Chlorophyll distribution in a temperate estuary: The Strait of Georgia and Juan de Fuca Strait

Data collected during 7 years of seasonal surveys are used to investigate the distribution of phytoplankton biomass within the estuarine waters of the Strait of Georgia and Juan de Fuca Strait. Variability of the chlorophyll distribution is examined in relation to the density stratification, light availability and nutrient concentration. In the Strait of Georgia, both the horizontal and vertical distribution of chlorophyll are found to be linked to the presence of a near-surface layer of increased density stratification. Despite important year-to-year variability, the seasonal cycle of chlorophyll in the Strait of Georgia is dominated every year by relatively large near-surface concentrations in the spring that are linked to the seasonal increase in solar radiation onto the stratified near-surface layer. In the vertical, a sub-surface peak is observed around 10 m depth, corresponding to the depth of maximum water column stability. Nutrients within the euphotic zone are in general abundant, with the exception of the Strait of Georgia in summer where phytoplankton growth is potentially limited by low nitrate concentration near the surface. The depth of the euphotic zone is estimated along the thalweg of the estuary from transmissometer profiles. It appears to vary relatively little within the estuary from a minimum of 20 m in spring, near the mouth of the Fraser River, to an autumnal maximum of about 30 m in the northern Strait of Georgia. Finally, the estimated self-shading contribution to light attenuation is shown to be generally significant (5–10%) in the surface waters of the Strait of Georgia, during spring and summer, reaching values as high as 35% during the spring bloom.