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Figure 14. — Observed concentrations of inorganic phosphate (/jg-at./ 

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



Figure 15. — Observed concentrations of dissolved silicate ((jg- 

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



at./ 



between 0.30 and 0.20/zg-at./liter, versus a winter high of 

 3.0 to 3.5 pg-at./liter (Fig. 14). 



Short-term increases in phosphate concentrations in 

 the top 5 m from May through August did not have a 

 strong coincidence with periods of increased runoff. 

 Hence, freshwater input was discounted as a significant 

 cause of increases in nutrients, including phosphate, over 

 short time periods (3 to 7 days). Studies by Curl, Iver- 

 son, and Conners 5 showed that tidal effects in Auke Bay 

 were not sufficient to mix nutrient-rich water into the 

 euphotic zone, but that wind-induced mixing through 

 the pycnocline could be caused by winds of 4 m/s per- 

 sisting for 24 h or longer. 



Although the causes of fluctuation in phosphate levels 

 are not clear, the most probable causes are regeneration 

 of inorganic phosphate from organically bound phos- 

 phate and wind-induced vertical mixing of inorganic 

 phosphate through the pycnocline. Laboratory 

 measurements of the rate of phosphate regeneration 

 (Cooper 1935) indicate that up to one-half of the total 

 phosphorous content of decomposing plankton appears 

 in soluble form within 24 h. 



'Curl, H. C, Jr., R. L. Iverson, and H. B. O'Conners, Jr. 1971. Pela- 

 gic ecology of biological production in Auke Bay, Alaska. Final report to 

 National Marine Fisheries Service, Auke Bay Fisheries Laboratory, Auke 

 Bay, Alaska, Contract 14-17-0005-207. 



The silicate cycle in Auke Bay (Fig. 15) generally 

 paralleled the phosphate cycle; this similarity was also 

 demonstrated by Richards (1958) in the western Atlan- 

 tic. Studies in the English Channel by Atkins (1930) 

 showed that silicate was depleted and regenerated simul- 

 taneously with phosphate. 



While phosphate and silicate in Auke Bay had the 

 same general seasonal patterns of distribution, some dif- 

 ferences appear when the data are examined in detail: 

 short-term variation in concentration of phosphate oc- 

 curred primarily in the upper 5 m of the water column 

 (Fig. 14); the greatest variations in silicate occurred 

 between 10 and 20 m (Fig. 15). Variability in the con- 

 centration of silicate throughout the water column in 

 Auke Bay is generally greater than it is for phosphate. 



Minimum silicate concentrations of 2 to 4 yug-at./liter 

 occurred in the surface water of Auke Bay during July, 

 August, and September (Fig. 15). Short-term increases 

 occurred in late June and early July, when the propor- 

 tion of dinoflagellates (phytoplankton which do not 

 utilize silicate) to diatoms was quite high. We inter- 

 preted this increase as a reflection of the difference in 

 rate of uptake by the relatively small number of diatoms 

 present and the rate of in situ regeneration, plus advec- 

 tive input. 



Maximum silicate concentration of about 60 /ug- 

 at./liter occurred throughout the water column from 

 December through the middle of March — a high concen- 



