Abstract
Optical models have been proposed to relate spectral variations in the beam attenuation (cp) and optical backscattering (bbp) coefficients to marine particle size distributions (PSDs). However, due to limited PSD data, particularly in the open ocean, optically derived PSDs suffer from large uncertainties and we have a poor empirical understanding of the drivers of spectral cp and bbp coefficients. Here we evaluated PSD optical proxies and investigated their drivers by analyzing an unprecedented dataset of co-located PSDs, phytoplankton abundances and optical measurements collected across the upper 500 m of the Atlantic Ocean. The spectral slope of cp was correlated (r>0.59) with the slope of the PSD only for particles with diameters >1 µm and also with eukaryotic phytoplankton concentrations. No significant relationships between PSDs and the spectral slope of bbp were observed. In the upper 200 m, the bbp spectral slope was correlated to the light absorption by particles (ap; r<-0.54) and to the ratio of cyanobacteria to eukaryotic phytoplankton. This latter correlation was likely the consequence of the strong relationship we observed between ap and the concentration of eukaryotic phytoplankton (r=0.83).
Highlights
Microscopic organic particles play a key role in the ocean biological carbon pump, transferring carbon from the surface to the deep ocean [1], which helps modulate the Earth’s climate [2,3].These particles encompass a variety of living organisms and detrital matter that vary in concentration, size and composition, both spatially and temporally
In the submicron range, peaks centered around 0.6 μm were observed in some particle size distributions (PSDs) at the surface and at the deep chlorophyll maximum (DCM), even though these peaks could not be fully resolved because of the detection limits of the Coulter Counter (Fig. 2)
All PSDs at the surface and DCM showed low uncertainties within each size bin up to 5 μm for AMT26 and up to 7 μm for AMT expeditions carried out in October-November 2012 (AMT22), while uncertainty increased for PSDs collected in the mesopelagic zone (Supplement 1, Fig. S3)
Summary
Microscopic organic particles play a key role in the ocean biological carbon pump, transferring carbon from the surface to the deep ocean [1], which helps modulate the Earth’s climate [2,3] These particles encompass a variety of living organisms (i.e., viruses, bacteria, phytoplankton and zooplankton) and detrital matter (e.g., debris and faecal pellets) that vary in concentration, size and composition, both spatially and temporally. It can be used to distinguish phytoplankton groups from detritus over a broad size range by exploiting the natural fluorescence properties of individual phytoplankton cells [7] Both these techniques have an invaluable analytic power, but are discrete measurements, involving specialized equipment and trained operators. They are not well-suited to capture high spatial and temporal variations in particle size and composition
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