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



Figure 1. — Great Central Lake showing principal sam- 

 pling stations (1, 2, and 3) and area of nutrient en- 

 richment ( crosshatched ) . 



and west at rates of up to 6 miles per day or 

 ca. 10 km/day (Parsons et al., in press). Thus 

 while Station 2 was generally under the most 

 immediate influence of the nutrient additions, 

 Stations 1 and 3 also received an accumulative 

 enrichment. Fertilizer was added at the rate of 

 5 tons per week from June through to October 

 1970. During May 1970 approximately 2 tons of 

 fertilizer were added in experiments to deter- 

 mine the rate of mixing and distribution of nu- 

 trients in the vessel's wake. 



RESULTS 



LAKE MORPHOMETRY 



Great Central Lake is located on Vancouver 

 Island, B.C., at lat 49°20' N on an east/ west axis 

 between long 125°00' W and 125°25' W (Figure 

 1). It is a long narrow lake (ca. 33 X 1.5 km) 

 with steep sides and a mean depth of 200 m. 

 The yearly mean discharge is approximately 6 

 X lO^mVday with a range from 0.4 x 10^ to 

 32 X lO^mVday. 



TEMPERATURE 



The temperature structure at Station 1 is 

 shown in Figure 2. The results are representa- 

 tive for the open waters of the whole lake, and 

 it is apparent that the lake was isothermal dur- 

 ing January and February; a thermocline 

 started to form during March and was well 

 established by May. Maximum surface tem- 

 perature during July was 21.2° C; surface cool- 

 ing started in September but a thermocline of 



Figure 2.— Temperature (°C) stratification at Station 1. 



10° persisted through October and the lake did 

 not become isothermal until January of the fol- 

 lowing year. 



RADIATION 



Changes in photosynthetically active radiation 

 (PAR) at the lake surface are shown in Figure 3 

 together with the mean radiation for the water 

 column to 20 m, calculated -on a 24-hr basis. 



Figure 3. — (A) Surface photosynthetically active ra- 

 diation (PAR) and (B) 15-day mean PAR in the first 

 20 m. 



From the latter results it is apparent that radi- 

 ation in the water column increased by 50% from 

 the beginning of May until the middle of June; 

 the decrease in radiation during the second part 

 of June was due to a combination of higher ex- 

 tinction coefficients and lower surface radiation. 

 The average radiation remained virtually con- 

 stant during July and decreased by 20% during 

 the latter half of August. 



CHLOROPHYLL A 



Surface chlorophyll a concentrations are pre- 

 sented in Figure 5 in the same way and for the 

 same stations and years as SD data in Figure 4. 

 The two figures have some mirrored similarities ; 



15 



