Abstract
The rate of phytoplankton excretion of organic matter was measured by acidification of water samples incubated in the laboratory following the Steemann Nielscn method (1952). After the release of inorganic 14C,14C-organic carbón was estimated through Berman's methodology (1976).
 Percentages of excretion of organic carbón ranged bctween 8 and 18% (mean value 13,5%) were the lowest observed, supporting the hypothesis that the cxcretion is lower in eutrophic waters. Thesc percentages incrcascd during winter and decresed during summer: instead rates of release of organic matter showed a bimodal distribution with two máximum valúes, one in winter and other in summer (57,6 and 72,0 mg C • m-3 • h-1, respectively). Percentages of cxcretion showed and inverse relationship with phytoplankton photosynthesis and chlorophyll-a concentrations (P < 0,01). Scasonal fluctuations of excretion were influenced by light intcnsity, with higher light rates during winter than in summer.
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