Horizontal flux of nutrients and plankton across and along the British Columbia continental margin

We report rate estimates for the horizontal transport of realized and potential “new” production across and along the Vancouver Island continental margin. Measurements consisted of three summer-season surveys (1993–1995) of water properties, chlorophyll and dissolved nutrient concentrations, zooplankton biomass and community composition. Sampling was done along paired 350-km station lines extending parallel to and approximately 25 km seaward of the shelf break. Horizontal transport of nutrients and plankton biomass was estimated from cross-products of concentration fields with cross-shore and alongshore geostrophic velocity fields and with space- and time-averaged estimates of Ekman volume transport. Because concentrations of nutrients and phytoplankton were low in the upper 30–50 m, their horizontal flux within the Ekman layer was relatively small (order 10% of geostrophic transport). Geostrophic transport was strongly localized and was correlated vertically with concentration gradients, and horizontally with eddies and meanders of the alongshore geostrophic currents. Net geostrophic transport was a small difference between larger localized seaward and shoreward components. Upper layer (0–50 m) transports of nutrients and phytoplankton biomass were of roughly similar magnitude. Both were much larger than transport of zooplankton biomass. Total cross-shore flux was a small fraction (<10%) of the estimated total productivity shoreward of the sampling lines. Direction and magnitude varied among survey periods, but for all 1990s surveys appear to have been weaker than in the mid-1980s, when summer-season averaged upwelling-favorable winds were stronger and the shelf-break current was faster.