Downward fluxes of sinking particulate matter in the deep Ionian Sea (NESTOR site), eastern Mediterranean: seasonal and interannual variability
In order to assess seasonal and interannual variability in the export of particulate matter and its main constituents, sediment traps were deployed at five successive depths from February 2006 to March 2010 in the deepest basin of the Mediterranean (SE Ionian Sea, NESTOR site). The average total mass fluxes were 66, 58, 54, 34, and 52 mg m −2 d −1, at 700, 1200, 2000, 3200, and 4300 m, respectively. The temporal variations of the mass flux showed similar seasonal signal at all sampling depths with higher values in spring–summer and lower in autumn–winter. Changes in the main constituents of the mass flux (organic carbon, carbonates, opal, and lithogenic matter) largely followed the same temporal variability with total mass flux, revealing mechanisms of rapid vertical (top-down) transport from 700 m down to 4300 m depth. Lateral inputs at the deepest trap are probably of importance, attributed to the influence of the deep Adriatic water, characterized by relatively higher turbidity than overlying water masses. Two major processes seem to control the seasonal mass flux variability: (a) primary productivity at the euphotic zone; and (b) episodic dust input events. Primary productivity shows two maxima during late winter/early spring and late spring/summer, as witnessed by the organic carbon, carbonate, and opal fluxes in the mesopelagic and bathypelagic layers, whereas the influence of dust inputs is evidenced by enhanced lithogenic fluxes occurring during spring and summer. The interannual variability generally shows a gradual increase of fluxes during the time frame of the experiment. Both seasonal and interannual variability of mass flux are associated with variations in the intensity and position of the neighboring Pelops anticyclonic gyre, which appears to affect the upwelling of intermediate, nutrient-rich waters and subsequently the surface productivity. Combination of estimated satellite and algorithm-generated primary production data for the Ionian Sea, calculated POC fluxes out of the euphotic layer and POC fluxes measured by sediment traps at the mesopelagic and bathypelagic layers of NESTOR site during our 4 yr experiment, reveal that only a small portion of primary production (0.46%) reaches at 3200 m, corresponding to a mean annual carbon export of 1.12 g C m −2 yr −1.