Blue and yellow fluorescence of filtered seawater in a frontal zone (Ligurian Sea, northwest Mediterranean Sea)

Bulk fluorescence of filtered seawater was measured in the blue (BF) and yellow (YF) regions of the spectrum (emission centered at 452 and 550 nm respectively) at stations along a transect through the Liguro-Provençal Frontal System (northwestern Mediterranean Sea). Layers of fluorescence enhanced above the background level (peaks) were found in the 0–1100 m column at all the stations. They are attributed to fluorescent dissolved organic matter (DOM) release by various populations on the basis of coincidences between the depths of fluorescence peaks and those of some of the other parameters used (Chl a, nutrients, accoustic data and distributions of Mediterranean Winter Water and Mediterranean Intermediate Water). Several fluorescent DOM producing processes were involved at variable levels of intensity according to their position on the transect and depth in the water column. (1) Production by plankton (phyto- and zooplankton) was predominant within the main subsurface Chl a maximum in all the three delineated zones of the frontal system (marginal, frontal and peripheral). (2) Correlation between fluorescence and nutrients is visible over the whole data set for the transect. Regeneration was one of the major processes in the frontal zone: in the euphotic layer in the form of an isopycnal fluorescent layer below the main subsurface Chl a layer (rapid regeneration), as well as in the aphotic layer. (3) Significant fluorescence increases in two deep-scattering layers point to the contribution of populations of midwater organisms. (4) Inputs originating from the continent and from sediments were detected in the peripheral zone. An overall similarity between respective BF and YF intensities was found, but discrepancies occurring locally in the water column point to the particular contribution of YF to the bulk seawater fluorescence.