Microbial processes involved in water biogeochemistry are linked to physical and chemical variability of water masses. While the occurrence of hydrological changes in the Eastern Mediterranean Sea has been documented, its effects on microbial dynamics and processes have been poorly investigated. An interdisciplinary survey was carried out in November 2011 to depict recent scenarios in the Ionian Mediterranean Sea. This study provides a snapshot of the hydrology and biogeochemistry of this basin, with particular emphasis on the abyssal waters, where two cores of the Eastern Mediterranean Deep Water, both of Adriatic origin, were found. The first core, located at about 3000 m, overlies the second one, which has a recent origin. Both water masses have Adriatic origin and the upper one is older than the lower one. A novel approach has been adopted to relate changes in microbial abundance and metabolism to new thermohaline and biogeochemical findings. A close relationship between surface processes driven by air–sea interaction, microbial variables, physical and chemical parameters (nutrients, dissolved organic carbon and oxygen) was observed. Being intimately coupled to DOC patterns, the microbial respiratory activity was likely the most responsive microbial parameter to hydrological changes, confirming this variable as a sentinel of environmental changes. © 2018, Springer International Publishing AG, part of Springer Nature.

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