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Importance of wind and river discharge in influencing nutrient dynamics and phytoplankton production in summer in the central Strait of Georgia

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A cruise was conducted during August 6-14, 1991 to investigate the dynamics of nutrients and phytoplankton production in the central Strait of Georgia, British Columbia, Canada, during a period when strong stratification resulted in nitrogen-limited primary productivity. High resolution vertical profiles of salinity, temperature, fluorescence and nutrients (nitrate and phosphate) were taken daily along a transect. A wind event occurred on August 7 and a rapid increase in the Fraser River discharge took place from August 8 to 14. The wind event mixed the water column and nutrients increased at the same time. Phytoplankton responded to the increase in nutrients and a bloom occurred soon after the wind event. The rapid increase in river discharge caused the entrainment of nitrate in the estuarine plume and, as a result, a subsurface maximum of chi a was developed. Our results clearly demonstrated that summer phytoplankton productivity in the central Strait of Georgia is fueled by a supply of nutrients from the nitracline through vertical mixing induced by the interaction of winds, river discharge and tidal cycles. Of these 3 factors, winds are the most variable and therefore a summer with frequent wind events could result in higher than normal productivity. The mechanism for this is that part of the nitracline was maintained above the euphotic zone due to various physical processes in spite of the strong stratification, and therefore, nutrients were frequently available for phytoplankton uptake caused by across- pycnocline mixing due to wind, river discharge and tides.

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