Hunt 



Chapter 21 



Oceanographic Processes and Marine Productivity 



In coastal waters, strong winds cause upwelling by two 

 mechanisms. In the first, water is displaced from near the 

 coast by winds blowing parallel to the coast from the north. 

 In the northern hemisphere, if water depths are sufficient, 

 surface waters will move at approximately 90 degrees to the 

 right of the direction of the surface wind because of the 

 Coriolis Effect (Ekman transport). When this occurs near the 

 coast, the displaced water is replaced by nutrient rich water 

 from depth. In the second, winds blowing from the shore 

 cause inshore upwelling. If the water is sufficiently shallow, 

 the Ekman transport is effectively canceled by friction with 

 the bottom, and surface flows will be in the same direction 

 as the wind. When strong land breezes blow surface water 

 away from a lee shore, inshore surface waters are replaced 

 with water from greater depth. 



Along the open coasts of California, Oregon, and 

 Washington, localized nearshore upwelling due to Ekman 

 transport and offshore winds blowing water away from lee 

 shores provides regions of enhanced primary and secondary 

 production. These upwelling processes, and fronts associated 

 with river discharges are expected to be the most important 

 physical features in determining murrelet foraging 

 opportunities. Ainley and others (this volume) provide one of 

 the only examples of the sort of mesoscale studies needed to 

 link murrelet foraging distributions to physical and biological 

 processes that result in exploitable concentrations of prey. 



Sheltered Waters of Sounds, Inlets 

 and Bays 



The physical and chemical oceanographic processes 

 controlling primary production in the fjords and estuaries of 

 the Gulf of Alaska and the British Columbia coasts are 

 reviewed by Burrell (1987) and Reeburgh and Kipphut (1987). 

 In these fjords, freshwater input, primary production, and 

 other biogeochemical processes are highly seasonal. 

 Freshwater, less dense than saltwater, forms a surface layer 

 in the fjords and is discharged from these upper layers into 

 the Gulf of Alaska; waters from the Gulf of Alaska circulation 

 episodically penetrate the fjords to replace intermediate and 

 deep resident waters (Burrell 1987). These exchanges 

 influence the availability of nutrients to, and the residence 

 time of, phytoplankton. Both factors also affect the timing 

 and magnitude of primary production in the fjords. Coastal 

 frontal zones associated with shallow areas with increased 

 water flow can be the site of elevated primary production 

 because of enhanced vertical flux of nutrients (Parsons and 

 others 1983, 1984). High tidal ranges present in British 

 Columbia and along the coast of the Gulf of Alaska would 

 promote these enhanced vertical fluxes in the vicinity of sills 

 at the mouths of fjords (Burrell 1987). In late summer and 

 early fall, turbidity from river inflows may progressively 

 limit primary production in the upper ends of fjords (e.g., 

 Goering and others 1973). 



Fjords may support one of two generalized trophic 

 pathways (Burrell 1987, Matthews and Heindal 1980). In 

 shallow fjords or those with shallow sills, the pathway may 

 lead from small phytoplankton to small copepods to jellyfish. 

 In deeper fjords, and fjords with deep sills, the trophic 

 pathway may include large net phytoplankton (primarily 

 diatoms), large copepods and finfish. Apparently, the depth 

 of the sill is a critical feature; if it intercepts the pycnocline 

 (the layer in which water density changes rapidly with 

 depth, and which inhibits vertical mixng of water), the 

 upper layer of out-flowing fresh water inhibits the recruitment 

 of large calanoid copepods from outside the fjord. The 

 ontogenetic migration to the upper water column of 

 Neocalanus plumchrus and related oceanic copepod species 

 in the North Pacific occurs at the same time as the coastal 

 convergence mentioned above (Burrell 1987). Their presence 

 in the upper water column at this time allows them to be 

 transported into adjacent fjord environments according to 

 observations by R. T. Cooney, as cited by Burrell (1987). 

 These large copepods are likely to be important prey for the 

 small fish taken by Marbled Murrelets. One might 

 hypothesize, then, that murrelets would be more likely to 

 forage in fjords supporting populations of large copepods 

 than in fjords lacking this component of trophic transfer. 

 Additionally, one might expect that Marbled Murrelets 

 would be more likely to forage at the seaward ends and near 

 the sills of these fjords, rather than at their inner ends. 



In the inland waters of the sounds and channels of 

 Washington, British Columbia, and Alaska, tidal processes 

 are likely the most important determinants of localized 

 foraging opportunities for marbled murrelets and other 

 seabirds. Upwellings can be caused by currents impinging 

 on an obstruction and being driven to the surface, such as 

 when strong tidal currents encounter a sill and flow over it. 

 In these circumstances planktonic organisms are driven to 

 the surface (Brown and Gaskin 1988; Vermeer and others 

 1987), or may be concentrated at depth where their ability to 

 swim against a gradient is matched by an opposing current 

 (e.g., Coyle and others 1992). 



Superimposed on the physical mechanisms that enhance 

 primary production and concentrate prey are the seasonal 

 variations in production and the movements of prey of suitable 

 size. We know of few studies of physical processes and fish 

 movements at temporal or spatial scales appropriate for 

 understanding murrelet foraging, and none for which murrelets 

 were a focus of the study. This paucity of data makes it 

 difficult to assess, in oceanographic terms, the characteristics 

 of habitats critical for foraging murrelets. 



Acknowledgments 



I thank Dan Anderson, Larry Spear and C. John Ralph 

 for helpful comments on an earlier version of this manuscript. 



222 



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



