Sensitivity analysis of CDOM spectral slope in artificial and natural samples: an application in the central eastern Mediterranean Basin

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.

Spatio-seasonal variability of chromophoric dissolved organic matter absorption and responses to photobleaching in a large shallow temperate lake

“…Newly produced autochthonous CDOM typically has a higher S CDOM coefficient compared to humic-rich allochthonous material (Bracchini et al, 2010). Photobleaching also results in an increase in S CDOM .…”

Evolution of dissolved and particulate chromophoric materials during the VAHINE mesocosm experiment in the New Caledonian coral lagoon (south-west Pacific)

“…Nonetheless, recent studies have highlighted some degree of de-phasing between the dynamics in phytoplankton and that of CDOM at the global, regional or seasonal scale (Siegel et al, 2002;Morel et al, 2010;Xing et al, 2014). Whilst photobleaching is now considered as a major degradation process of CDOM in surface waters Helms et al, 2008;Bracchini et al, 2010;Swan et al, 2012), the main source of CDOM in open ocean is still a matter of debate, particularly for its "humic-like" component, which absorbs light over a broad range of UV and visible wavelengths and fluoresces in the visible domain (Andrew et al, 2013). Some works suggest that this humic-like CDOM is in part a remainder of terrestrial matter that has been diluted and transformed during transit to and within the ocean Hernes and Benner, 2006;Murphy et al, 2008;Andrew et al, 2013).…”

Sensor Capability and Atmospheric Correction in Ocean Colour Remote Sensing

“…These issues could be solved through the use of improved satellite-based sensors and atmospheric correction algorithms, which build upon these advancements. Sensors that include bands in the near UV [18,21,92] can aid the correction of absorbing aerosols [87,91], as well as the characterisation of CDOM concentrations [133,134]. A promising avenue for hyperspectral approaches is to adaptively choose which bands are most useful depending on the task or the optical properties of the water and atmosphere.…”

“…A band in the near ultraviolet (UV) at 350 nm was also recommended. Bands in the near UV can be used to characterise concentrations of CDOM [133,134] and nitrates [38], as well as being useful in accounting for absorbing aerosols in the atmospheric correction process [135]. The PACE report also recommended a spatial resolution of 1 km and a revisit time of two days [91].…”