UBC Theses and Dissertations
Wind-driven upwelling and nutrient supply in a productive estuarine sea Moore-Maley, Benjamin Lee
Marine ecosystems are under increasing pressure due to climate change. Wind-driven circulation in the upper ocean is one of the primary ways that climate determines ecosystem behavior. One example is wind-driven upwelling of nutrients to the euphotic zone. In small enclosed seas where stratification is often strong and wind sporadic, the significance of nutrient upwelling is not well-established. These small systems can be important habitats for juvenile migrating fish such as Pacific salmon. In this thesis, wind-driven upwelling and the effect on surface nitrate availability were investigated in the Strait of Georgia on the Canadian Pacific coast using a high-resolution, coupled biophysical ocean model. This investigation was conducted in three parts. First, the model skill was evaluated against observations from local monitoring programs, and the sensitivity of tuning the surface wave breaking parameterization using wave model results from the region was tested. Second, principal component analysis (PCA) was performed on five years of modelled hourly surface nitrate and temperature fields to identify the wind-driven upwelling modes and determine their significance relative to other processes. Spectral analysis of the principal component (PC) loadings and correlations between the PC loadings and the surface wind stress were used to attribute the dominant PCA modes to physical phenomena. Third, depth of upwelling estimates from cross-shore density transects during a comprehensive set of wind events across the five-year simulation were compared to a theoretical cross-shore upwelling model depending only on wind stress, stratification and cross-shore bottom slope to identify the physical parameters that control upwelling. Upwelling accounted for approximately one-third of summer surface nitrate variance based on the PCA results. Modelled upwelling depth generally agreed with the theoretical prediction and was thus interpreted to depend primarily on wind stress and stratification. The deepest upwelling and strongest nitrate anomalies occurred in the northern Strait of Georgia, which is consistent with the presence of an along-axis stratification gradient due to the Fraser River. These results establish a direct link between climate forcing and the factors that determine upwelling in the Strait of Georgia, with strong implications for summer productivity in the region.
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