Τριανταφύλλου Μαρία, Καθηγήτρια, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, ΕΚΠΑ
The main objective of the present study is the investigation of coccolithophore export production in the NE Mediterranean Sea and the comparison of sediment trap fluxes. Coccolithophore export production has been studied in sediment trap samples collected during various time intervals from three Eastern Mediterranean sites: the mesotrophic North Aegean Sea (Athos Basin) at 500 m depth (June 2011-November 2015), the ultra-oligotrophic South Aegean/Cretan Sea at 1500 m depth (February 2001-January 2002, December 2014-December 2015) and the oligotrophic SE Ionian Sea at 2000 m depth (January 2010-September 2015). The data are compared in order to define the spatial and seasonal variability in assemblage composition and in coccosphere fluxes at different depths. Through the present study we can observe the water column biogenic sedimentation as reflected to the coccolithophore export production.
During December 2014-December 2015, coccolithophore fluxes reveal a highly seasonal pattern, with a peak during February to March (N. Aegean: max. 8.56 x 106 coccospheres m-2 day-1, Ionian Sea: max. 0.81 x 106 coccospheres m-2 day-1 and Cretan Sea: max. 3.37 x 106 coccospheres m-2 day-1). The flux maxima occurred during the phase of water column mixing and coincide with the interval of decreased sea surface temperatures. Coccosphere fluxes are dominated by E. huxleyi (75% in average in the North Aegean, slightly exceeding 50% in average, in Cretan and Ionian Seas), followed by Syracosphaera spp. (14% in average in the North Aegean, ~10% in average in Cretan and Ionian Seas) and Algirosphaera robusta (up to 14% in average in the Ionian Sea; more than 45% in February 2015); Umbilicosphaera sibogae is the most important out of the minor species, particularly in the Cretan Sea (13% in average, max. flux 1.01 x 106 coccospheres m-2 day-1 in March 2015).
Apparently, N. Aegean total coccosphere flux is considerably higher, due to the prominent seasonal peak of E. huxleyi (>95% of coccolithophore assemblage in January 2015, max. E. huxleyi flux: 7.92 x 106 coccospheres m-2 day-1). A comparable seasonal signal in coccosphere fluxes was also observed within previously sampled time-series at the same locations; however, a trend towards increasing coccosphere fluxes during the last years is observed in the Cretan Sea (max. 0.4 x 106 coccospheres m-2 day-1 in 2001) and the N. Aegean Sea (max. 0.29 x 106 coccospheres m-2 day-1 in 2011). Overall, the fluxes are strongly dependent on the regeneration of nutrients by vertical mixing in winter in the North Aegean and/or the fertilizing influence of episodic dust input events in the South Aegean and Ionian Seas, as this is evidenced by the lithogenic and nutrient fluxes.
A comparative study on E. huxleyi coccolith morphology and degree of calcification was made through SEM images from SE Ionian and Cretan Sea sediment trap samples, revealing higher relative tube width values (~0.3 in the Cretan Sea, >0.35 in the Ionian) during the increased coccolithophore productivity time intervals. Several samplings from the Aegean water column, have shown that E. huxleyi has slightly larger and significantly more heavily calcified coccoliths during winter – early spring when lower temperatures and higher productivity take place, than in summer. The obtained data from the present sediment trap samplings reveal highest relative tube width values in the Ionian sediment trap observed in February 2015, in accordance with the minimum Sea Surface Temperatures. Similarly, the maximum relative tube width values occurred within April 2015 in the Cretan Sea time-series, exposing one month delay in respect to the minimum SSTs, but overall keeping with the E. huxleyi seasonal calcification pattern in the NE Mediterranean.
The combination of the results can provide us a clear picture of the spatial and seasonal variability of the coccolithophore fluxes in the Aegean and Ionian Seas, as well as the response of these organisms to the increasing CO2 of the atmosphere.
coccolithophores, export production, fluxes, sediment traps, NE Mediterranen, coccolithophore calcification, ocean acidification