Particle sources over the Danube River delta, Black Sea based on distribution, composition and size using optics, imaging and bulk analyses

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 alpha concentration: chl alpha, 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 17-27 m. 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 similar to 40 mu m, 20 mu m and 5 mu m. PMC, POC, and chl alpha 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 alpha 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. (C) 2013 Elsevier B.V. All rights reserved.