Some aspects of U.S. GLOBEC's proposed studies in the Oregon and Northern California region will be done in collaboration with the Coastal Ocean Processes (CoOP) program's investigation of cross-shelf exchanges in wind-driven regions. CoOP proposes to use modeling and intensive process studies in Regions I and II of the CCS to examine the processes that control the cross-margin transport of biological, chemical and geological materials in a strongly wind-driven system (Smith and Brink 1994). Although the alongshore coastal winds are the dominant forcing from the northwest tip of Washington (48°N) to Point Conception (35°N) in southern California, there is a significant difference north and south of about 40°N. During summer, the alongshore winds are strongly favorable for coastal upwelling but more variable north of about 40°N. During winter, low pressure systems from the Gulf of Alaska cause a strong northward component in the coastal winds and downwelling along the coast of Oregon and Washington. South of San Francisco (37°N) upwelling generally continues intermittently, interrupted by occasional winter storms. These differences in forcing and response form a natural laboratory within which processes responsible for wind-driven cross-shelf transport can be studied intensively and incorporated into theoretical, numerical and laboratory models of these systems. Since coastal upwelling and downwelling are the ubiquitous coastal responses to surface boundary layer transports forced by local alongshore winds, these processes have more than regional importance in understanding cross-margin transport (Huyer, 1990).
CoOP has recommended that parallel studies north and south of about 40°N be made, with possible locations being central Oregon and northern California (Smith and Brink, 1994). The logistical ease, the oceanographic background from previous studies, and the relative simplicity of these regions (lack of major riverine, topographic, or tidal effects) makes them especially attractive for a CoOP study of wind-driven processes affecting cross-margin transport. For the past three years, the U.S. GLOBEC program has been discussing with CoOP ways to couple our studies of the CCS with those planned by that program. Linking U.S. GLOBEC and CoOP studies, where possible, will bring more expertise and greater resources to this effort than would be possible by either program alone. CoOP could bring detailed nearshore physical oceanography and larval studies (especially if meroplankton of adult nearshore species are used as tracers of cross-shelf exchange, as proposed by CoOP) to the broader spatial, temporal, and ecological studies proposed by U.S. GLOBEC.
U.S. GLOBEC's CCS process studies will be structured to address four questions:
The important biological processes include primary production; growth, mortality, fecundity and the
genetic composition of zooplankton; transport, retention and recruitment; and links between secondary
production, especially reproductive output, and the subsequent recruitment success of fish and
benthos. Mesoscale physical processes expected to control these biological variables, and subject to
change with changing climate, include frontal dynamics, upwelling, locally intense cross-shelf
transport, eddy recirculation, stratification and vertical shear. Mesoscale features are the dominant
variability in the Northern California section (Region II) of the CCS. In Region II, productivity and
zooplankton abundances are probably highest inshore of the meandering jet region, in the region of
most intense upwelling, but the habitat used by the salmon off of Northern California is not known.
Nearshore circulation in Region I is less complex than in Region II. The upwelling zone extends only
ca. 50 km from the shore, but upwelling is most intense within 15 km of the shore. Phytoplankton,
zooplankton, fish larvae and juvenile salmonids are most abundant within the nearshore region of
greatest upwelling.
Recent evidence (Huyer et al. 1991; Washburn et al. 1993) suggests that much of the advection within Region II of the CCS occurs in mesoscale features, which also may affect the local intensities of upwelling, downwelling, mixing and primary productivity. Large scale forcing, operating through both the ocean (e.g., advection from the north and south) and the atmosphere (variations in wind intensity, direction or duration), must be important to these mesoscale features, but the mechanistic linkages between the large- and meso-scales are not known. Satellite SST images off central California led Schwing et al. (1991, p. 57) to conclude:
"The impression gained from a series of satellite images of the study area is that several general water masses, indicated by surface temperature, are present at all times, but their relative and absolute location can change on short subsynoptic time scales not detected by traditional ship-survey methods. These changes could profoundly affect the biota in the region..."
Mesoscale features (i.e., eddies, jets, etc.) may be retention sites (via physical means) or aggregation sites (via behavioral means) for zooplanktonic populations (Huntley et al. 1995). If individual demographic parameters (e.g., the vital rates of birth, growth and death) differ inside and outside these features, this can have significant impacts on population growth rate and production. Frontal zones associated with the mesoscale features may be sites of enhanced primary production and concentration of planktonic prey, and therefore favor zooplankton growth and fecundity. Conversely, if predators accumulate at the fronts to better utilize their prey, zooplankton survival may decrease. Upwelling, especially its intensity and persistence (or conversely, intermittence), can have important impacts on the productivity of the nearshore ecosystem, and the ability of secondary consumers to efficiently utilize upwelling enhanced primary production (Attwood and Peterson 1989; Peterson et al. 1988). Timing of the spring transition in relation to the period of nearshore spawning of benthic invertebrates and the arrival of salmon from their natal streams (or hatcheries) may be critical in determining growth and survival.
