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Plankton blooms of the British Columbia northern shelf : seasonal distributions and mechanisms influencing their formation Perry, Richard Ian


The first description is presented of spatial and seasonal distributions of plankton blooms, and physical oceanographic conditions underlying their formation, on the British Columbia northern shelf. This region includes Queen Charlotte Strait and Sound, Hecate Strait, Dixon Entrance and their contiguous waters. Its biological oceanographic characteristics are virtually unknown, yet it has valuable petroleum and fishery resources. Samples were collected between March 1978 and August 1980 using a coastal oil tanker as a ship of opportunity. Fourteen cruises were conducted, concentrated during spring and summer. Samples for salinity, nutrients, phytoplankton composition and pigments were collected from the sea water intake system (depth 3 m) while underway. Zooplankton were collected with a Miller sampler towed from the stern; temperature profiles were obtained with an XBT system. Light penetration and vertical mixing characteristics were the principal physical properties leading to initiation of phytoplankton blooms in spring and summer. The spring (diatom) bloom was predicted by a critical - mixed depth model calculated from historical data. Summer blooms were predicted by a tidal front model, which compares tidal velocities with water depth. Bathymetry is the common feature of these two mechanisms, limiting the mixed depth in spring and maintaining tidal mixing in shallow regions during summer. The spring bloom progressed northward with increasing irradiance, occurring during February - March in the Strait of Georgia and during May in Dixon Entrance. However, it was predicted and observed earlier in Hecate Strait (April), where a shallow shelf limits the mixed depth, than in more southerly Queen Charlotte Sound. The bloom in Queen Charlotte Sound first developed along its southeastern coast, apparently due to runoff-related stratification. On the scale of sampling, zooplankton blooms in spring occurred in the same areas as phytoplankton. During summer, observations confirmed low biomass (flagellates) and well-mixed conditions in shallow western Hecate Strait, and high biomass (diatoms) and stratified conditions on its eastern side. However, mean mixed layer light intensities were similar, and near-surface nutrient concentrations low, on both sides of the strait. Phytoplankton on the shallow side were probably limited by rates of nutrient resupply, while the eastern side received nutrients from upwelling or sporadic mixing by storms. This may distinguish the effect of tidal mixing in shelf seas and straits, with advection of nutrients to the mixed region of straits reduced by the shallow bottom and adjacent land boundaries.

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