Here we report on an investigation of the three-dimensional excitation-emission-matrix (EEM) fluorescence spectra of unconcentrated water samples collected in 1996, 1998 and 1999 at a site particularly propitious for macro-algae development. The degradation of these macro-algae was studied to determine the influence of their exudates on natural water EEM fluorescence spectra. This work demonstrates that biological activity is one of the major factors involved in the formation of the blue-shifted fluorescence band observed in marine waters (β component Ex/Em = 310-320 nm/380-410 nm); our study also shows that fluorescence can be used to evaluate the biological activity both quantitatively and to determine its different phases. \textcopyright 2000 Elsevier Science Ltd.