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UBC Theses and Dissertations

Promontory induced tidal mixing in a narrow channel : effects on nutrient concentrations, primary productivity and zooplankton standing stock St. John, Michael A.


Horizontal mapping, aerial photography, and current meter deployment have been used to identify a tidally induced mixing event about a promontory in the Strait of Georgia, British Columbia, Canada. Mixing plumes were observed to commence, downstream of Shingle Spit during ebb tides when the mean velocity for the water column reached a minimum of 12.7 cm.s⁻¹. Mixing plumes on ebb tides, were characterized by increased surface salinities as well as increased nitrate + nitrite and phosphate concentrations. Increases in concentrations of nitrate + nitrite of 2.6 μg at .1⁻¹ and phosphate of 0.45 μg at.1⁻¹ were observed in the euphotic zone associated with the mixing plumes. Nutrient additions were correlated to an increase in primary production of 13.8 mg C.m⁻³ in the mixed water as determined by the uptake of ¹⁴CO₃²⁻. Estimates of total volume of upwelling during the stratified months of June and July 1986 were performed allowing an estimation of the net flux of new nutrients into the euphotic zone during this period. The total increase in primary production due to mixing occurring downstream of Shingle Spit was determined experimentally to range between 910 and 2.2x10⁴ kg of "new production" compared to the Redfield stoichiometric estimate of from 1.4x10³ to 3.0x10⁴ kg of "new production" during June and July, 1986. Measurements of net flow in Lambert Channel allowed determination of the destination of the increases in primary production. It is suggested that utilization of increases in primary production caused increases in the zooplankton standing stock in the region south of Lambert Channel and Hornby Island. Gut contents of adult Qncorhynchus kisutch caught in the region contained zooplankton groups which were components of the increase of zooplankton standing stock. The presence of these zooplankton groups in the gut contents suggests utilization of the increases in zooplankton standing stock by predators further up the food chain.

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