tration even for inshore waters. Offshore waters usually 

 contain an order of magnitude less silica, although the 

 level is quite variable. 



The nitrate cycle in Auke Bay paralleled in general the 

 phosphate and silicate cycles. Maximum concentrations 

 of 24 to 28 ,ug-at. /liter were present throughout the water 

 column during the winter period (Fig. 16). The reduc- 

 tion of nitrate in the top 5 m from 24-28/^g-at./liter to 0.5 

 /ig-at./liter with the outburst of phytoplankton growth in 

 April is more drastic than in phosphate and silicate. 



The short-term fluctuations that occurred in concen- 

 trations of phosphate and silicate did not occur in nitrate 

 concentrations. The top 5 m were essentially depleted of 

 nitrate by May and remained impoverished through 

 August. We attribute this to a slow regeneration rate of 

 nitrate from organic combinations, which according to 

 the experiments by Brand and Rakestraw (1941) requires 

 3 or 4 mo. Riley (1967) presented indirect evidence that 

 the nitrogen cycle is significantly slower than the phos- 

 phate cycle. 



The summer pycnocline is enough of a barrier to pre- 

 vent nitrate from reaching the surface in any significant 

 quantity (this is true also for phosphate and silicate), ex- 

 cept during periods when wind velocity is at least 4 m/s 

 for at least 24 h. Periodic measurements of the nitrate 

 content of fresh water entering Auke Bay show that only 

 negligible amounts of nitrate enter the bay from these 

 sources." 





























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SUMMARY 



This study has described temporal variation in 

 oceanographic conditions over an 8-yr period and 

 provides background information on environmental 

 variability for future detailed studies of the ecology of 

 Auke Bay. Salient environmental features of an annual 

 cycle in Auke Bay are summarized below. 



Auke Bay is about 130 km from the open ocean and is 

 relatively isolated from direct oceanic moderation and 

 from major circulation patterns of the main passages and 

 straits in southeastern Alaska. The bay is strongly in- 

 fluenced by land runoff and by the local climate. The an- 

 nual oceanographic cycles closely follow local atmos- 

 pheric cycles. 



By the end of the second week in April, the water 

 column began to stratify because of increased absorp- 

 tion of solar radiation in the upper 5 m and the ac- 

 cumulation of fresh water on the surface from increased 

 runoff and precipitation. As the spring-summer season 

 progressed, stratification increased until a strongly 

 developed pycnocline essentially isolated the upper 12 to 

 20 m from the deeper water. 



Full development of the brackish surface layer in Auke 

 Bay occurred in late July or August. In September, cool- 

 ing of surface water coupled with wind mixing due to fall 

 storms broke the density stratification developed during 



'Unpublished data on file Northwest and Alaska Fisheries Center Auke 

 Bay Laboratory, NMFS, NOAA, P.O. Box 155, Auke Bay, AK 99821. 



Figure 16. — Observed concentrations of dissolved nitrate (pg-at./ 

 liter) at selected depths in Auke Bay, Alaska, 1963-67. 



the spring-summer period. Nutrient-rich water below the 

 pycnocline was mixed into the euphotic zone to replenish 

 the nutrient-impoverished surface water, and a fall 

 phytoplankton bloom occurred. 



As the season progressed into fall, decreasing 

 temperatures and increasing salinities in the surface 

 layer of water increased the density of the surface water, 

 resulting in thermohaline circulation which thoroughly 

 mixed the water column. Hence, during the winter, the 

 water column was essentially homogeneous, and the con- 

 centrations of phosphate, silicate, and nitrate 

 throughout the water column were high. A well-mixed 

 water column and low solar radiation prevented any sig- 

 nificant phytoplankton growth during the winter; con- 

 sequently, the distribution of biologically active dis- 

 solved components depended almost totally on physical 

 processes during the winter. 



The seasonal pattern described was repeated each year 

 during the 8-yr period that routine observations were 

 made. 



LITERATURE CITED 



ATKINS. VV. R. G. 



1930. Seasonal variations in the phosphate and silicate content of 

 sea-water in relation to the phytoplankton crop. Part V. Novem- 

 ber 1927 to April 1929, compared with earlier years from 1923. 

 J. Mar. Biol. Assoc. U.K. 16:821-852. 



10 



