Sensitivity analysis of CDOM spectral slope in artificial and natural samples: an application in the central eastern Mediterranean Basin |
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Authors: | Luca Bracchini Antonio Tognazzi Arduino Massimo Dattilo Franco Decembrini Claudio Rossi Steven Arthur Loiselle |
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Institution: | 1. Biosystem Chemistry Group, Dipartimento Farmaco Chimico Tecnologico, Università di Siena, Via Aldo Moro 2, 53100, Siena, Italy 2. Italia and Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI), Via della Lastruccia 3, 50010, Sesto Fiorentino, Florence, Italy 3. CNR-IAMC, Istituto Talassografico, Spianata S. Raineri 86, 98122, Messina, Italy
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Abstract: | In the past two decades, optical properties of chromophoric dissolved organic matter (CDOM) in marine environments have been
extensively studied. Many of these studies report CDOM properties for the offshore environment where this complex mixture
of optically active compounds is strongly diluted. Nevertheless, autochthonous and allochthonous sources have been identified
and sinks related to photodegradation and bacterial activity have been demonstrated. The calculation of the spectral slope
of the CDOM absorption curve has been proven to be useful and is often reported. However, a rigorous uncertainty analysis
of the slope calculation is rarely reported. In this paper, we propose a method to evaluate the uncertainty of CDOM spectral
slope calculated between 270 and 400 nm, using both naturally sampled and artificial solutions. We use these results to study
the ultra-oligotrophic waters of the Mediterranean Sea (central eastern basin), where little is known about CDOM spatial distribution.
We show that dilutions of both artificial and natural samples produce a Gaussian distribution of spectral slopes, indicating
that consistent values may be determined, with a typical uncertainty of ±0.0004 nm−1 when absorption at 300 nm was greater then 0.1 m−1 (0.1 m pathlength). Comparing the distribution of spectral slopes from central eastern basin samples to a Gaussian distribution,
we show differences between measurements that were significantly different. These values allow us to distinguish possible
sources (algal derived CDOM), sinks (e.g. photo-bleaching) at different depths. We propose a subdivision of CDOM compounds
into refractory and semilabile/refractory pools and evaluate the CDOM spectral slope of algal derived CDOM released at or
near deep chlorophyll maximum. |
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