growth of fish; therefore, we measured solar 

 radiation and water temperature in our limnol- 

 ogy studies. Many data were collected, but only 

 general comparisons were made among the sys- 

 tems. 



Thermal stratification and development of 

 thermoclines are most pronounced in lakes pro- 

 tected from the mixing action of strong and 

 persistent winds. During our study, thermoclines 

 usually were well devel.'>;/ed in the upper lakes 

 of the Wood system, which are generally pro- 

 tected from strong winds ; they were less well 

 defined in the lower, more exposed lakes. The 

 larger, broadly exposed lakes, such as Iliamna 

 and those of the Naknek system, were generally 

 without thermoclines. Thermoclines were com- 

 mon in the smaller lakes, such as Chignik and 

 Karluk, but were easily destroyed by the strong 

 persistent winds which result from the lakes' 

 exposure to the ocean climate. 



PRIMARY PRODUCTIVITY AND STANDING CROP 

 OF PHYTOPLANKTON 



The measure of primary productivity used in 

 this study was the rate of carbon fixation by 

 phytoplankton per 4-hour midday incubation 

 period throughout the water column, calculated 

 on the basis of a square meter of the surface 

 and a cubic meter in the euphotic zone. The rate 

 of carbon fixation was determined by the in situ 

 light- and dark-bottle C" technique (method I 

 of Steemann Nielsen, 1952) . The measure of the 

 standing crop of phytoplankton was the concen- 

 tration of chlorophyll a. In general, rates of car- 

 bon fixation were lower in 1961 than in 1962, 

 but the values for 1962 rather than the averages 

 for 1961-62 are used here to compare lakes be- 

 cause productivity measurements were available 

 for more lakes in 1962. These data along with 

 concurrent measures of the transparency of the 

 water (Secchi disk extinction values) are given 

 in table 6. 



The results of our measures of basic produc- 

 tivity are discussed by system. 



Mainland Systems 



Among lakes of the Mainland Systems, those 

 of the Nuyakuk rank highest in concentrations 

 of total dissolved solids and alkalinity; yet in 

 the single series of observations in two lakes of 

 the system (Tikchik and Chauekuktuli) , phy- 

 toplankton production and standing crop were 

 low. The average Secchi disk reading of more 



than 15 m. in Lake Chauekuktuli was the high- 

 est in any of the systems — a further indicator 

 of low phytoplankton density; and the rate of 

 carbon fixation per cubic meter had the lowest 

 average for any of the lakes. Deficiencies of 

 trace elements may limit productivity. 



The single series of observations of photosyn- 

 thetic rates in Ualik Lake of the Igushik system 

 and Lake Nunavaugaluk of the Snake system 

 indicated that they were intermediate in the 

 ranking among all systems tested and were simi- 

 lar to those of the Wood system. The standing 

 crops of phytoplankton, however, were lower. 



The lakes of the Wood system had carbon 

 fixation rates, chlorophyll a concentrations, and 

 Secchi disk readings intermediate in the range 

 of values for all systems. The photosynthetic 

 rate and chlorophyll a concentration in Little 

 Togiak Lake were slightly higher than in the 

 other lakes. The differences among lakes in the 

 Wood system were rather small. 



Rates of carbon fixation were only slightly 

 lower in lakes of the Kvichak system than in 

 lakes of the Wood system. The photosynthetic 

 rate per cubic meter in the euphotic zone and 

 the chlorophyll a concentrations were as high 

 in Lake Clark as in Iliamna Lake, but the total 

 carbon fixed in the water column was less in 

 Lake Clark because of the restricted depth of 

 the euphotic zone (caused by rock flour). The 

 Secchi disk extinction value was only 3.8 m. in 

 Lake Clark, in contrast to 9.8 m. in Iliamna Lake 

 and 8.7 to 12.6 m. in lakes of the Wood system. 



Peninsula S\'Stems 



Among lakes of the Peninsula Systems, those 

 of the Alagnak system ranked near the bottom 

 in basic productivity and also had low standing 

 crops of phytoplankton. 



Lakes within the Naknek system had a wide 

 range in productivity. Shallow Coville Lake is 

 very productive and ranks next to lakes of the 

 Chignik and Karluk systems in primary pro- 

 ductivity and standing crop of phytoplankton, 

 whereas deep and clear Brooks Lake is among 

 the least productive systems. The Secchi disk 

 readings in each lake were corre.spondingly low 

 and high. 



In the Ugashik system the primary productiv- 

 ity and .standing crop of phytoplankton in Low- 

 er Ugashik Lake (the only lake examined) were 

 about as low as in the Alagnak system. 



Lakes of the Chignik system had by far the 



416 



U.S. FISH AND WILDLIFE SERVICE 



