On 7 May the team ran a westward coastal transect aboard R/V Sagitta III, departing Villefranche-sur-Mer and following the coast to the mouth of the Var near Nice, in collaboration with the Laboratoire d’Océanographie de Villefranche (CNRS-SU). Heavy rainfall over the preceding days had pushed a turbid plume out of the river, and the campaign was timed to catch it against the clear Mediterranean water alongside.
That timing is what made the day worth the fuel. A river plume and the open sea beside it are the two ends of the optical range an instrument has to cover, and on 7 May they sat within one transect of each other, which is the rare case where the same sensor reads both without changing anything about how it is run.
The gradient was steep. Secchi depth measured 0.46 m in the core of the plume and 16 m offshore, a span most optical instruments are not asked to cross in a single day. The LIFL-11 resolved the plume against that gradient, recorded distinct subsurface scattering layers carried by the river water, and profiled vertical optical structure down to roughly 30 m in the clear water offshore.
The transect returned 29,791 lidar depth profiles at 2 Hz and 210,680 surface-mode fluorescence measurements at 10 Hz across eight stations, with continuous surface maps of chlorophyll-a at 685 nm and CDOM at 440 nm. Between the plume and the offshore water, both chlorophyll and CDOM varied by more than an order of magnitude. Conventional casts, CTD, fluorometric profiles, Niskin sampling and Secchi, were taken alongside as the reference the lidar is checked against.
The instrument suite ran together, with the mFL-12 and the mHR-2 operating beside the lidar, and the results are documented in the LIFL VarPlume field report. Our thanks to the Laboratoire d’Océanographie de Villefranche and to the crew of R/V Sagitta III for the support.