificus oceanicus) enter Region I from the Subarctic Pacific or the West Wind Drift (Fleminger 1964; Fleminger and Hulseman 1973) and rarely extend south of this region.
The dominant physical characteristics of Region II, which extends approximately from Cape Blanco to Point Conception, are the coastal promontories. Recent research suggests that energetic coastal jets, filaments, and meanders are associated with these promontories (Davis 1985; Kosro & Huyer 1986; Huyer & Kosro 1987; Strub et al. 1991). Current jets commonly extend 200-300 km offshore and may lead to relatively short residence times for plankton in the coastal zone. The strongest equatorward wind stress and, hence, coastal upwelling also occur in Region II (Nelson 1977; Huyer 1983; Strub et al. 1987). Although wind stress varies seasonally, the seasonal mean is always directed toward the equator. The strong coastal upwelling in this region supplies "new" nutrients into the euphotic zone, leading to elevated primary production rates and high standing stocks of phytoplankton (Dugdale & Wilkerson 1989). Satellite imagery suggests that many of the high-chlorophyll features found in Region II are associated with jets, eddies, and other mesoscale features (Flament et al. 1985). Zooplankton biomass varies seasonally (Roesler & Chelton 1987). Zooplankton species composition can shift relatively abruptly at the frontal boundaries associated with mesoscale jets and eddies. Among the most dramatic biological characteristics of Region II is the latitudinal minimum in spawning of pelagic fishes. Whereas epipelagic fishes spawn extensively in Region III and to some extent in Region I, those in Region II appear mainly to brood their eggs or larvae (e.g., rockfishes) or to use nearshore embayments as spawning and nursery grounds (e.g., Pacific herring) that appear to reduce the probability of offshore transport of pelagic larvae (Parrish et al. 1981).
The dominant physical characteristic of Region III is that, because of the coastline bend at Point Conception, local wind stress is relatively weak on the scales of seasons and events (Nelson 1977; Halliwell and Allen 1987). Thus local upwelling is weak in spring and summer, and wind- and wave-induced mixing is relatively weak year-round. Winter storms occur only occasionally. Freshwater input is insignificant. Interleaving of differing water masses occurs in Region III, making it particularly sensitive to large-scale, long-time-scale environmental perturbations such as ENSO (Hickey 1979; Lynn & Simpson 1987,1990; Tsuchiya 1980). Seasonal cycles in zooplankton biomass are relatively weak (Roesler & Chelton 1987). Deep overwintering of calanoid copepods occurs (Alldredge et al. 1984), but it may involve only part of a population while another part grows and reproduces year-round (Mullin & Brooks 1967). The boundary between Region II and III is a biogeographic boundary for some species of nearshore benthic marine invertebrates and pelagic fishes. Region III is the preferred spawning site for over 90% of the epipelagic fish biomass (hake, sardine, anchovy) in the southern part of the CCS.
In addition to these latitudinal patterns, strong cross-shore variations occur in the CCS. For example, the wind field has strong cross-shore gradients at most latitudes, with maximum winds occurring seaward of the continental shelf (Nelson 1977). Vertically integrated primary production rates tend to decrease in the cross-shore direction (P.E. Smith, pers. comm.; F. Chavez pers. comm; Perry et al. 1989). A zone of maximum variability in dynamic height begins approximately 200-300 km offshore (Lynn & Simpson 1987); this zone has been called an eddy alley. The long-term maximum in macrozooplankton biomass occurs offshore in some areas (Roesler & Chelton 1987). This maximum is sometimes dominated by gelatinous zooplankton such as salps and doliolids (Berner 1967).
H1: Nearshore eddies, jets, and current meanders are significant dispersal mechanisms for coastal populations.
H2: Frontal zones associated with these mesoscale features are sites of enhanced production and concentration of planktonic prey.
H3: Offshore mesoscale eddies are retention sites that reduce spatial losses and enhance population growth rates of some planktonic populations.
