Regulation of phytoplankton communities by physical processes in upwelling ecosystems

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Abstract
Sinking rates of particles were superimposed on the x-z-t current field observed at 15S off the coast of San Juan, Peru in March-May, 1977 to calculate particle trajectories in the upwelling circulation. Vertical velocities were calculated by a modified variational objective analysis technique using the measured onshore and longshore currents in conjunction with the physical constraint of mass continuity. The calculated vertical flow showed considerable temporal and spatial variability, with the mean vertical transport varying by 2 orders of magnitude over the 16 km wide continental shelf. Changes in direction occurred rapidly (within 24 h) as has been observed for horizontal circulation in this region. The vertical velocity of water was much greater than the sinking rates of particles during the 52 day period, so that the net vertical transport of particles was controlled by the vertical velocity of the water. Reseeding of sinking particles from the surface offshore-flowing layer into the deeper onshore flow could not be demonstrated for this period, which may explain why the measured biomass and primary productivity were anomalously low in 1977. Vertical mixing greatly increased the possibility of reseeding by transporting material downward into the onshore flow. Recirculation of particles may normally occur in the Peruvian upwelling system, but that the continuity probably involves movement in the longshore as well as the cross-shelf direction.