Particulate matter spatial–temporal distribution and associated surface sediment properties: Thermaikos Gulf and Sporades Basin, NW Aegean Sea

Abstract

Particulate matter (PM) distribution was studied in the NW Aegean Sea (Eastern Mediterranean) over a series of four seasons. PM concentrations in the surface and bottom nepheloid layers exhibit persistent maxima close to the river mouths; the rivers are identified as the major sources of PM. In general, higher particulate matter concentration (PMC) values appear in the shelf of the Thermaikos Gulf, whereas the deep Sporades Basin represents the area of lowest PM concentration. PMC during winter is much higher than in summer. This strong seasonal feature is attributed to the variation of the fresh-water supply, which is maximised during winter and spring, and almost negligible during the dry seasons. PM is mainly transported southwards and preferentially along the western coastline of the study area, due to a well-defined and dominant cyclonic circulation. Lateral transport of the PM prevails, whereas vertical particle transport is relatively limited. BNL formation is mainly attributed to sediment resuspension forced by the near-bottom currents. Surface sediment grain-size distribution is characterised by the presence of relict sands in the central and eastern shelf, while fine-grained muddy sediments cover the western shelf and the Sporades Basin. Relict sands were deposited during the Holocene sea-level rise. The general water mass circulation and the distance from the PM sources seem to favour PM deposition close to the river mouths and along the western coastline, while PM hardly reaches the eastern part of the area. As a result, relict sands remain uncovered or partly mixed with modern fine sediments. The shelf traps most of the PM, while some portion escapes to the deep Basin, probably from the western side of the continental slope.