Chapter 21 



Oceanographic Processes and Marine Productivity in Waters 

 Offshore of Marbled Murrelet Breeding Habitat 



George L. Hunt, Jr. 1 



Abstract: Marbled Murrelets (Brochyromphus marmoratus) oc- 

 cupy nearshore waters in the eastern North Pacific Ocean from 

 central California to the Aleutian Islands. The offshore marine 

 ecology of these waters is dominated by a series of currents roughly 

 parallel to the coast that determine marine productivity of shelf 

 w aters by influencing the rate of nutrient flux to the euphotic zone. 

 Immediately adjacent to the exposed outer coasts, wind driven 

 Ekman transport and upwelling in the vicinity of promontories and 

 other features create zones of enhanced primary production in 

 which primary and secondary consumers may aggregate. In the 

 more protected waters of the sounds, bays and inlets of British 

 Columbia and Alaska, tidal processes dominate the physical mecha- 

 nisms responsible for small-scale variation in primary production 

 and prey aggregations. 



In North America, Marbled Murrelets (Brachyramphus 

 marmoratus) occupy coastal marine waters from central 

 California to the Aleutian Islands of Alaska. To understand 

 factors controlling marine resources in the habitats occupied 

 by Marbled Murrelets, it is useful to review the coastal 

 oceanography of the region between California and Alaska. 

 For the purposes of this review, I focus on three types of 

 habitat: shelf waters, influenced primarily by the major long- 

 shore current systems; inshore waters of the open coasts; and 

 the relatively sheltered waters of sounds, inlets and bays. 



This chapter provides an overview for the non-marine 

 specialist of the types of habitats, and the processes that 

 determine the distribution and abundance of marine resources 

 used by Marbled Murrelets. 



Determinants of the Shelf Circulation 



The major offshore currents off the west coast of northern 

 North America originate as eastward flowing currents crossing 

 the North Pacific Ocean. One of these, the North Pacific 

 Current, divides into two branches west of the continental 

 shelf off the British Columbia coast (Reed and Schumacher 

 1987. Thomson 1981 ). The northern branch curves northeast 

 as the Alaska Current, and forms a counterclockwise rotating 

 gyre in the Gulf of Alaska (fig. 1). The second branch of the 

 North Pacific Current turns southeast as the California Current 

 and flows along the edge of the continental slope off 

 Washington. Oregon and California. The division of the 

 North Pacific Current is seasonally variable; it is most abrupt 

 in winter, and most diffuse and spatially variable in summer 

 (Thomson 1981). 



' Professor. Department of Ecology and Evolutionary Biology, Uni- 

 versity of California, Irvine. CA 92717 



The Alaska Current is relatively wide (400 km) and 

 slow (30 cm/s) as it moves through the eastern Gulf of 

 Alaska (Reed and Schumacher 1987). As the Alaska Current 

 passes Kayak Island in the northern Gulf of Alaska, it forms 

 a strong (>50 cm/s), clockwise rotating gyre in the island's 

 lee (Royer and others 1 979). A branch of the Alaska Current, 

 the Alaska Coastal Current, diverges from the gyre and 

 approaches the Kenai Peninsula coast (ftg. 1). In fall, the 

 Alaska Coastal Current shows a marked increase in velocity, 

 apparently as a result of both increased freshwater runoff 

 and easterly winds that constrain the current in a narrow 

 coastal stream and produce coastal convergence (movement 

 of water toward the coast, with attendant downwelling) (Royer 

 1979, 1983; Schumacher and Reed 1980). Much of this flow 

 passes through Kennedy Entrance, south of the Kenai 

 Peninsula, and thence into either Cook Inlet or westward 

 into Shelikof Strait between Kodiak Island and the Alaska 

 Peninsula. The main Alaska Current exits the Alaska Gyre 

 to the west as the Alaska Stream, flowing along the Alaska 

 Peninsula and the south side of the Aleutian Islands. West of 

 Kodiak Island, it becomes narrow (100 km) and swift (-100 

 cm/s) (Reed and Schumacher 1987). Although these currents 

 are for the most part seaward of the distribution of Marbled 

 Murrelets in the Gulf of Alaska (Piatt and Ford 1993), the 

 currents are important to marbled murrelets because they 

 influence the transport of plankton into coastal waters and 

 also because they can play an important role in the transport 

 of oil slicks when spills occur (Piatt and others 1990). 



The California Current varies in its intensity, definition, 

 and direction of flow geographically and seasonally (fig. 2) 

 (Mooers and Robinson 1984; Thomson 1981 ). It is relatively 

 weak off the Washington and Oregon coasts, where it has a 

 southward flow only 20 percent of the time. In contrast, off 

 California, the current is usually well defined and flows 

 southward about 50 percent of each month. The California 

 Current is most often southward and strongest between March 

 and September. 



Changes in the direction and intensity of flow of the 

 California Current have important effects on offshore marine 

 production (Chelton 1981. Chelton and others 1982). When 

 the current moves strongly southward, water throughout the 

 water column moves away from the coast (offshore transport) 

 due to the Coriolis Effect. In addition, offshore transport of 

 surface water, also related to the Coriolis Effect (Ekman 

 transport), results when north and northwest winds force 

 increased surface flow to the south. Water transported offshore 

 is replaced by the upwelling of deep, cold, nutrient rich 

 water that supports enhanced productivity. These seasonal 

 and interannual fluctuations in the California Current system 

 and its productivity have been linked to changes in the 



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



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