BACKGROUND

Clearwater Lake is a large ovoid shaped lake located approximately 25 km northwest of The Pas It has a surface area of approximately 289 km² and a mean depth of 13 m. Intensive water quality surveys of Clearwater Lake were conducted in June 1989 and August 1996. These surveys included 50 sites in 1989 and 23 sites in 1996 (Sorba and Hughes, 1997). Water was analyzed for 37 parameters including general chemistry, nutrients, fecal coliform bacteria, and heavy metals. Also, analyses was done for eight of the most commonly used pesticides at 10 of the sites in the 1989 survey. None of the pesticides were detectable and therefore they were not sampled in 1996. In 1989 there were six exceedences of water quality objectives for copper, and one each for lead, manganese and total phosphorus. In 1996 there were no exceedences of water quality objectives for metals but there were 18 out of 23 exceedences of the objectives for total phosphorus. The high phosphorus values in 1996 appeared to be unusual and it was postulated that phosphorus may have been re-suspended from the sediment due the stormy weather at the time of sampling. Therefore, further sampling was initiated in 1997 to provide more information on total phosphorus and other related parameters.

METHODS

During 1997 water quality sampling was carried out at five sites (Figure 1). Aside from total phosphorus water samples were analyzed for total Kjeldahl nitrogen, total ammonia (ammonia and ammonium) nitrogen, nitrate-nitrite nitrogen, turbidity and pH both during the winter and summer months. In addition, during the summer analyses for chlorophyll a was also carried out and Secchi disk depths were measured.

Sample collection differed between summer and winter samples. On February 23, 1997 water samples were collected at the surface, one metre, and five metre depths at water quality sites WQ1201, WQ1208, and WQ1209 (Figure 1). Summer water samples were collected at sites WQ1201, WQ1208, WQ1214, and WQ1224 on July 7, August 5 and 17, and September 4, 1997. Sampling procedure during the summer included measuring the Secchi disk depth. The Secchi disk is a 20 centimetre diameter black and white disk which is lowered into the water to a point where it disappears from sight and the depth at which this occurs is recorded. Samples were collected where possible throughout the water column to a depth of twice the Secchi disk depth. A depth of twice the Secchi disk depth is the approximate depth of the euphotic zone, or zone of light penetration where algal photosynthesis can occur.

RESULTS AND DISCUSSION

The physical and chemical data for each site sampled in 1997 are listed in Appendix 1 and summarized in Table 1. Table 2 includes average annual values for 1997 data along with comparable data from 1943 to 1996. The main emphasis of the water quality sampling in 1997 was to determine if there were any total phosphorus values which exceeded water quality objectives. This appeared to have been a potential problem in August of 1996 (Sorba and Hughes, 1997).

Fig. 1: Clearwater Lake Map

Tables 1 and 2

Nutrients

Nutrients required for the growth of algae and other plants include phosphorus and nitrogen compounds. Analyses were conducted for total phosphorus, total Kjeldahl nitrogen, ammonia-ammonium nitrogen (also known as total ammonia), and nitrate-nitrite nitrogen. Also, un-ionized ammonia can be calculated from the ammonia-ammonium nitrogen if pH and temperature at time of sampling are known.

Phosphorus often limits algae growth since it is likely to be the least abundant nutrient. Total phosphorus values exceeding 0.025 mg/L in lakes are considered to be in exceedence of water quality objectives (Williamson, 1988). In 1997 phosphorus results were all low and the total phosphorus never exceeded the objectives (Appendix 1). In comparison to other years, the average 1997 total phosphorus data was also one of the lowest (Figure 2). It appears that the high exceedence of objectives rate noted in August 1996 was a very unusual occurrence and not indicative of a trend toward reduced water quality.

Figure 2. Average phosphorus in Clearwater Lake.

Figure 3. Average nitrogen compounds in Clearwater Lake.

Nitrogen analytes generally showed low average values with an unusual peak in 1988 (Figure 3). The water quality objective relating to drinking water for nitrate and nitrite nitrogen is 10 mg/L and this was never exceeded. Un-ionized ammonia values and un-ionized ammonia objectives were calculated for three samples in 1989, 23 samples in 1996 and eight samples in 1997. The objective for un-ionized ammonia relates to the protection of aquatic life and varies depending on temperature and pH. Un-ionized ammonia was well below objective levels in all cases.

Turbidity

Turbidity, measured during a number of years, was very low although there was an unusual peak in 1962 (Figure 4).

Figure 4. Average turbidity in Clearwater Lake.

Figure 5. Average transparency in Clearwater Lake.

Secchi disk transparency provides a visual estimate of water clarity. Two Secchi disk depths of approximately 10 m were recorded in 1943. These Secchi disk measurements were taken in deep areas of Clearwater Lake and show excellent water clarity (Figure 5). The average Secchi disk transparencies in 1970 and 1997 were lower at 5.36 and 5.72 m, respectively (Table 2). However, in 1997 at water quality site WQ1201, there was not sufficient depth to provide a proper estimate of transparency since the lake bottom could usually be seen. If data from site WQ1201 is excluded the average Secchi disk depth in 1997 would be 6.79 m (Table 1). The Secchi disk was visible at 9.5 m on one occasion at the deepest water quality site WQ1214 (Figure 1 and Appendix 1).

pH

Other analytes examined in 1997 were pH and chlorophyll a. The pH never exceeded the upper and lower water quality objectives of 6.5 and 9.0 recommended for the protection of aquatic life (Appendix 1). The pH was higher during the summer than during the winter of 1997, probably due to the activity of algae in the summer (Table 1).

Chlorophyll a

Chlorophyll a was analyzed from samples collected during the summer of 1997 in water samples collected through the water column within the euphotic zone. A depth of twice the Secchi disk depth is the approximate depth of the euphotic zone, or zone of light penetration where algae photosynthesis can occur. Chlorophyll a is extracted from the algae in the water samples. The chlorophyll a results were low, generally less than the detection limit (Table 1 and Appendix 1).

CONCLUSIONS

• Water quality in Clearwater Lake remains excellent. High phosphorus observed in 1996 and reported by Sorba and Hughes (1997) appear to be anomalous and do not reflect a general decline in water quality. Sampling should be conducted at periodic intervals in the future to ensure that this high level of water quality is maintained.

ACKNOWLEDGMENTS

We are grateful for the assistance of Steve Davis and Andreas Oertel of Manitoba Environment and members of the Clearwater Lake Cottage Owners Association especially Jim Atkins, Robin Carpenter, Mary Carpenter and Jim Coyne who were involved with water sample collections.

REFERENCES

Sorba, E.A. and C.E. Hughes. 1997. Water quality assessment of Clearwater Lake, Manitoba, Canada. Manitoba Environment Report # 97-04. Manitoba Department of Environment. Winnipeg Manitoba.

Williamson D.A. 1988. Manitoba surface water quality objectives. Manitoba Department of Environment and Workplace Safety and Health. Winnipeg Manitoba.