Wahleach Project Water Use Plan Wahleach Reservoir ... - BC Hydro

4.3 Water Chemistry
The mean pH was 7.5 in 2004, 6.6 in 2005 and 7.0 in 2005 (Table 2). The mean pH
for the study period was 7.0, which was similar to previously reported values by
Perrin and Stables (2000). The pH was the same for the surface and deep samples.
Table 2. Mean concentration or measurement of chemical parameters and Secchi
depth in Wahleach Reservoir, 2004-2006.
Parameter
Mean concentration or measurement
2004 2005 2006
1 m 20 m 1 m 20 m 1 m ML*
Secchi depth (m) 4.6 - 4.4 - 3.9 -
Chl a (μg·L -1 ) 2.4 - 2.1 - 6.4 -
pH
DO
7.5 7.5 6.6 6.5 7.0 7.1
Alkalinity (mg·L -1 ) - - 10.9 10.6 10.6 11.0
Silicic Acid (mg·L -1 ) 5.1 4.9 5.1 5.1 2.9 2.8
TP (μg·L -1 ) 6.5 4.7 4.1 4.2 6.4 6.5
TDP (μg·L -1 ) 4.0 4.8 2.5 2.0 3.1 3.5
SRP (μg·L -1 ) 1.8 2.0 2.7 2.5 1.4 1.9
TN (μg·L -1 ) 156.7 146.7 167.0 197.0 149.2 158.5
NO3-N (μg·L -1 ) 38.8 56.5 33.7 78 48 52
TN:TP 24.2 31.5 40.8 47.0 23.2 24.4
* Mixed layer depth as determined by temperature profiles
The inorganic nutrients, phosphorus and nitrogen, are known to control lake
productivity (Schindler, 1977, Hecky and Kilham 1998, Guildford and Hecky 2000)
and many forms (total and inorganic) of both nutrients need to be studied when water
monitoring is completed. Total phosphorus concentrations are an indicator of
biomass and trophic state and in Wahleach Reservoir they ranged from 4.1-6.5 μg·L -
1 , which is indicative of an oligotrophic system (Wetzel 2001). There was a modest
decrease in TP in 2005 to 4.1 μg·L -1 likely due to reduced fertilizer loading (Table 2).
While TP is a measure of biomass and trophic state, it is not the form of phosphorus
which is biologically available to phytoplankton. SRP concentrations ranged from
1.4-2.7 μg·L -1 , which are below the concentrations known to limit phytoplankton
growth. This suggests that the phosphorus added to the epilimnion as fertilizer is
immediately taken up by phytoplankton.
Epilimnetic concentrations of total and dissolved forms of nitrogen were also low
(Table 2). Total nitrogen concentrations ranged from 149 ± 20 μg·L -1 in 2006 to 167
± 19 μg·L -1 in 2005, which is extremely low and indicative of ultraoligotrophic
conditions (Wetzel 2001). Of the utilizable forms of nitrogen by phytoplankton,
Wahleach Reservoir Fertilization Project, 2004-2006.
17