Annual Physical and Chemical Oceanographic Cycles 

 of Auke Bay, Southeastern Alaska 



HERBERT E. BRUCE, DOUGLAS R. McLAIN, and BRUCE L. WING 1 



ABSTRACT 



The annual cycles of physical and chemical oceanographic conditions in Auke Bay, a small 

 estuary in southeastern Alaska, showed a consistent pattern over an 8-yr period (1961-68). The cycles 

 closely followed seasonal climatological and atmospheric events. Increased insolation in the spring 

 caused general warming of the surface water and the air, which in turn increased the freshwater in- 

 put into Auke Bay from melting snow and ice. The fresh water lowered surface salinities and together 

 with warming of the surface waters caused a density stratification of the water column, which in- 

 creased as the spring-summer season progressed. Maximum stratification occurred in August, fol- 

 lowed by a general decay of stratification in September. Vertical mixing of the top 20 m of the water 

 column by fall storms in September and cooling of surface water resulting from decreased insolation 

 set up a thermohaline circulation that continued through the fall and early winter. The water column 

 became homogeneous by January and remained thoroughly mixed from January through March or 

 early April. 



Auke Bay was rich in the inorganic nutrients phosphate, silicate, and nitrate. Spring 

 phytoplankton blooms followed the onset of stratification and drastically reduced the concentration of 

 all three nutrients in the surface water. Nitrate was essentially depleted and remained so throughout 

 the summer. Low nitrate availability was undoubtedly one of the important factors limiting primary 

 production in Auke Bay. 



INTRODUCTION 



Auke Bay is one of the many small bays along the 

 major straits and passages of the inside waters of 

 southeastern Alaska. Although these bays represent a 

 small percentage of the total water mass of the area, they 

 are important as spawning and nursery grounds for 

 several species of fish and shellfish. Streams tributary to 

 Auke Bay are spawning grounds for four species of salm- 

 on: pink, Oncorhynchus gorbuscha; chum, 0. keta; 

 coho, 0. kisutch; and sockeye, 0. nerka. Herring, Clupea 

 harengus pallasi; king crabs, Paralithodes camtschatica 

 and P. platypus; snow crab, Chionoecetes bairdi; and 

 other species of commercial and recreational value 

 spawn in the bay itself. 



Several reports describe aspects of physical and 

 chemical oceanography of small bays and estuaries in 

 southeastern Alaska. Barnes et al. (1956) made an in- 

 tensive survey (including some biological observations) 

 of Silver Bay on Baranof Island near Sitka before con- 

 struction of a pulp mill. Powers (1963) examined Little 

 Port Walter at the southern end of Baranof Island. The 

 U.S. Federal Water Pollution Control Administration 

 (1966) made surveys of Gastineau Channel and Fritz 



'Northwest and Alaska Fisheries Center Auke Bay Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, P.O. Box 155. Auke Bay, AK 

 99821. Herbert E. Bruce's present address: Bering Sea-Gulf of Alaska 

 Project Office, Environmental Research Laboratories, NOAA, P.O. Box 

 1808, Juneau, AK 99802. Douglas R. McLain's present address: Pacific 

 Environmental Group, National Marine Fisheries Service, c/o Fleet 

 Numerical Weather Central. Monterey, CA 93940. 



Cove near Juneau and of Silver Bay and Ward Cove near 

 Ketchikan in the summer of 1965. McLain (1968) sur- 

 veyed Traitors Cove on Revillagigedo Island. McLain 

 (1969) described seasonal changes in oceanographic con- 

 ditions in Lynn Canal. Nebert and Matthews (1972) 

 described the circulation of Endicott Arm. Most of these 

 studies were less than 2-yr duration, not long enough to 

 separate short-term and protracted hydrographic dis- 

 turbances known to occur in natural environments. 



Therefore, in our study of Auke Bay, the objective was 

 to describe the annual cycles of oceanographic con- 

 ditions in the bay over several years. In this way the 

 complete annual cycle and the range of natural environ- 

 mental variability could be identified. From this infor- 

 mation the biological consequences of normal cycles and 

 anomalous oceanographic conditions could be assessed. 



DESCRIPTION OF STUDY AREA 

 Physiography and Geology 



Auke Bay is one of a large number of small embay- 

 ments off a system of large fiords connecting with the 

 open ocean (Fig. 1). It is located at lat. 58°22'N and long. 

 134°40'W, 19.3 km northwest of Juneau, Alaska, and 

 about 130 km inland from the open Gulf of Alaska. The 

 bay covers an area of about 11 km 2 and contains several 

 small islands and reefs. 



The bottom topography of the bay is irregular, and 

 submarine gulleys and mounts are quite common (Fig. 



