River-induced particle distribution in the northwestern Black Sea (September 2002 and 2004)

Observations from two comprehensive oceanographic surveys on the northwestern Black Sea are analyzed to reveal the hydrological characteristics of the shelf area around the Danube delta in relation to particulate matter dynamics. These processes have broader implications on flows through the adjacent complex strait system. The surveys cover both a period of high (September 2002) and low (September 2004) discharge of the Danube River. The resulting low-salinity waters generally occupied the upper 10-15 m within the 20-30 m thick warm layer that was separated by deeper cooler waters through a sharp thermocline. The buoyant plume was influenced by the discharge conditions and the prevailing wind patterns. The plume structure was identified using in situ bottle data, continuous beam transmission and fluorescence profiles, and satellite remotely sensed data. Particulate matter concentration values correlated with low-salinity riverine waters and their maxima were recorded in the immediate vicinity of the Danube branches, decreasing rapidly toward the open sea. The preferred particle transport pathway in 2002 was along a narrow strip near the coast, with a south-southwest direction, favored by the strong northeasterlies which predominated prior to the cruise. Low-salinity waters and riverine particles in 2004 were characterized by mostly offshore removal, attributed to calmer northwesterlies prior to the survey. Changes in ambient stratification due to wind-driven vertical displacements of the seasonal thermocline appeared to also influence the plume dynamics. The downwelling (upwelling) conditions that prevailed in 2002 (2004) were revealed in both the in situ vertical profile temperature measurements and the satellite-derived sea surface temperature patterns.