A New Glider-Compatible Optical Sensor for Dissolved Organic Matter Measurements: Test Case from the NW Mediterranean Sea

Abstract

The MiniFluo-UV is a new glider-compatible optical sensor for measurements of dissolved organic matter (DOM) in natural waters. The working principle, sensor design and challenges faced during the validation phase are reported. The first in situ application of the sensor during three glider deployments in the NW Mediterranean sea (spring, summer, and fall) are also presented. For these campaigns, the two channels of the sensor were adjusted to target Tryptophan-like (excitation/emission wavelengths λ Ex /λ Em : 275/340 nm) and Phenanthrene-like (λ Ex /λ Em : 255/360 nm) fluorescence. These were chosen because they represent fluorophores of interest commonly found in seawater. While Tryptophan (an amino-acid believed to be a by-product of biological activity) is naturally found in the ocean, Phenanthrene (a polycyclic aromatic hydrocarbon) is mainly introduced in the environment by human activities. The addition of these variables to more common physical and biogeochemical glider measurements reveals new features of DOM dynamics in the Mediterranean Sea. For example, the temporal and spatial decoupling between Tryptophan-like and Chl-a fluorescence suggests that the former is not only a marker of phytoplankton activity, but could also give more subtle information on the microbial processes occurring. The identification of a Phenanthrene-like layer just below the pycnocline at all seasons also raises questions on the mechanisms driving its presence in the Mediterranean. Knowing that the role of ocean DOM on atmospheric carbon sequestration is becoming clearer, the high spatiotemporal resolution possible with this new sampling strategy may represents a key step toward our deep understanding of DOM dynamics and its role on the biological pump.

A Glider-Compatible Optical Sensor for the Detection of Polycyclic Aromatic Hydrocarbons in the Marine Environment

“…Based on this first prototype, a second generation of the MiniFluo was developed during the period 2012-2015. This new submersible version is now compatible with underwater gliders SeaExplorer, and exhibits significant improvements in term of opto-electronic architecture (this version is first presented in Cyr et al, 2017). In addition to these improvements, the number of fluorophores to be detected has been extended with the addition of Fluorenelike (Flu-like) and Pyrene-like (Pyr-like) fluorophores.…”

“…Two configurations are presented here: the "MiniFluo-1" for the detection of Naph-like and Phe-like, and the "MiniFluo-2" for the detection of Pyr-like and Flu-like. A first study using the actual version of the sensor was already published (Cyr et al, 2017), but the sensor was used to detect natural dissolved organic matter (DOM) fluorophores, and the study limited to MiniFluo-1 configuration without the emphasis on PAHs detection. The present study focuses on PAHs detection and the utilization of the MiniFluo/glider combination in the case of industrial contamination and/or risk-assessment applications.…”

“…Basic working principles relevant for this new application are recalled here, but we refer to previous studies for a more complete description of the sensor architecture and functioning (Tedetti et al, 2013;Cyr et al, 2017). The MiniFluo has two optical channels, enabling the simultaneous detection/quantification of 2 fluorescent compounds of interest (1 compound per optical channel).…”

“…As reported in an earlier study (see Cyr et al, 2017, for a complete description), the conversion from relative unit (RU) signal of the MiniFluo (C RU = C c −N D C m −N D ) to mass concentration (C in ng L −1 ) is done through the equation:…”

“…When equipped with specific sensors, gliders could thus be used as powerful assessment tools to track dissolved PAHs in natural waters. One of these sensors is the MiniFluo-UV, hereinafter simply MiniFluo (Tedetti et al, 2013;Cyr et al, 2017), now fully operational on SeaExplorer gliders. Through laboratory work on the sensor alone and 3 scenarios of increasing complexity with the glider, it is shown below that gliderbased optical measurements with the MiniFluo is a promising environmental assessment tool to monitor PAH concentrations in natural waters.…”