Phytoplankton productivity on the Canadian Shelf of the Beaufort Sea

For the first time, the seasonal cycle of phytoplankton productivity on a broad, seasonally ice-covered arctic shelf (the Canadian Shelf of the Beaufort Sea) is examined. This shelf is the most riverine of the panarctic shelves. During 5 surveys between April and September 1987, field observations (salinity, temperature, nutrients, photosynthetically active radiation [PAR], chlorophyll a) and shore-based data (air temperature, river discharge, sea-ice cover, PAR) were collected. Productivity was measured using C-14 and converted to daily production with PAR and vertical light extinction values. Due to inflow from the Mackenzie River, phosphorus limits production over the inner shelf (depth z<20 m) whereas nitrogen limits production over the middle (20< z< 80 m) and outer (shelf-break; z>80 m) shelf. Light limitation due to landfast ice delays the onset of watercolumn productivity in spring in the inner shelf by about 1 mo compared to the outer shelf. In spring, maximum productivity occurred near the surface and decreased exponentially with depth. In summer, as nutrients became depleted in the upper mixed layer, a deep chlorophyll maximum developed toward the bottom of the winter mixed layer (20 to 40 m). Productivity during spring (ice cover) was about 10 mg C m(-2) d(-1), rising typically to about 200 mg C m(-2) d(-1) by late July. Maximum productivities, similar to600 mg C m(-2) d(-1), were observed above the head of Kugmallit Canyon, suggesting enhanced upwelling. The total production for 1983 was estimated at 12 to 16 g C m(-2), characteristic of oligotrophic waters. This range is consistent with new-production estimates based on nutrient draw-down over the shelf and on vertical carbon fluxes at the shelf break measured by sequential traps or Th-234 disequilibria. Light limitation due to snow and ice cover may control the timing of primary production, but the availability of nutrients ultimately sets the annual limit. Nutrient availability is determined partly by cumulative vertical convection during winter, and partly by upwelling. Projected loss of ice cover over the Arctic's marginal seas is likely to have its greatest impact by widespread initiation of conditions for shelf-break upwelling.