Edouard Leymarie

Seasonal variability of nutrient concentrations in the Mediterranean Sea: Contribution of Bio‐Argo floats

In 2013, as part of the French NAOS (Novel Argo Oceanic observing System) program, five profiling floats equipped with nitrate sensors (SUNA-V2) together with CTD and bio-optical sensors were deployed in the Mediterranean Sea. At present day, more than 500 profiles of physical and biological parameters were acquired, and significantly increased the number of available nitrate data in the Mediterranean Sea. Results obtained from floats confirm the general view of the basin, and the well-known west-to-east gradient of oligotrophy. At seasonal scale, the north western Mediterranean displays a clear temperate pattern sustained by both deep winter mixed layer and shallow nitracline. The other sampled areas follow a subtropical regime (nitracline depth and mixed layer depth are generally decoupled). Float data also permit to highlight the major contribution of high-frequency processes in controlling the nitrate supply during winter in the north western Mediterranean Sea and in altering the nitrate stock in subsurface in the eastern basin.

Underwater Radiance Distributions Measured with Miniaturized Multispectral Radiance Cameras

Miniaturized radiance cameras measuring underwater multispectral radiances in all directions at highradiometric accuracy (CE600) are presented. The camera design is described, as well as the main steps of its optical and radiometric characterization and calibration. The results show the excellent optical quality of the specifically designed fish-eye objective. They also show the low noise and excellent linearity of the complementary metal oxide semiconductor (CMOS) detector array that is used. Initial results obtained in various oceanic environments demonstrate the potential of this instrument to provide new measurements of the underwater radiance distribution from the sea surface to dimly lit layers at depth. Excellent agreement is obtainedbetweennadirradiancesmeasuredwiththecameraandcommercialradiometers. Comparisonofthe upwelling radiance distributions measured with the CE600 and those obtained with another radiance camera also shows a very close agreement. The CE600 measurements allow all apparentoptical properties (AOPs) to be determined from integration of the radiance distributions and inherent optical properties (IOPs) to be determined from inversion of the AOPs. This possibility represents a significant advance for marine optics by tying all optical properties to the radiometric standard and avoiding the deployment of complex instrument packages to collect AOPs and IOPs simultaneously (except when it comes to partitioning IOPs into their component parts).

Assessing the Variability in the Relationship Between the Particulate Backscattering Coefficient and the Chlorophyll a Concentration From a Global Biogeochemical‐Argo Database

Characterizing phytoplankton distribution and dynamics in the worlds open oceans requires in situ observations over a broad range of space and time scales. In addition to temperature/salinity measurements, Biogeochemical-Argo (BGC-Argo) profiling floats are capable of autonomously observing at high frequency bio-optical properties such as the chlorophyll fluorescence, a proxy of the chlorophyll a concentration (Chla), the particulate backscattering coefficient (bbp), a proxy of the stock of particulate organic carbon, and the light available for photosynthesis. We analyzed an unprecedented BGC-Argo database of more than 8,500 multi-variable profiles collected in various oceanic conditions, from subpolar waters to subtropical gyres. Our objective is to refine previously established Chla vs bbp relationships and gain insights into the sources of vertical, seasonal and regional variability in this relationship. Despite some nuances in the relationship considering one or another water column layer or region, a general covariation occurs at a global scale. We distinguish two main contrasted situations: (1) concomitant changes in Chla and bbp that correspond to actual variations in phytoplankton biomass, e.g. in subpolar regimes; (2) a decoupling between the two variables attributed to photoacclimation or changes in the relative abundance of non-algal particles, e.g. in subtropical regimes. The variability in the bbp:Chla ratio in the surface layer appears to be essentially influenced by the type of particles and photoacclimation processes. The dense BGC-Argo database helps identifying the spatial and temporal scales at which this ratio is predominantly driven by one or the other of these two factors.