PARSONS. STEPHENS, and TAKAHASHI : LAKE FERTILIZATION. I. 



MAY I JUNE I JULY ' AUG I SEPT IQCT 

 197 



Figure 6. — Nitrate (^g at./liter) profiles at Station 1, 

 May to October 1969 and 1970. 



1200 



^, 1000 - 



E 



°- 800 

 ■o 



0) 



'£ 600 



Z. 400 



E 



200 



• N utilized in the woter column 

 A N added as fertilizer 

 O total N utilized 



F'MA'M'J'JASO 



Figure 7. — N utilization at Station 1. 



of inorganic nitrogen added as fertilizer (ex- 

 pressed per m- for the entire 51 km- lake sur- 

 face) is also shown; since this was utilized with- 

 in hours following each addition, the total ni- 

 trogen budget is represented as the sum of the 

 natural and added inorganic nitrogen. Some 

 mixing occurred during September and October, 

 and the utilization of inorganic N during this 

 period is shown as an indefinite extrapolation of 

 the nitrogen utilized by the end of August. From 



these curves and Figure 5 it may be seen that 

 the fertilizer was the principal source of new 

 nitrogen during the period July-August when 

 the lake nitrate was practically exhausted in the 

 euphotic zone. 



Ammonia values tended to show sporadic in- 

 creases during 1970, and at times ammonia may 

 have been the principal inorganic form of ni- 

 trogen in the lake, probably through being re- 

 cycled as excretory products of the zooplankton 

 (Beers, 1962). However, due to analytical dif- 

 ficulties with this radical, further investigation 

 of its seasonal behavior is required, especially 

 with reference to the verification of high values. 

 Phosphate showed similar variations to nitrate 

 although the depletion of phosphate was less reg- 

 ular. Seasonal concentrations ranged from 

 <0.01 to 0.04 fxg at. P/liter with about 3% of 

 the values falling in a much higher range of 0.1 

 to 0.6 /Jig at./liter. A determination of phos- 

 phate utilized and phosphate added (similar to 

 the inorganic N budget shown in Figure 7) was 

 difficult to describe because of the unpredictable 

 occurrence of phosphate throughout the summer; 

 this may have been due to phosphate regener- 

 ation. As an overall assessment, however, if a 

 winter level of 0.03 fig at. P/liter were complete- 

 ly utilized in the water column to 30 m, the 

 addition of 100 tons of 27-14-0 would increase 

 the supply of phosphate over the whole lake 

 by a factor of about 450% compared with the 

 increase in the inorganic nitrogen budget of ap- 

 proximately 100% (Figure 7). 



From winter to summer, silicate concentra- 

 tions ranged from about 1.8 to 3.0 mg silica/ 

 liter. According to Lund (1965) silicate be- 

 comes rate limiting for diatoms at about 0.5 mg/ 

 liter, which is considerably lower than the sea- 

 sonal range for Great Central Lake. 



BACTERIA 



Plate counts of bacterial colonies per 100 ml 

 are shown in Figure 8, together with the range 

 of counts obtained on several days when areal 

 surveys were made. During May, the total num- 

 ber of colonies per 100 ml was generally below 

 the mean value of ca. 9,000 reported by Henrici 

 (1940) for oligotrophic lakes; however, there is 



17 



