Other Nutrients and Trace Elements 



We measured all of the dissolved chemicals 

 our facilities permitted because more are either 

 known or suspected to be important in the pro- 

 duction of phytoplankton. In addition to those 

 measurements previously discussed, we de- 

 termined the concentrations of nitrogen, phos- 

 phorus, molybdenum, silica, iron, aluminum, 

 boron, and copper. 



Because nitrogen is essential for photosyn- 

 thesis, it is commonly considered as a factor 

 that may limit the production of phytoplankton. 

 Although the ultimate source of nitrogen is the 

 atmosphere, the combined nitrogen in lake water 

 is derived primarily from inflowing waters 

 (Hutchinson, 1957). Goldman (1960) found a 

 nitrate-nitrogen deficiency in Brooks Lake in 

 July and August 1957. In the lakes we studied 

 concentrations of nitrate nitrogen were highest 

 in the Mainland Systems and lowest in the 

 Peninsula Systems (table 3). The lower values 

 in the Peninsula Systems may be due in part to 

 greater use of nitrate nitrogen in the higher 

 production of phytoplankton." 



Some investigators have considered phos- 

 phorus as a possible limiting factor in the pro- 

 duction of phytoplankton in sockeye salmon 

 lakes (Krokhin, 1957; Nelson and Edmondson, 

 1955; Nelson, 1958; Goldman, 1960). Carcasses 

 of adult salmon are considered an important 

 source of replenishment of phosphorus to sock- 

 eye salmon lakes (Krokhin, 1957; Nelson and 

 Edmondson, 1955). The limit of detectability 

 of phosphate in our analysis of water samples 

 was about 0.01 to 0.02 p. p.m. In general, con- 

 centrations of phosphorus were below these 

 limits. Since a large part of the total phosphorus 

 is bound in plankton, and phytoplankters are 

 known to store more phosphorus than they need, 

 the rate of phosphorus turnover largely con- 

 trols the level in solution in water and, hence, 

 determines its availability. Additional measure- 

 ments and nutritional experiments are needed 

 to establish whether availability of phosphorus 

 is a factor that seriously limits production of 

 phytoplankton in the lakes under study. 



From limited nutritional experiments in 1962, 

 Goldman (1964) concluded that low concentra- 

 tions of molybdenum were limiting the phyto- 



' The low rates of productivity in the Ugashik and Alagnak systems 

 are based on only one and two observatii)ns, respectively. 



plankton growth in many lakes, including some 

 sockeye salmon lakes of Bristol Bay. We meas- 

 ured molybdenum by colorimetric and spectro- 

 graphic methods. Concentrations were low in all 

 lakes. Comparison of spectrograph ic analyses of 

 filtered water and phytoplankton from lakes of 

 the Wood, Kvichak, and Chignik systems in 

 1964 indicates that molybdenum was generally 

 detectable in water only. This finding indicates 

 that phytoplankters were not concentrating the 

 element. Experiments are needed to clarify the 

 role of molybdenum in the production of phy- 

 toplankton. 



Silica is necessary for the growth of diatoms 

 and, "the main mechanism of loss of silica from 

 lake waters is its utilization by diatoms. . . ."' 

 (Hutchin.son, 1957) . The peninsula lakes, except 

 for Kukaklek and Lower Ugashik, had the high- 

 est concentrations of silica. Lakes of the Wood 

 system had moderate levels ; Karluk Lake, al- 

 though highly productive, had considerably 

 lower concentrations than the other lakes. It is 

 likely that large amounts of silica are bound in 

 the phytoplankton, especially in Karluk Lake, 

 where Hilliard (1959) found that diatoms were 

 the dominant phytoplankters throughout the 

 year. 



Some differences were found also between 

 systems in concentrations of the other elements 

 detected regularly in spectrographic analyses of 

 water samples. Lakes in the Chignik system had 

 the highest concentrations of iron, aluminum, 

 boron, and copper ; the Kvichak systerh followed. 

 The concentrations of these elements were lower 

 but fairly uniform in the lakes of the several 

 systems tributary to Nushagak Bay. High con- 

 centrations of copper can inhibit growth of 

 phytoplankton, but the concentrations found in 

 this study were low. 



Dissolved Oxygen 



Concentrations of dissolved oxygen were near 

 saturation whenever lakes were sampled, re- 

 gardless of depth or season of year ; winter sam- 

 ples through the ice were taken only in Naknek 

 and Bi'ooks Lakes. 



THERMAL CHARACTERISTICS 



Variation in both photosynthetic rates and 

 water temperature during the summer is largely 

 due to variation in solar radiation. Photosyn- 

 thesis is at the base of primary productivity, 

 and temperature greatly influences the rate of 



SOCKEYE SALMON IN MAJOR RIVER SYSTEMS IN SOUTHWESTERN ALASKA 



415 



