Pacific Coastal Biome
399
1998–1999, when a major El Niño event occurred, associated with another possible
decadal climate shift, after which the character of the California returned to the pre-1976
condition. In 2002, another El Niño event was apparently associated with an anomalous
intrusion of subarctic water.
Regional Response of the Pelagic Ecosystem
The effects of wind-stress curl on nutrient dynamics are similar in each of the four
principal eastern boundary currents (Bakun and Nelson, 1991). Though the paradigm
of eastern boundary current nutrient dynamics is usually described simply as “coastal
upwelling,” the reality is far more complex and a variety of enrichment processes occur
in (i) coastal upwelling cells, caused by Ekman divergence at the coast, usually of small
dimension and locked to topography; (ii) continued upwelling in offshore-trending filaments of cool upwelled water; (iii) cyclonic eddies shed from meanders of the coastal
current; (iv) and the offshore divergent front at the shear zone between inshore poleward
and offshore equatorward flows.
Here, it is clear that the offshore jet of the California Current separates inshore,
eutrophic water having a high concentration of chlorophyll from oligotrophic offshore
water (Collins et al., 2003). There is a striking discontinuity in properties at the sea
surface so that, where it is parallel to the coast, the offshore jet runs along the junction
between low oceanic chlorophyll values (<05 mg m
−3 ) and higher values inshore. Along
the same line, nitrate values are of order < 1 M liter
−1 rising to around 20 M liter
−1
near the coast. Offshore squirts and dipoles do not disrupt this pattern, for the jet
current continues around their margins, enclosing high-productivity water. Such a pattern
suggests that an amendment is required to the earlier model of Chelton et al. (1982),
who found that nutrient input in the offshore California Current is principally advective
in the cool, low-salinity, high-nutrient core of the offshore jet.
However this may be, off both northern California and Oregon–British Columbia, a
winter-spring bloom occurs that appears to be unrelated to the effects of coastal upwelling
plumes and filaments. Here, in March–April, chlorophyll values >10 mg chl m
−3 occur
over wide areas as far as 300 km from the coast. Between these two regions, chlorophyll
values offshore are generally lower even at this season, and a “blue hole” region (covering
several degrees of latitude) is persistent offshore, centered off the Columbia River. Satellite
images clearly show that this is continuous with high surface chlorophyll at this season
right across the North Pacific Ocean.
Because the processes leading to upwelling are not the same throughout the coastal
area, it may be useful to review the processes that are characteristic of four compartments:
Oregon–British Columbia, Point Conception to Cape Mendocino, the Southern California
Bight, and Baja California.
Oregon–British Columbia (42–48
N ): Winter storms are strong and frequent and seasonal current reversal occurs regularly; primary production is strongly seasonal.
Upwelling occurs in summer at a coastwise front about 10 km offshore as well as
at the coast itself, and response to wind events typically results, after 4 or 5 days, in
the development of an upwelling cell, whose seaward front moves progressively offshore during development and returns shoreward during the subsequent relaxation
of upwelling. During this process, mean offshore flow is restricted to the surface
20–30 m (Brink, 1983; Smith, 1981a,b). Upwelling occurs in persistent, topographically locked gyres during summer, as at Juan de Fuca. During both summer and
winter, frequent cold tongues of upwelled water on the scale of hundreds of kilometers extend westward from the continent across and beyond the relatively broad
continental shelf. Relatively high levels of nutrients occur throughout this region
offshore, entrained toward the south from the subarctic zone.
399
1998–1999, when a major El Niño event occurred, associated with another possible
decadal climate shift, after which the character of the California returned to the pre-1976
condition. In 2002, another El Niño event was apparently associated with an anomalous
intrusion of subarctic water.
Regional Response of the Pelagic Ecosystem
The effects of wind-stress curl on nutrient dynamics are similar in each of the four
principal eastern boundary currents (Bakun and Nelson, 1991). Though the paradigm
of eastern boundary current nutrient dynamics is usually described simply as “coastal
upwelling,” the reality is far more complex and a variety of enrichment processes occur
in (i) coastal upwelling cells, caused by Ekman divergence at the coast, usually of small
dimension and locked to topography; (ii) continued upwelling in offshore-trending filaments of cool upwelled water; (iii) cyclonic eddies shed from meanders of the coastal
current; (iv) and the offshore divergent front at the shear zone between inshore poleward
and offshore equatorward flows.
Here, it is clear that the offshore jet of the California Current separates inshore,
eutrophic water having a high concentration of chlorophyll from oligotrophic offshore
water (Collins et al., 2003). There is a striking discontinuity in properties at the sea
surface so that, where it is parallel to the coast, the offshore jet runs along the junction
between low oceanic chlorophyll values (<05 mg m
−3 ) and higher values inshore. Along
the same line, nitrate values are of order < 1 M liter
−1 rising to around 20 M liter
−1
near the coast. Offshore squirts and dipoles do not disrupt this pattern, for the jet
current continues around their margins, enclosing high-productivity water. Such a pattern
suggests that an amendment is required to the earlier model of Chelton et al. (1982),
who found that nutrient input in the offshore California Current is principally advective
in the cool, low-salinity, high-nutrient core of the offshore jet.
However this may be, off both northern California and Oregon–British Columbia, a
winter-spring bloom occurs that appears to be unrelated to the effects of coastal upwelling
plumes and filaments. Here, in March–April, chlorophyll values >10 mg chl m
−3 occur
over wide areas as far as 300 km from the coast. Between these two regions, chlorophyll
values offshore are generally lower even at this season, and a “blue hole” region (covering
several degrees of latitude) is persistent offshore, centered off the Columbia River. Satellite
images clearly show that this is continuous with high surface chlorophyll at this season
right across the North Pacific Ocean.
Because the processes leading to upwelling are not the same throughout the coastal
area, it may be useful to review the processes that are characteristic of four compartments:
Oregon–British Columbia, Point Conception to Cape Mendocino, the Southern California
Bight, and Baja California.
Oregon–British Columbia (42–48
N ): Winter storms are strong and frequent and seasonal current reversal occurs regularly; primary production is strongly seasonal.
Upwelling occurs in summer at a coastwise front about 10 km offshore as well as
at the coast itself, and response to wind events typically results, after 4 or 5 days, in
the development of an upwelling cell, whose seaward front moves progressively offshore during development and returns shoreward during the subsequent relaxation
of upwelling. During this process, mean offshore flow is restricted to the surface
20–30 m (Brink, 1983; Smith, 1981a,b). Upwelling occurs in persistent, topographically locked gyres during summer, as at Juan de Fuca. During both summer and
winter, frequent cold tongues of upwelled water on the scale of hundreds of kilometers extend westward from the continent across and beyond the relatively broad
continental shelf. Relatively high levels of nutrients occur throughout this region
offshore, entrained toward the south from the subarctic zone.