Measurements
are made with a Secchi disc to determine the depth of effective
light penetration into the lake. The greater this depth, the
clearer the water. If the Secchi disc reading is less than
3 metres, the lake is considered "enriched" (too
many nutrients). Data available back to 1971 indicate LBL measurements
have been consistently in the enriched range at less than 3 metres,
but improving since 1990. Clarity sampling was expanded in 1994
to include Kendricks Bay and the area north of Black Jack Island,
in addition to Oak Bay (the area south of the Narrows near the
mouth of Morton Creek). In 1998, measurements were initiated at
the Upper Lake dam in Delta to monitor input into LBL, and indicate
Upper Beverley Lake water quality is generally slightly better.
Results
deteriorated in 1998, due in part to unusually high water levels
in early July that resulted in increased watershed runoff. Clarity
improved in 1999 consistent with a regional trend attributed to
low rainfall and nutrients staying on land and in wetlands. Measurements
in 2000 display a slight decline (still better than 1998), but
still in the enriched range at less than 3 metres, reflecting
higher water levels/watershed runoff and unusually heavy algal
blooms in August.
The
annual Secchi disc data represent the average of weekly samples
taken through each summer. It is interesting to see the change
in water clarity as the lake progresses through its seasonal cycle.
Experience in 2000 demonstrated better clarity through July, but
deteriorated significantly in August reflecting high water levels,
increased watershed runoff and severe algal blooms.
