Abstract
Chromophoric dissolved organic matter (CDOM) is the fraction of the Dissolved Organic Carbon (DOC) mainly absorbing UV and blue radiation, influencing water optical properties, light availability for primary production, and water-leaving radiance. In open seas, phytoplankton is the main source of organic carbon and CDOM. Despite this, the direct or indirect phytoplankton role in CDOM production is not yet fully clarified. From studies about the relationship between CDOM and phytoplankton biomass as Chlorophyll a (Chl) in the epipelagic layer, positive correlations have been highlighted with regional differences and high levels of variability. Seven years of seasonal dynamic and vertical distribution of CDOM in the Ligurian Sea continental shelf waters have been analyzed in order to evidence the main environmental and/or biological factors determining CDOM dynamic, focusing on the CDOM/Chl relationship. CDOM optical properties (absorption at 440 nm, aCDOM (440), and spectral slope, S) allowed to distinguish different pools and to debate their origin. Four different pools were characterized and two of them were directly or indirectly related to phytoplankton biomass and taxonomic composition. Nevertheless, CDOM/Chl confirm a high level of variability These findings suggest some inputs to improve Mediterranean satellite estimates of Chl and CDOM, such as the seasonal differentiation of optical properties, especially S and CDOM/Chl relationships.
Highlights
Chromophoric dissolved organic matter (CDOM or ‘yellow substance’) is operationally defined as the group of absorbing substances that passes filters of 0.2 μm pore size [1]
CDOM/Chlorophyll a (Chl) confirm a high level of variability These findings suggest some inputs to improve Mediterranean satellite estimates of Chl and CDOM, such as the seasonal differentiation of optical properties, especially S and CDOM/Chl relationships
CDOM is an important fraction of the Dissolved Organic Matter (DOM) in natural waters (10–90%, [2]) and it plays an important role in the ocean Carbon cycle [3]
Summary
Chromophoric dissolved organic matter (CDOM or ‘yellow substance’) is operationally defined as the group of absorbing substances that passes filters of 0.2 μm pore size [1]. Most of the organic substance dissolved in the sea derived from phytoplankton is biologically labile and has a rather short life span, a portion is not immediately mineralized and is transformed (biologically and abiologically) into resistant organic material and creates the large oceanic reservoir of CDOM Thanks to this property, the fraction that absorbs UV has been and is used as a tracer of the water masses. Morel and Gentili [18] confirmed this hypothesis assessing indirectly CDOM concentrations by the analysis of the errors in Chl estimates, which were attributed to a higher CDOM concentration with respect to the open ocean These differences can be ascribed to the inputs of terrigenous CDOM, linked to the characteristics of the semi-enclosed basin of the Mediterranean Sea [18]. Particular attention was focused on: relationships between CDOM and phytoplankton biomass, taxonomic, and size structure composition; CDOM optical properties in the context of an improvement of Mediterranean regional satellite algorithms
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