Paleolimnological elucidation of the historical water quality of Sooke reservoir, Victoria, British Columbia
Date
1995
Authors
Barraclough, Cori L.
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Abstract
Following impoundment, reservoirs commonly undergo a rapid increase in planktonic primary productivity, known as "trophic upsurge," followed by a slower return to a more stable level of productivity. This upsurge is fueled by a pulse of nutrients, particularly phosphorus, derived from inundated soils and decomposing vegetation. This upsurge can be of concern in drinking water supplies where algal blooms may cause taste and odour problems. Sooke Reservoir, which supplies potable water to 80% of the population (312,500) in the Greater Victoria region, was created by damming the outflow of Sooke Lake in 1913 and raising the water level 3.7 m. The water level was subsequently raised 6.3 m in 1970 by the construction of a second dam. Future plans call for raising the water level an additional 6.3 m in 1998 but it is not known what effect this may have on the quality of water in the reservoir. Since neither historical phytoplankton nor water quality data are available, this paleolimnological study of sedimentary diatoms was undertaken to determine if changes in planktonic species composition or abundance, geochemistry or sedimentation rate, which occurred post-European settlement, were evident within Sooke Reservoir and could be attributed to changes in the watershed which have occurred in the past 125 years. An examination of the diatom microfossil record and geochemical and age profiles of deep water sediment were used to assess these changes and their relationship to known disturbances in the watershed, especially the effects of previous dam construction or other ecosystem perturbations.
Diatom fossils revealed that the composition and abundance of the phytoplankton community underwent a rapid change beginning in 1890. By the mid-1920's the evidence of this shift had disappeared. The onset of a change in the phytoplankton community pre-dated reservoir construction and is thought to be a result of fish removal and altered trophic interactions. Other watershed disturbances including logging, road construction and fire have not resulted in significant changes in the phytoplankton community; however, the composition of this community has become increasingly variable over time. Increased variability may be the result of varying seasonal water levels and/or nutrient loading due to exposure of shoreline sediments as the reservoir is drawn down and refilled. Increasing the storage capacity of Sooke Reservoir may benefit the phytoplankton by reducing the magnitude of drawdown and stabilizing both water levels and nutrient inputs.
The sedimentation rate in Sooke Reservoir ranged between 1.1 and 1.7 mm/yr and was highest in the eighteenth century and lowest from 1898-1918. It has not increased as a result of recent watershed disturbance. Metal concentrations in the sediments were generally consistent with concentrations found in other Vancouver Island lakes.
Though the phytoplankton community of Sooke Reservoir is extremely sensitive to change in the short term, historically it demonstrated resilience and was able to integrate change and regain equilibrium within a few decades. What cannot be ascertained from this study is the threshold for alteration beyond which irreversible change will occur. This question must be addressed in-situ under present conditions. Raising the water level of the reservoir in the future may not have the same effect as it did in the past, because ecosystem response to disturbance is often non-linear. Caution must therefore be exercised when creating any disturbance within the watershed.