Optical Properties and Biochemical Indices of Marine Particles in the Open Mediterranean Sea: The R/V Maria S. Merian Cruise, March 2018

Spyros Chaikalis,Aristomenis Karageorgis,Afrodite Androni,Dagmar Hainbucher,Emmanouil Proestakis,Vanessa Cardin,Alexandra Gogou,Toste Tanhua,Jürgen Möbius,Dimitris Velaoras,Sarantis Sofianos,Constantine Parinos,Vassilis Amiridis

doi:10.3389/feart.2021.614703

Abstract

A rich data set on particulate matter optical properties and parameters (beam attenuation coefficient, volume concentration, particle size and PSD slope), accompanied by measurements of biochemical indices (particulate organic carbon, particulate nitrogen and their stable isotopic composition) was obtained from the surface to deep waters across the Mediterranean Sea, in March-April 2018. A decrease of beam attenuation coefficients, total particle volume concentrations, particulate organic carbon and nitrogen concentrations was noted towards the eastern Mediterranean Sea (EMed) in comparison to the western Mediterranean Sea (WMed). LISST-derived optical properties were significantly correlated with water mass characteristics. Overall, the most turbid water mass identified in the Mediterranean Sea was the Surface Atlantic water (AW), and the most transparent was the Transitional Mediterranean Water (TMW) in the Cretan Sea, whereas a general decrease in particulate matter concentration is observed from the WMed towards the EMed. Relatively depleted δ13C-POC values in the particle pool of the open Mediterranean Sea can be attributed to contribution from terrestrial inputs, mainly via atmospheric deposition. Throughout the entire water column, a significant positive correlation between particle beam attenuation coefficient and particulate organic carbon concentration is observed in the open Mediterranean Sea. Such relationship suggests the predominance of organic particles with biogenic origin. POC concentration and particle median diameter D50 are significantly and negatively correlated both in the WMed and the EMed Sea, confirming that small particles are POC-rich. At depth, a prominent decrease of most measured parameters was observed, with the exception of particle median diameter that increased substantially in the EMed towards the deep sea, suggesting potentially enhanced aggregation processes. The low particle size distribution slope ξ observed in the EMed, corresponding to larger particle populations, supports the above notion. Basin-wide Rayleigh-type isotopic fractionation in vertical profiles of δ15N-PN across the Mediterranean Sea, underlines the differences in the trophic characters of the two sub-basins and highlights the role of circulation changes on biogeochemical parameters and the redistribution of particulate matter as a source of nutrients in the water column.

Highlights

The vertical distribution, horizontal distribution, and export at depth of particulate matter (PM) in the world ocean have been the subject of numerous studies over the past decades (Volk and Hoffert, 1985; Honjo et al, 2008; Jeandel et al, 2015)
3,000 m depth was obtained across the Mediterranean Sea during cruise MSM72 in March-April 2018, a relatively high productivity period for the region, accompanied by standard CTD, biochemical (POC and particulate nitrogen (PN) contents) and isotopic data (δ13C-particulate organic carbon (POC) and δ15N-PN) of PM
A direct relationship between hydrological parameters, water masses and LISST-derived particle optical properties was observed throughout the Mediterranean Sea

Summary

Introduction

The vertical distribution, horizontal distribution, and export at depth of particulate matter (PM) in the world ocean have been the subject of numerous studies over the past decades (Volk and Hoffert, 1985; Honjo et al, 2008; Jeandel et al, 2015). During its transit towards the seafloor, most of the particulate organic carbon (POC) particulate nitrogen (PN) and particulate organic phosphorus (POP) is returned to their inorganic forms and redistributed into the water column. This redistribution determines the surface concentration of dissolved CO2 and inorganic nutrients and the rate at which the ocean can absorb CO2 from the atmosphere. Processes involved in particle formation, remineralization and transport from the epipelagic to the mesopelagic and bathypelagic ocean are tightly connected to the biochemistry of marine organisms and the biodiversity and health of marine ecosystems

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