Downward fluxes of sinking particulate matter in the deep Ionian Sea (NESTOR site), eastern Mediterranean: seasonal and interannual variability

Abstract

Abstract. 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 .

Composition and sources of sedimentary organic matter in the deep eastern Mediterranean Sea

“…The range of lithogenics, carbonates and opal contents recorded in the investigated samples are similar to those previously reported for the eastern Mediterranean Sea (Emelyanov and Shimkus, 1986;Bethoux, 1989;Cros, 1995;Kemp et al, 1999;Rutten et al, 2000;Struck et al, 2001). OC contents reach values slightly above 1 % and are also comparable to those found in the eastern Mediterranean Sea (0.56-1.51 %, Danovaro et al, 1993;0.23-0.99 %, Bianchi et al, 2003;0.30-0.82 %, Gogou et al, 2000;0.25-1.73 %, Polymenakou et al, 2006) and relatively lower than those found in the western Mediterranean Sea (0.80-1.60 %, Kaiser et al, 2014;0.47-1.53 %, Masqué et al, 2003;0.23-1.85 %, Roussiez et al, 2006).…”

“…Stavrakakis et al, 2000Stavrakakis et al, , 2013Theodosi et al, 2013). In the deep EMS, the distribution of pelagic carbonates (second PC) seems to be mainly influenced by planktonic contributions.…”

“…The overall sedimentation rate in the deep areas of the open EMS is low (i.e. 2-5 cm kyr −1 ) (Van Santvoort et al, 1996Garcia-Orellana et al, 2009;Stavrakakis et al, 2013).…”

Spatiotemporal variability of alkalinity in the Mediterranean Sea

“…However, this process is poorly quantified for the Mediterranean Sea (only sparse measures of calcifiers abundances are available Siokou-Frangou et al, 2010;Oviedo et al, 2015) so it was not possible to include it in the model. Nevertheless, an offline estimate derived by combining information from global studies and models (Poulton et al, 2007;Gregg and Casey, 2007;Jansen et al, 2002), data on sedimentation (Stavrakakis et al, 2013) and our results indicates extra terms of the alkalinity budget up to −0.007 mmol m −3 d −1 in the upper part of the water column and up to about +0.004 mmol m −3 d −1 for the intermediate layer.…”

Enhanced carbon export to the abyssal depths driven by atmosphere dynamics

“…Despite the macronutrient and micronutrient (mainly N, Si, and Fe) inputs from the atmosphere, the antiestuarine circulation of the EMS, with nutrient‐depleted Atlantic Water flowing in through the Strait of Sicily and nutrient‐rich Levantine Intermediate Water (LIW) flowing out at intermediate depths, results in a net outflow of nutrients and ultraoligotrophic conditions [e.g., Ribera d ' Alcalà , ; Huertas et al ., ]. This leads to a phytoplanktonic community dominated by heavily grazed picophytoplankton and nanophytoplankton [ Yacobi et al ., ; Zohary and Robarts , ; Christaki , ; Siokou‐Frangou et al ., ] and drives primary production rates below 200 mg C m −2 d −1 [ Psarra et al ., ; Moutin and Raimbault , ], which translates into very low amounts of OC, from 0.01 to 9.66 mg m −2 d −1 , reaching the EMS deep sea [ Warnken , ; Koppelmann et al ., ; Stavrakakis et al ., ; Gogou et al ., ].…”