Experiments of water enrichments with natural populations of phytoplankton were carried out at the region of Ubatuba (Lat. 23º30'S - Long. 45º06'W) during winter and summer time. Special attention was given to the influence of nitrogen and phosphorus on the standing-stock (chlorophyll-α) and 14C assimilation and dark fixation in situ conditions. The phytoplankton populations was analysed in terms of its composition and hydrographical parameters were made according to Strickland & Parsons (1968). The enrichments with surface waters reveal that nitrogen may be the major limiting factor for phytoplankton biomass sensu Liebig.