The role played by heterotrophic microplankton in the synthesis and flux of organic matter was studied in the Punta San Juan Coastal upwelling region off Peru in April and May 1977 . The data from a drogue study show that the main component of the planktonic community in the freshly upwelled water is the microheterotroph component . The biomass of bacteria (49 mg C/m') in the newly upwelled water exceeded by two orders of magnitude the biomass of phytoplankton . Total respiration of the microheterotrophs (335 g C/m 2 /day) exceeded by three-fold the pnmary production, indicating that the heterotrophic respiration was dependent on the content of organic matter preexisting in the upwelling waters . In the upwelling at Punta San Juan the biomass of protozoa was I g/ m3 (wet weight) at the depth of the maximum concentration ; this concentration is the highest ever observed in sea water . During transects on a section normal to the coastline an abundant population of dinoflagellates (5 to 40 x Io 3/1) of the genera Gymnodinium and Prorocentrum were found in anoxic waters at 50 to too m. Strong red tide water coloration was observed as a result of a bloom of the autotrophic ciliate Mesodinium rubrum; the biomass of the ciliate at the surface in calm weather reached 50 to 70 g/ m3 (wet weight) and the cell density was 2 to 4 x 106/1 .

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