During the spring and summer, species with long pelagic larval stages are likely to be transported substantial distances southward (the "mean" flow direction). Mesoscale features, which persist temporally, or are spatially predictable, may be one mechanism for maintaining larvae near their source (Barth and Smith, in press). Another mechanism may be the interaction of behavior with transport processes. For instance, some marine zooplankton, e.g., Calanus marshallae, in Region I employ vertical migration behavior and ontogenetic changes in vertical distribution that interact with vertical current shear to reduce offshore transport and increase their residence time in the coastal zone (Peterson et al. 1979). U.S. GLOBEC studies in this region should examine how behavioral attributes of the resident fauna control their retention in a region with strong alongshore advection.
Most of the biological impacts of mesoscale features and dynamics discussed above relate to spatial and temporal variability in the patterns of secondary (e.g., zooplankton) production. These processes may be equally important to consumer species (i.e., salmon). There are related questions that could also be addressed by U.S. GLOBEC studies off of Oregon and California. What are the implications of the mesoscale structure (and variation in the structure) in permitting different suites of higher trophic level organisms (e.g., perhaps predators of salmon) to occupy the nearshore regions? For instance, in some El Niño years, the advection of warmer waters from the south permit warm-water predators to "invade" normally cold habitats off northern California and Oregon. Warmer offshore waters brought inshore by eddies may have similar impacts, either directly (by introducing additional predators) or indirectly (by compressing favorable salmon habitat to smaller regions, which can be more effectively exploited by nearshore consumers). In addition to these impacts on the growth and survival of the target species (i.e., salmon), there may be direct impacts on higher trophic level productivities, distribution, and abundance. Another issue that could be addressed deals with genetic and demographic differences in the populations. Coho and chinook salmon exhibit both inter- and intra-specific life-history variation (Groot and Margolis, 1991; Mangel, 1994). Some of the variability (e.g., age at maturity, timing of spawning, migration behavior) may be linked to genetic and geographic factors. Perhaps some is also controlled by environmental variability; for example, by the certainty of encountering, or the location, of salinity fronts at the mouths of coastal streams as salmon emerge from the estuaries. Small-scale environmental conditions may interact with a range of genetically programmed responses, resulting in specific "habitat selection" behaviors. The last two questions (above) consider whether salmon survival during the ocean phase of the life history is controlled from the bottom-up (through food availability) or from the top-down (through predation relations) during the juvenile period in the coastal ocean.
Pearcy (1992) recently reviewed the ecology of juvenile coho salmon in the nearshore waters of Oregon and Washington (Region I). Studies of juvenile coho conducted over five years in the 1970's indicate that they are not highly migratory, remaining nearshore within the upwelling zone through much of their first summer in the ocean. Ocean survival was positively correlated with upwelling during the period immediately after ocean entrance before the 1976-77 shift in ocean conditions. Most juvenile coho appear to be swept southwards in the mean coastal flow during May and June upwelling, when the mean coastal flow is strong to the south, and when the smolts are smallest and weak swimmers. Later in the summer, most of the juvenile coho, now larger and stronger swimmers, are able to swim northward against the weaker southward flow (Pearcy and Fisher, 1988). It also appears that the critical period that determines eventual year-class strength most likely occurs early in ocean life, perhaps within the first month of ocean residence. This is suggested by the high correlation of eventual year-class strength with early returns of precocious males (jacks) (Fisher and Pearcy, 1988). The mechanistic coupling of coho salmon survival with coastal upwelling is unknown at present and will be a focus of U.S. GLOBEC studies in the CCS.
In Region II (south of Cape Blanco), little is known about the marine habitats used by coho juveniles. After the spring transition, southward velocities in the jet off Oregon are commonly 10-20 km/day and can reach 50 km/day (Barth and Smith, submitted). Trajectories of drifters released into the jet north of Cape Blanco vary seasonally. Drifters deployed in May (during well developed upwelling) are rapidly advected southward and offshore in the vicinity of Cape Mendocino and Point Arena (Fig. 11); during an August period of rather weak equatorward (upwelling favorable) winds, drifters moved offshore into either a recirculating eddy region or a quiescent region, later returning to shore at nearly the same latitude as they departed the coast (Barth and Smith, submitted). This spatial and temporal variation in the strength of alongshore transport can have implications on early salmon survival; food and predator environments of the salmon juveniles may differ depending on phasing of salmon entry to the ocean and the spring transition. Mesoscale features in physics, primary production, and zooplankton distribution and production may be important to the growth and survival of juvenile coho and chinook in this region of the CCS.
Studies of mesoscale processes and their impacts on zooplankton distribution, and productivity (see above), will include measurements of juvenile salmon distributions, growth (and survival from marked fish) at selected sites in Region I (Newport, OR) (Year 1), Region II (Pt. Reyes/Arena or Monterey region) (Year 3), and as a Lagrangian flow through study of both regions I and II (Year 5). All elements (zooplankton and salmon juveniles) will be studied in each process study. These studies of the CCS may include the following components:
