Bio-optics – chlorophyll-a and turbidity of surface seawater

Fluorometers combined with scatter meters are installed on the large German research vessels RV SONNE and RV MARIA S MERIAN to determine chlorophyll-a and turbidity. These active sensors emit blue light pulses to excite chlorophyll-a and detect chlorophyll-a fluorescence at 695 nm. At 700 nm, an additional light pulse is emitted and the backscattered light is measured, from which the turbidity is then determined. For both parameters, calibration coefficients have been determined in advance by the manufacturer in the laboratory, which can be used to convert the sensor output into chlorophyll-a in µg/L and turbidity in NTU. These conversion factors are subject to significant fluctuations in nature, as they depend on the specific optical properties of the particles and phytoplankton species. For this reason, additional field calibration using reference samples is required to obtain robust absolute values for these parameters.

The Sea-Bird Scientific (ECO-FLNTU) sensors are installed in a flow-through system together with the TSG. Seawater near the surface is continuously pumped through the system via an inlet in the ship’s hull. Two sensors are always used, which take measurements alternately so that the inactive measuring container can be cleaned in between.

Chlorophyll-a is used as a proxy for biomass and thus provides information about the total mass of phytoplankton. Turbidity significantly influences the availability of light in the ocean. The data can be used to compare and supplement satellite data, also in connection with global biogeochemical models, and is particularly valuable in regions with high cloud cover.

The data steward responsible for the bio-optical data is Julia Oelker from the Institute for Chemistry and Biology of the Marine Environment at Carl von Ossietzky University Oldenburg, who has a background in environmental physics with a focus on ocean colour. Her main tasks include quality assurance of the raw data, for example through sensor monitoring, comprehensive quality control of the data in accordance with international standards, management of standardised metadata and provision of the final data sets. She is also the central contact person for questions regarding the bio-optical sensor technology of the flow-through systems.

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