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

Frédéric Cyr,Andrea M Doglioli,Marc Tedetti,Florent Besson,Laurent Beguery,Anne A Petrenko,Madeleine Goutx

doi:10.3389/fmars.2017.00089

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 λEx/λEm: 275/340 nm) and Phenanthrene-like (λEx/λEm: 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 spatio-temporal 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.

Highlights

Oceanic dissolved organic matter (DOM) is operationally defined as all organic material that can pass through 0.2–0.7 μm pore size filters
Environmental Blank Value One limitation when using the sensor on the glider is the need to adjust the blank B from its laboratory value (Blab) to an environment-specific value (Benv)
It is worth noting that the error on the blank for water accommodated fraction (WAF) calibrations is on the same order of magnitude than the value itself

Summary

Introduction

Oceanic dissolved organic matter (DOM) is operationally defined as all organic material that can pass through 0.2–0.7 μm pore size filters. The DOM fraction supporting these microbial populations is biologically labile, and mineralized into CO2, or converted into biomass. Another fraction of DOM, which is biotically or abiotically transformed to biologically recalcitrant material, escapes mineralization and accumulates in surface waters. This refractory DOM may be exported to deep ocean layers, contributing to the carbon biological pump, i.e., the biologically driven sequestration of atmospheric carbon into the deep ocean waters (Hansell et al, 2009). With less than 10% of marine DOM characterized (Jiao et al, 2010), the role of DOM on carbon export is unclear

Results

Discussion

Conclusion