Continuing influence of the changed thermohaline circulation in the eastern Mediterranean on the distribution of dissolved oxygen and nutrients: Physical and chemical characterization of the water masses

Abstract

Changes in the vertical distribution of the physical and chemical parameters observed during this study point to the continuing spatial and temporal evolution of the Eastern Mediterranean Deep Water (EMDW). These changes influenced primarily the water column below 800 m and confined the old EMDW of Adriatic origin to the 1000–2000 m in the Levantine, uplifting the minimum‐oxygen/maximum‐nutrient (MinOx/MaxNut) from 2500 m in 1995 to 1500 m in 1999. The denser and younger EMDW of Aegean origin (EMDWAeg) appeared at the bottom layer, more evident at the central eastern Mediterranean and moving prevalently into the Levantine basin. Younger water still intruded the central area at intermediate depths. In the Levantine Intermediate Waters (LIW) oxygen decreased and nutrients increased westward in agreement with its pathway across the basin. However, a temporal decrease in oxygen and concurrent increase in nutrient was found in the LIW because of its confinement in the Levantine by anticyclonic features. Temporal changes were also found in the EMDW at the western Ionian and eastern Cretan passage. The differences in the vertical placement of the MinOx/MaxNut layer among the different areas were explained by the physical processes, but the relative displacement of the extreme points within the same area were probably a result of oxidation of particulate matter poorer in nitrogen and phosphorus in the Levantine than in the Ionian. Maximal concentrations of silicic acid were located deeper than the other nutrients because of the slower chemical dissolution of silicious tests. The N:P molar ratios at the EMDW were high (26.2–30.7), highest in the younger EMDWAeg. These differences could not be explained by different dissolved inorganic nutrients concentration at the sources nor by the composition of particulate matter in the euphotic zone. It was suggested that DOM may have a significant role in determining those differences; however, data on DOM concentration and composition across the basin is lacking in order to test this hypothesis.