Calibration data for a high dynamic range and autonomous ocean-light instrument (OptiCAL)

The OptiCAL (Optical Chain And Logger) is an autonomous ice-tethered observatory (ITO) for measuring the underwater light climate. It comprises of a spar buoy (containing Iridium and GPS antenna, logger/controller, Iridium modem and batteries) and a chain with “nodes” at certain depths. Each node carries three upward and three downward looking light sensors. For each direction two sensors have a gel filter, one blue and one orange while the third sensor is unfiltered.

For calibration, a Polarcirkel workboat belonging to the Norwegian Research vessel R/V Helmer Hanssen was used as platform for deploying a 50m long chain of the OptiCAL and a Satlantic PAR-LOG ICSW Irradiance-Cosine-in-Water sensor. The OptiCAL chain was controlled by a handheld logger. The profiling Satlantic sensor was mounted on a Sea-Bird CTD and measured several depth profiles (for most stations 10).

The resulting co-temporal and near co-spatial optical data can be used for calculating a calibration factor for converting OptiCAL raw data from millihertz [mHz] to EPAR [µmol m-2s-1]. See Schartmüller et al. (submitted for publication) for a detailed description of the instrument and the calibration procedure for the unfiltered sensor.

Six stations were sampled at three locations in the Svalbard archipelago, Kongsfjorden, Smeerenburgfjorden and Rijpfjorden between 8th and 12th September 2020. The targeted depths for the OptiCAL light sensors were 1,2,3,4,5,6,7,14,21,32 and 50m. OptiCAL data are archived in a netCDF file as 'counts' from the sensors and are in units of millihertz [mHz]. EPAR data from the Satlantic sensor are archived in a netCDF file in the unit [µmol m-2s-1]. See the metadata in the netCDF files for more info.

The work was supported by three grants from the Norwegian Research Council: Arctic ABC (project no 244319, Arctic ABC Development (project no 245923), Deep Impact (project no 300333), and the Centre of Excellence AMOS (project no 223254).