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UBC Theses and Dissertations

Mixing processes from CTD profiles using a lake-specific equation of state : Quesnel Lake James, Christina


Quesnel Lake, is a deep (511m maximum depth) fjord-type lake in northeast British Columbia, Canada. Mixing processes in the lake exchange deep-water with surface water and contribute to the renewal of surface-water nutrients and oxygenated deep-water. These processes are of great consequence to the lake's trophic dynamics and understanding them will enable better management of the large salmon resources in Quesnel Lake. To better understand large-scale convective processes, a lake-specific equation of state was developed. Water samples were collected at locations around Quesnel Lake and analysed for ionic and non-ionic composition as well as other quantities that are integral to determining the lake's equation of state including pH, alkalinity and specific conductance. A relationship was developed to find lake water salinity from CTD data. Salinity was in turn related to density using a modified form of a general limnological equation of state. The equation of state developed for Quesnel Lake gives densities accurate to ± 0.0018kg/m³ whereas the general equation of state (based on seawater composition) is only accurate to ± 0.0158kg/m³ for Quesnel Lake water samples. The lake-specific equation of state was used to identify gravitational instability in density profiles estimated from CTD data. In order to compare water parcel density within a profile, the hydrostatic pressure effect must be removed. The three quantities that are used for this purpose, potential density, quasi-density and standard density, were compared. Quasi-density was found to be most appropriate for Quesnel Lake's deep water which is near the temperature of maximum density. Quesnel lake water column stability was quantified using the Brunt-Vaisala frequency calculated using quasi-density.

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