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Abstract: Optical measurements provide substantial information on the dynamics and composition of particulate matter in the open ocean and coastal waters. When calibrated with the analysis of simultaneously collected discrete bottle samples, (particulate matter concentration: PMC, particulate organic carbon concentration: POC, chlorophyll α concentration: chl α, particle volume concentration and particle size distribution measured in situ), optical proxies increase the vertical resolution of changes in particle properties in the water column. We report relationships of inherent optical properties (beam attenuation at 2 wavelengths, fluorescence) and bulk particle properties obtained in the NW Black Sea during October 2007. The Danube River delta area was heavily stratified at that time, mainly due to a sharp thermocline at 17–27m. Beam c p and fluorescence were significantly correlated and showed highest values near the coast, with a decreasing trend offshore. In situ measured particle size distributions were characterized by modes at ~40μm, 20μm and 5μm. PMC, POC, and chl α exhibited wide ranges of spatial variation, a common feature being the gradual decrease in concentrations from the coast to offshore. The POC:PMC and POC:chl α ratios suggested a general predominance of biogenic material over terrigenous particles throughout the study area. The commonly accepted sequence of large phytoplanktonic species transitioning to smaller ones during summer–autumn was confirmed by light microscopy and SEM observations. Detritus of Chaetoceros sp. and other diatoms was the dominant component of particulate matter. The small percentage of terrigenous particles was surprising given the high riverine sediment loads suggesting that most of the sediment load flocculated and was deposited before reaching the delta. Given the lack of previous data in this area, our study may serve as a baseline or background to look for changes in future bio-optical and/or biogeochemical measurements. [Copyright &y& Elsevier]

Abstract: 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 c p), 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: 0.02–0.85mgL−1), with values substantially higher in the euphotic zone during spring. The deep waters (>200m) in the Eastern Mediterranean exhibit extremely low PMCs, well‐below 0.1mgL−1. Total chlorophyll α concentrations ranged from 0.003 to 0.28μgL−1 in spring and from 0.08 to 0.19μgL−1 in summer, verifying the ultra‐oligotrophic character of the area. A significant correlation of beam c p 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: 31–230μm; median ~85μm) 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. [Copyright &y& Elsevier]