H1: Seeding from dormant stages is more important to population growth in Region II, where offshore transport is more frequent, than in Region I, where transport is more frequently alongshore.
H2: A significant fraction of the primary and secondary production in Region II is advected off the shelf and is unavailable to pelagic fishes that normally inhabit the continental shelf and slope (e.g., Pacific hake and northern anchovy).
H1: There is an optimal wind speed that maximizes primary and hence secondary production. The optimal speed is about 7-8 m s-l.
H2: Survival is greatest for fish larvae that hatch during calm conditions.
H1: Large-scale advection from the north alters the species composition and secondary production of CCS zooplankton assemblages.
H2: Large-scale advection from the south during El Niño/Southern Oscillation alters the species composition and secondary production of CCS zooplankton assemblages.
H1: Some pelagic species need the poleward-flowing California Undercurrent and Davidson Current to complete their life history.
Each of these processes may fluctuate on a variety of time scales. For example, the frequency or intensity of upwelling-favorable winds may change within a single season, as well as over decades (Bakun 1990).
Figure 6. Migration of the Pacific hake, Merluccius productus, from Bailey et al. 1982; interpretation of timing modified by D. M. Ware. Inset illustrates the geographic shift in spawning area from 12975 to 1978 (P. E. Smith, pers. comm.). The northward displacement of spawing occurred at a time of warming in the California Current system (A. D. MacCall, pers. comm.).
The second research strategy involves selecting species (or sibling species) that occur broadly throughout the CCS. It is hypothesized that the same species are governed by different processes in different regions. This strategy might be viewed as an Eulerian mode of study. In the different regions occupied by "metapopulations," or subpopulations, of the species, the differing effects of processes such as offshore transport, food limitation, vertical mixing, or large-scale advection can be quantified.
A simplified description of the population growth rate for a metapopulation within each of the three regions of the CCS can be expressed as follows:
where the subscript R designates the region of interest. The terms in the equation are each rather complex and nonlinear functions of other processes and will vary within (as well as between) regions. Nevertheless, in this Eulerian approach, strong regional contrasts in the importance of these terms should make it possible to identify the most significant population control mechanisms. For example, for cyprid larvae of barnacles, the advection term may predominate in Region II, and the death rate term may predominate in Region I.
Although the spawning regions of the Pacific hake and northern anchovy overlap broadly, the two species show markedly different interannual variations in recruitment (Fig. 7). Hake have occasional very strong year classes, while northern anchovy tend to have runs of weaker or stronger year classes. This difference is a likely topic for GLOBEC studies.
Figure 7. Comparative recruitment time series of Pacific hake (Merluccius productus; from Hollowed and Bailey 1989) and northern anchovy (Engraulis mordax; from Jacobson and Lo 1989).
With respect to euphausiids, Euphausia pacifica is distributed throughout the CCS, from the Gulf of Alaska to Baja California (Brinton 1962). It is the dominant species at many localities and makes an excellent candidate for contrasting studies in different regions of the CCS. The northern metapopulations of E. pacifica have shorter growing seasons and greater age and size at maturity than the southern metapopulations. Other potential candidates include Thysanoessa spinifera and congeners. Since euphausiids and copepods are the dominant prey for the target fish species identified above, we expect to advance understanding of the coupling between physical processes, zooplankton production, and fish recruitment.
Salps, and to a lesser extent doliolids, have extraordinarily rapid growth rates and colonizing abilities. Historical evidence suggests that they may predominate in some regions of the CCS (Berner 1967). It is also known that major ENSO events affect the total thaliacean (salp, doliolid, pyrosome) biomass much more than the copepod and euphausiid biomass (Smith 1985). Both observations suggest that a salp or doliolid species should also be a focus of study. Experiments should be designed expressly to understand the contrasting processes that select for either thaliacean or crustacean dominance, and the resulting consequences for the pelagic ecosystem.