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



mg C/mg Chi a/hr were 1.12 and 1.98. At Sta- 

 tion 2 in 1970, however, the scatter of points is 

 so great that 95% confidence hmits become very 

 large. The probable reason for this is that the 

 station was sometimes in the area to which nu- 

 trients were first added, and sometimes the move- 

 ment of water containing freshly added nutrients 

 was away from Station 2 (Figure 1) . If in fact 

 it is assumed that there were only two alter- 

 natives in such a narrow lake (i.e., movement 

 of nutrients towards or away from Station 2) 

 then the 50% confidence limits for the asymp- 

 totic value of 4.17 mg C/mg Chi a/hr were 2.26 

 and 6.07. 



DISCUSSION 



The principal purpose of this report is to 

 establish the effect of inorganic nutrient enrich- 

 ment on the primary production of Great Central 

 Lake. From data in Figure 10 it is quite ap- 

 parent that primary productivity was increased 

 in surface samples during 1970 compared with 

 1969, both at Station 1 and particularly at Sta- 

 tion 2, which was very close to the area of re- 

 peated enrichment. However, while the effect of 

 nutrient enrichment was apparent to the extent 

 of a tenfold increase in surface primary pro- 

 ductivity, the integrated productivity for the 

 water column only showed an approximate dou- 

 bling in primary productivity during the first 3 

 months of nutrient enrichment (see Parsons 

 et al, in press, for primary production depth pro- 

 files) . This result is in keeping with the fact 

 that the total inorganic nitrogen addition to the 

 lake (Figure 7) was only sufficient to approx- 

 imately double the natural reservoir of inorganic 

 nitrogen in the upper 10 m, based on winter ni- 

 trate levels. However, it does not take into ac- 

 count nitrogen fixation by the blue-green alga, 

 Chroococcus, which may have taken arvantage 

 of the increased supply of phosphate to become 

 one of the predominant summer plankters. 



The question is, whether some factor other 

 than fertilization could have accounted for the 

 increased primary productivity? Firstly, it is 

 apparent that since the largest increase in pri- 

 mary productivity occurred at the surface, it can- 

 not be argued that the increased primary produc- 



tivity was due to greater enrichment of the water 

 column from the hypolimnion, especially in view 

 of the high degree of stratification (Figure 3) 

 and apparent nitrate depletion in the epilimnion 

 (Figure 6). It might be argued that the in- 

 creased productivity was due to an increase in 

 standing stock of primary producers and in- 

 creased radiation. Data in Figure 5 indicate 

 that the standing stock of primary producers 

 at Station 2 was generally higher than at Station 

 1 during 1969, although the effect is within a 

 95% probability of being due to within-lake 

 variations in standing stock of chlorophyll a. 

 However, in order to examine this question in 

 more detail, primary productivity data for Sta- 

 tions 1 and 2 in 1970 and Station 1 in 1969 have 

 been expressed as the production per unit chlor- 

 ophyll ft and plotted against the calculated light 

 intensity at various depths (Figure 11). This 

 presentation of data has been used by Ichimura 

 and Aruga (1964) to compare the productivity 

 of oligotrophic, mesotrophic, and eutrophic lakes 

 under conditions of different standing stocks of 

 primary producers, light conditions, and photo- 

 synthesis. From their findings it was concluded 

 that oligotrophic lakes had a productive index 

 of between 0.1 and 1.0 mg C/mg Chi a/hr, which 

 is very similar to the range of values computed 

 from the data in Figure 11 for Station 1 during 

 1969. The computed range for Station 1 during 

 1970 was appreciably higher, however, and en- 

 ters the classification for mesotrophic lakes 

 which have a photosynthetic index of up to 2 mg 

 C/mg Chi a/hr; finally the asymptotic value 

 (4.17) from Station 2 in 1970 is within Ichi- 

 mura's and Aruga's (1964) range for eutrophic 

 lakes, which the authors report as having photo- 

 sjnithetic indices of up to 6 mg C/mg Chi a/hr. 

 Since the only basis for this classification is the 

 effect of nutrient enrichment in enhancing the 

 photosynthetic response, it may be concluded 

 that our observed increase in primary produc- 

 tivity was determined by the artificial addition 

 of fertilizer. 



Secondary effects of nutrient enrichment may 

 also have influenced the primary formation of 

 particulate material through a heterotrophic 

 cycle. Unfortunately, our evidence for this is 

 not substantial and rests mainly on the increase 



21 



