Primary production cycle in an upwelling center

Abstract

The cycle of nitrogen and carbon productivity of phytoplankton in an upwelling center at 15°S on the coast of Peru was studied during the JOINT-II expedition of the Coastal Upwelling Ecosystems Analysis program. The productivity cycle was characterized by repeated stations at various locations in the upwelling plume, a time series of stations in mid plume, and stations located along drogue tracks. Four zones of physiological condition were distinguished along the axis of the upwelling plume. In Zone I phytoplankton upwelled with nutrient-rich water were initially ‘shifted-down’; in Zone II they underwent light induced ‘shift-up’ to increased nutrient uptake, photosynthesis, and synthesis of macromolecules. In Zone III ambient nutrient concentrations were rapidly reduced, there was a rapid accumulation of phytoplankton biomass in the water column, and rate processes proceeded at maximal rates. In Zone IV ambient nutrient concentrations were significantly decreased, phytoplankton biomass remained high, and limitation of phytoplankton processes was beginning to be observed. Phytoplankton responded to the altered environment by undergoing ‘shift-down’ to lower rates of nutrient uptake, photosynthesis, and macromolecule synthesis. The time and space domain where this entire sequence occurs was relatively small; the cycle from initial upwelling to ‘shift-down’ was completed in 8 to 10 days within 30 to 60 km off the coast.