DISTRIBUTION OF SEAWATER FLUORESCENCE AND DISSOLVED FLAVINS IN THE ALMERIA-ORAN FRONT (ALBORAN SEA, WESTERN MEDITERRANEAN-SEA)

Seawater fluorescence in the blue region of the spectrum (excitation a t 360 nm) due to fluorescent dissolved organic matter (FDOM), and diss olved flavins were investigated in the Almeria-Oran geostrophic front (western Mediterranean) in the 0-200 m layer. Seawater fluorescence in creased with depth from a minimum in the jet divergence, increasing to wards the oligotrophic waters located outside the jet zone, and reachi ng a maximum in the right side of the jet, a convergence zone. Compari sons with other recorded parameters suggested both physical and biolog ical factors were involved in its distribution along the transect. Pho todegradation due to light-penetration and seawater enrichment with FD OM due to biological activity appeared as driving factors of fluoresce nce distribution. Fluorescence increase along the secondary circulatio n of the jet was attributed to the combined effects of aging of a bloo m (where it was suggested that both auto- and heterotrophic population s were involved) and photodegradation. FDOM of deeper waters (found in the divergence zone) was inferred to be less photodegradable than tha t generated in the productive layers (the convergence zone). From thes e data fluorescence in the oligotrophic sites was deduced to originate from prevaling biological activity. Three flavins were investigated: riboflavin and its photoproducts (lumichrome and lumiflavin). The vert ical distribution of flavins was marked by a stratification into two l ayers of enhanced concentrations. The upper one was found to coincide with the upper chlorophyll layer (DCM or DCM1), the lower one with the lower chlorophyll layer (DCM2, where it occurred) and/or with the bas e of the halocline. From these depth coincidences both auto- and heter otrophic populations were inferred to be sources of flavins although t heir respective contributions were hard to determine. As for fluoresce nce, an increase of flavins was found in the jet zone. However signifi cant differences were found between the oligotrophic sites. The effect s of subduction in the jet zone were visible from depth distribution o f riboflavin photoproducts. The contribution of flavins to seawater fl uorescence was found to be low (or negligible). Both these parameters appeared as promising tools for studying mesoscale hydrodynamic proces ses despite the fact that some interpretations appeared delicate.