Processing BGC-Argo CDOM concentration at the DAC level. Version 1.0. October 10th 2017.

Abstract

In the open ocean, Colored Dissolved Organic Matter (CDOM) is the fraction of the total Dissolved Organic Matter (DOM), composed by a mixture of chemically complex algal degradation products that interact with light. CDOM absorbs solar radiation in the UV and visible ranges (Bricaud et al., 1981) and, if illuminated, re-emits light as fluorescence (i.e., FDOM; Coble 1996). These optical measurements can be used as proxies of CDOM concentration. Whereas CDOM absorption measurements are given by the entire pool of the organic matter, FDOM measurements detect only sub-fractions. Depending on excitation and emission wavelengths of the fluorometer, FDOM can represent the fresh-material produced by microbial degradation of phytoplankton cells and/or from zooplanktonic excretion, or aged humic substances (Stedmon and Nelson, 2015; Nelson and Gauglitz, 2016). Whereas CDOM absorption measurements require laboratory facilities, FDOM detection can be easily implemented on autonomous Biogeochemical-Argo profiling floats.

Though being only a small part of the DOM, CDOM (and FDOM) plays an important role in the ocean carbon cycle (Mopper and Kieber, 2002). Its distribution varies from the surface to the ocean interior and across world’s oceanic regions (Nelson and Siegel, 2013). It can indicate presence of bacterial (Organelli et al., 2014) and zooplankton (Steinberg et al., 2004) activities, planktonic food-web interactions (Xing et al., 2014), or accumulate below the thermocline depending on water mass ventilation ages (Nelson et al., 2010). Relation between consumed oxygen and CDOM optical proxies is a consequence of this accumulation, and it can be linked to remineralization processes of marine organic particles in the deep ocean (Nelson et al., 2010; Nelson and Siegel, 2013). Knowledge of CDOM (and FDOM) spatio-temporal variability is also important for remote sensing applications as it can confound standard algorithms used for the retrieval of biogeochemical and bio-optical products from space (Organelli et al., 2016; Organelli et al., 2017).

FDOM sensors currently installed on BGC-Argo floats have excitation/emission wavelengths of 370 and 460 nm, respectively. Thus, FDOM measurements correspond to more aged refractory organic material (Nelson and Gauglitz, 2016). Hereafter, we refer to FDOM measurements as CDOM.

At the moment all fluorescence sensors implemented on floats are developed by the WET labs Company and are of the ECO serie. These CDOM fluorometers are not standalone sensors and combine the CDOM measurements together two measurements (ECO triplet) which generally are Chlorophyll-A fluorescence and backscattering at 700 nm. The present document is focused on the management of the CDOM fluorescence data flow acquired by those sensors (section 3). As soon as others sensors are implemented and successfully tested on floats, the present document would be accordingly updated.