Continuous monitoring of surface optical properties across a geostrophic front: Biogeochemical inferences

A system was designed for the continuous surface monitoring of hydrological , bio-optical, and biogeochemical properties in the vicinity of the Almeria -Oran jet-front system (Northwestern Mediterranean Sea). This system includ ed a thermosalinograph, a fluorometer, an optical particle counter, and an absorbance-attenuance meter (AC9), allowing the estimation of the absorptio n [a(lambda), (m(-1))] and scattering [b(lambda), (m(-1))] coefficients at nine wavelengths. More than 500 multivariate records were acquired over a t ransect crossing the jet-front system three times and encompassing the dive rsity of water types encountered in this area. A method is proposed to retr ieve relevant biogeochemical or bio-optical quantities, among which are the chlorophyll a concentration [from a(676)], the particulate carbon concentr ation [from b(555)], the colored dissolved organic matter [CDOM, from a(412 )], and the chlorophyll-specific scattering coefficient at 555 nm, b*(555) (m(2) mg Chi a(-1)). The analyses of these various quantities with respect to the surface hydrodynamic fields reveal two main features. First, the par ticle pool displays highly dynamic quantitative variations across the front al structure. Qualitatively, flagellate-dominated populations are associate d with moderate density waters and maximal jet velocity, while diatoms are recorded at the narrow limit between the jet-front and the Mediterranean wa ters, where jet velocity is minimal. Second, the CDOM pool presents remarka ble covariations with the surface-density field. In particular, the lowest a(CDOM)(412) values are reproducibly associated with the highest density va lues, which track upwelled flow associated with the frontal circulation; ac cumulation of CDOM is always observed inside the frontal waters, at a densi ty value of 27.2 kg m(-3) These CDOM variations suggest that the frontal sy stem is the place of enhanced biological activity.