Measurements of primary productivity and excretion of DOM in natural phytoplankton populations were carried out in surface waters from Lat. 17º14'S -Long. 039º13'W to Lat. 28º36'S -Long. 048º49'W. The relative rates of excretion varied between two and thirty percent from the total assimilated in the transparent bottle. In relative terms the excretion increased towards the pelagic zones, but in absolute results it followed the production values. Based on the predominat water masses and on the excretion values a classification of the stations under study is suggested.
