The fact that the annual cycle of peak T was in phase with DOC suggests that the DOC accumulated in Blanes Bay contained free and combined amino acids. In this sense, it is known that maintenance of the algal photosynthetic machinery after inorganic N and P exhaustion is accompanied by excretion of DOM and especially
RepSox (Norman et al., 1995). P limitation in the Mediterranean could lead to the accumulation of protein materials of little use under P starvation, despite the fact that it would be labile in other (P replete) environments. Therefore, despite the potential lability of the exuded materials, P limitation would prevent their utilisation by heterotrophic bacteria leading to DOC accumulation in summer (Thingstad et al., 1997). DOM accumulation is a common process that has also been evidenced in other oligotrophic aquatic environments (e.g., Sargasso Sea, Thingstad et al., 1997 and Siegel et al., 2002) as well as in experimental mesocosms (Norman et al., 1995). The accumulation of protein-like substances in summer was also observed in Florida Bay (Jaffé et al., 200
cool . Another plausible reason for the protein-like fluorescence maximum recorded in summer could be that photo-degradation of humic substances releases amino
acid moieties or changes its position in the protein throughout conformational changes of the macromolecule, exhibiting higher fluorescence in that new form (Lakowic, 2006). This has been hypothesised in an experimental study where it was reported an increase of peak T after exposure of open ocean ultrafiltered DOM to sunlight (Romera-Castillo, et al., in prep.).
