Zooplankton distribution and transport in the California Current off Oregon

Abstract

Transport and retention of zooplankton biomass in the shelf and slope regions off Oregon, USA, were studied in June 2002, using high-resolution measurements of temperature, salinity, depth, fluorescence, water current and zooplankton abundance. We employed 2 different analytical methods for minimizing divergence in the geostrophic current fields based on measurements from a vessel using an acoustic Doppler current profiler. We detected high zooplankton biomass in coastal upwelling areas on the shelf, and exchanges between shelf waters with high zooplankton biomass and offshelf waters with low zooplankton biomass via cross-isobath currents. In the shelf area of Heceta Bank off Newport bounded by the 153 m isobath, the standing zooplankton biomass was ~4 × 104 t C. The major flux of zooplankton biomass into the area occurred at the northern boundary at a rate of 1 × 103 to 2.5 × 103 t C d−1 or a specific rate of 0.03 to 0.06 d−1 based on 2 different analytical methods. The flux at the southern boundary was 1 order of magnitude lower, and a significant flux out of the area occurred at the 153 m isobath at a rate of ~0.8 × 103 to 3.7 × 103 t C d−1 or a specific rate of 0.02 to 0.09 d−1. These rates are comparable with the zooplankton growth and mortality rates of ~0.1 d−1 previously reported for this region. Offshelf transport of zooplankton contributes significantly to biomass losses in shelf ecosystems and in turn fuels offshelf ecosystems.