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

Abstract

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 included a thermosalinograph, a fluorometer, an optical particle counter, and an absorbance‐attenuance meter (AC9), allowing the estimation of the absorption [a(λ), (m−1)] and scattering [b(λ), (m−1)] coefficients at nine wavelengths. More than 500 multivariate records were acquired over a transect crossing the jet‐front system three times and encompassing the diversity of water types encountered in this area. A method is proposed to retrieve relevant biogeochemical or bio‐optical quantities, among which are the chlorophyll a concentration [from a(676)], the particulate carbon concentration [from b(555)], the colored dissolved organic matter [CDOM, from a(412)], and the chlorophyll‐specific scattering coefficient at 555 nm, b*(555) (m2 mg Chl a−1). The analyses of these various quantities with respect to the surface hydrodynamic fields reveal two main features. First, the particle pool displays highly dynamic quantitative variations across the frontal structure. Qualitatively, flagellate‐dominated populations are associated with moderate density waters and maximal jet velocity, while diatoms are recorded at the narrow limit between the jet‐front and the Mediterranean waters, where jet velocity is minimal. Second, the CDOM pool presents remarkable covariations with the surface‐density field. In particular, the lowest aCDOM (412) values are reproducibly associated with the highest density values, which track upwelled flow associated with the frontal circulation; accumulation of CDOM is always observed inside the frontal waters, at a density value of 27.2 kg m−3. These CDOM variations suggest that the frontal system is the place of enhanced biological activity.