Spatial and seasonal variability of particulate matter optical and size properties in the Eastern Mediterranean Sea

Particulate matter plays a paramount role in the biogeochemical processes taking place in the marine environment. We report seasonal (spring and summer 2008) distribution of particulate matter in the Eastern Mediterranean, along a transect extending from the open Ionian Sea to the North Aegean Sea, including measurements in the Levantine Sea. A suite of optical instruments measuring beam attenuation (beam cp), fluorescence and particle size, the latter obtained with the innovative in‐situ laser particle sizer LISST‐Deep are used in concert with traditional measurements of particulate matter concentration (PMC), and total chlorophyll α from bottle samples. PMCs were generally low during both seasons (range: 0.02-0.85 mg L− 1), with values substantially higher in the euphotic zone during spring. The deep waters (> 200 m) in the Eastern Mediterranean exhibit extremely low PMCs, well‐below 0.1 mg L− 1. Total chlorophyll α concentrations ranged from 0.003 to 0.28 μg L− 1 in spring and from 0.08 to 0.19 μg L− 1 in summer, verifying the ultra‐oligotrophic character of the area. A significant correlation of beam cp and fluorescence in spring suggests that sources of particles are primarily biogenic in the surface waters. Deep water formation triggered the development of a ~ 900‐m thick benthic nepheloid layer in the N. Aegean Sea. LISST‐Deep revealed valuable information on the particle volume concentrations and the median particle size. It is striking that large particles (range: 31-230 μm; median ~ 85 μm) predominate in the entire region, from the surface up to the deep waters. In addition, accumulation of particles in the pycnocline is observed during summer; however, it is possible that schlieren (increase in beam attenuation due to scattering off of density gradients) could be responsible for the high particle volume concentrations and large median particle diameters recorded. These measurements, conducted for the first time in the Eastern Mediterranean constitute valuable baseline information for future research.

► Particle dynamics in a 1300‐km transect in the E. Mediterranean. ► Beam cp and fluorescence are significantly correlated. ► LISST-Deep, in‐situ particle sizer was used up to 3000-m depth. ► Aggregates with median diameter ~ 85 μm (microflocs) predominate in the region. ► Deep waters of the E. Mediterranean exhibit very low particle concentrations.