Seasonal or Aperiodic Cessation of Oligotrophy in the Tropical Pacific Ocean

Abstract

Large regions of the tropical Pacific Ocean are probably in a permanent oligotrophic state. A monitoring of the sea surface chlorophyll concentration (SSCC) in cooperation with the crews of merchant ships shows however that the phytoplankton biomass increases in some of these regions during brief periods. Advection of nutrientcontaining surface waters which come from the equatorial and Peruvian upwellings causes the zone between 5°S and 15°S, at about 130°W, to be alternately in an oligotrophic or in a mesotrophic state; alternations occur at periods which are probably less than one month. In the southwestern tropical Pacific, the sea loses heat to the atmosphere each winter, and the depth of the mixed layer increases; the mixed layer then incorporates nitrate and new production is stimulated; the resulting increase of SSCC is observed every year from May to October, and extends northward to about 21°S. ENSO episodes are characterized by a rise of the thermocline in the western Pacific; during the 1982-83 ENSO, high SSCC was observed between the equator and 15°N, west of 170°E, from December 1982 to February 1983, caused by this rise of the thermocline and probably by vertical mixing. These observations show that the large scale coupling between physical processes and the phytoplankton can be mapped by very simple techniques, and suggest that other pulses of phytoplankton growth could be detected in oligotrophic areas using sea color satellite data.