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Introduction

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Cusheon Lake is located on Salt Spring Island. The watershed for this small lake is 7.24 km2 (Figure 1). The surface area of the lake is 26.9 ha and it has an average flushing rate of once in 0.8 years. The lake has a maximum depth of 9.14 m and a mean depth of 4.5 m (Figure 2).

Blackburn Creek flows southeast from Blackburn Lake and is the major inflow into Cusheon Lake. Also, there are several intermittent ditches and creeks that drain the north and south sides of the drainage basin. Cusheon Lake drains into Cusheon Creek that empties into the Pacific Ocean at Trimcomali Channel.

The Ministry of Environment, Lands and Parks monitored the water quality at various depths over the deepest point (9.14 m) of Cusheon Lake between 1974 and 1995. The data are stored on the provincial database, EMS (formerly SEAM), under station number 1100123 (Figure 2). The two purposes for monitoring the water quality of Cusheon Lake were to identify:
· long-term changes in water quality as a consequence of development within the watershed; and
· how these changes may impinge on certain uses of water from the lake.

The Capital Health Region collected water samples for bacteriological analyses from Beaver Beach on Cusheon Lake (Figure 2). Weekly sampling began in April each year and continued through the bathing season, ending in September. Fecal coliform results from five samples collected within a 30-day period were used to establish a geometric mean at the beginning of the season. A beach advisory notice, warning of the potential for increased risk to bathers' health, was considered for posting if the geometric mean exceeded 200 fecal coliforms/100 mL over a 30-day period. More intense sampling may have occurred if the results of a single sample exceeded 400 fecal coliforms/100 mL.

This report assesses 22 years of water quality data. These data consist of:

- four years (1974 -1976, 1980) of intensive water quality
sampling,

- 22 years (1974-1995) of spring overturn water quality
sampling (1974-99 for total phosphorus), and

- 13 years (1980-1995; no samples were collected in 1987
and 1989) of fecal coliform sampling.

The water quality data are plotted in Figures 3 to 28 and summarized in Tables 1 and 2.

The box plots in Figures 3 to 28 represent the variability of water quality indicators collected at the surface, mid-depth, and near the bottom of the lake. Each plot is comprised of a rectangle with the top portraying the upper quartile (75th percentile of the data series, Q(0.75)), the bottom portraying the lower portion (25 th percentile of the data series, Q(0.25)), and a horizontal line within the rectangle portraying the median. Vertical lines extend from the ends of the rectangle to the adjacent values, also known as "whiskers", and were defined by:

- computing the interquartile range, IQR=Q(0.75)- Q(0.25);
- defining the upper adjacent value as the largest observed
value between the upper quartile and the upper quartile plus 1.5 X IQR;
- defining the lower adjacent value as the smallest observed
value between the lower quartile and the lower quartile
minus 1.5 X IQR.

Values that fall outside the range of the adjacent values are defined as "outside values" and were plotted as asterisks (*). Values are defined as "far outside values" if they are located outside the outer range, which is defined as the upper quartile plus 3 X IQR or the lower quartile minus 3 X IQR. These values are plotted as empty circles (O).

Trends in water quality data collected at different depths and at different frequencies over time were assessed by comparing yearly changes in median values in conjunction with the size of sample variability. The size of sample variability is represented in the box plots by the rectangle, whiskers, and the two types of outliers. A change is observed when the median values and sample variability do not overlap.

Figure 2 Bathymetric map of Cusheon Lake

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