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UBC Theses and Dissertations

Mathematical modelling of the chlorophyll distribution in the Fraser River Plume, British Columbia De Lange Boom, Bodo Rudolf


The horizontal chlorophyll a distribution observed in the Strait of Georgia near the mouth of the Fraser River appears to reflect the influence of the river discharge. Mathematical models are developed to attempt to explain the observed distribution in terms of such factors as the velocity field, the available light and the grazing and sinking of the phytoplankton population. A steady state, two dimensional model is developed for the upper layer. The downstream velocity is modelled using a modified form of the downstream velocity in a jet; the vertical entrainment is represented by an empirical expression, while the cross-stream velocity is calculated from the vertically integrated continuity eguation. A vertically integrated conservation equation is written for the chlorophyll concentration by balancing advection against the source-sink term (net production minus grazing and sinking). Temperature effects are not modelled directly and nutrients are not considered as limiting. The first model is simplified by assuming: a constant depth of the upper layer, vertical entrainment proportional to the downstream velocity, and a uniform vertical distribution of chlorophyll. In model II the layer depth varies with distance from the river mouth, a more complex relation for the vertical entrainment is used and more realistic vertical profiles are employed for the horizontal velocity and the chlorophyll concentration. Although the observed downstream maximum in the horizontal chlorophyll distribution is not reproduced, the results indicate that the velocity field, the available light in the water column and the value of the maximum production rate (a function of water temperature) are the most important parameters influencing the distribution. Sinking is of secondary importance while grazing appears to be relatively unimportant.

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