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Size-fractionated chlorophyll and primary productivity and nutrient distributions off the west coast of Vancouver Island Harris, Shannon Lee


Spatial and temporal variability of nutrients, chlorophyll and primary productivity off the west coast of Vancouver Island are not well studied. This study examined how dissolved nutrients and size-fractionated biomass and primary productivity vary in time and space and evaluated the relative contribution of >5 μm size fraction of phytoplankton to total biomass and primary productivity. Size-fractionated primary productivity, and physical, chemical, and biological characteristics were studied during spring, summer and fall cruises for 1997 and 1998. Studies were conducted at four transects extending across the continental shelf, perpendicular to the west coast of Vancouver Island. Transects were over the La Perouse Bank, over Barkley Canyon, off Estevan Point and off Brooks Peninsula. Physical, chemical and biological characteristics of shelf regions were distinct from beyond shelf regions and showed a strong cross-shelf gradient. The shelf region was characterized by lower temperature and lower salinity. In addition, higher nitrate and silicic acid, and higher chlorophyll and primary productivity were observed in the shelf region compared to the beyond shelf region. Variability was very high off the west coast of Vancouver Island; often the mean and the standard deviation were similar. This study was conducted during a strong El Nino (1997/98) and La Nina (1998) event and interannual variation was evident. The mixed layer depth was shallower, nitrate, silicic acid and primary productivity were higher during El Nino. In contrast, phytoplankton biomass, diatom abundance and the relative contribution of >5 μm sized phytoplankton were higher during La Nifia. The >5 μm sized phytoplankton were dominated by the diatoms Chaetoceros spp. and Leptocylindrus danicus and contributed substantially to the biomass (62%) and primary productivity (57%) off the west coast of Vancouver Island. The relative contribution was higher in shelf regions than in beyond shelf regions. This study clearly showed that the contribution of the >5 μm size fraction was greatest at high biomass concentrations and high productivity rates supporting the idea that in order to reach high biomass and productivity, large cells are required.

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