l–1 (17th day). Low nutrient concentrations (Fig. 3B) in the
culture medium (37% of the initial concentration of nitrates
and 10% of the initial concentration of phosphates) have
coincided with the highest amounts of extracellular carbohydrates
present in the culture medium.
In Fig. 4C, the variation of extracellular carbohydrate
concentration in the culture medium of Navicula directa is
represented. The same pattern of exudation verified for the
former taxa, was noted in Navicula directa. Exudation was
more significant after 9 d of incubation when the stationary
growth phase was established (Fig. 4A), and the culture
medium was depleted of nitrates and phosphates (Fig. 4B).
The maximum concentration of extracellular carbohydrates
registered was 25.6 mg l–1.
Correlation analysis between nutrient concentration and
extracellular carbohydrate concentration was performed by
the application of the Kendall coefficient (Sokal and Rolf,
1987). This analysis (Table 2) revealed significant negative
correlations (* P < 0.01) for all the taxa under study.