Hunt 



Chapter 21 



Oceanographic Processes and Marine Productivity 



50 - 



170 



Figure 1 Major features of ocean circulation in the Gulf of Alaska. From Reed and Schumacher (1987), by permission. 



breeding success of seabirds (Ainley and Boekelheide 1990, 

 Ainley and others, in press) and in the numbers and distribution 

 of seabirds at sea (Briggs and others 1987). 



Inshore of the California Current, the Davidson current 

 flows northward seasonally from about 32 to about 50 N 

 (fig. 2). The onset of the Davidson Current usually occurs 

 in October, when the overall average movement of water in 

 the California Current system shifts toward the north until 

 March (Thomson 1981). When the northward flowing 

 Davidson Current prevails, upwelling is suppressed because 

 northward flowing water is deflected by the Coriolis Effect 

 toward the shore and downwelling is likely to prevail 

 (McLain and others 1985). The seasonal shifts in the flow 

 of the California Current system are largely the result of 

 changes in the direction of the prevailing winds. In spring 

 and summer, the winds blow from the northwest and move 

 the surface water southward, whereas in winter, prevailing 

 winds are from the southwest and surface water movements 

 are to the north. 



Off Vancouver Island, a northwestward coastal current 

 flows inshore of the southeastward flowing southern branch 

 of the North Pacific Current (Thomson 1981). This inshore 

 current originates in the outflow of the Strait of Juan de Fuca 

 and is confined in summer to within 15-20 km of the coast. 

 The speed of the coastal current is determined by the velocity 

 of the winds. In winter, the coastal flow merges with that of 

 the Davidson Current. 



Strong El Nino-Southern Oscillation events cause a 

 reversal of flow in the California Current System, the presence 



of a surface layer of warm, nutrient-depleted water, and the 

 replacement of coastal upwelling with downwelling (Johnson 

 and O'Brien 1990; Norton and others 1985; Rienecker and 

 Mooers 1986). A consequence of these events is a marked 

 reduction in primary production, followed by a reduction in 

 zooplankton populations and reduced survival of at least 

 some larval fish (Barber and Chavez 1984, MacCall 1986, 

 Pearcy and Schoener 1987). These events result in a marked 

 decrease in seabird reproductive success and in striking 

 changes in the offshore distribution and abundance of seabirds 

 (Ainley and Boekelheide 1990; Ainley and others, in press; 

 Briggs and others 1987). 



Inshore Waters of the Open Coasts 



Large oceanic currents determine regional marine habitat 

 types and are responsible for a major portion of the seasonal 

 variation in production on the shelf. However, marine waters 

 within a few kilometers of the shore are where Marbled 

 Murrelets spend most of their time. In these areas, currents 

 interacting with bathymetry can create fronts (boundaries 

 between water masses where convergences or upwelling 

 may occur) and upwellings that either enhance productivity, 

 or cause organisms to accumulate because of behavioral 

 responses to physical gradients. For example, upwelling 

 results when a current passes a promontory and draws away 

 surface water that is then replaced by water from depth 

 (Pingree and others 1978; Thomson 1981). Fronts associated 

 with these processes provide foraging sites for seabirds. 



220 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



