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Coccolithophore export production and response to seasonal surface water variability in the oligotrophic Cretan Sea (NE Mediterranean)

¹ Department of Historical Geology-Paleontology, Faculty of Geology, Panepistimiopolis, University of Athens, 15784, Athens, Greece
² Department of Paleoecology and Paleoclimatology, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, The Netherlands
³ Department of Oceanography, Institute of Marine Biology and Genetics (IMBG), Crete, Greece, email: mtriant{at}geol.uoa.gr

A single mooring with two sediment traps (trap A at 500 and trap B at 1700 m below sea surface) was deployed at the southern margin of the Cretan Sea at a depth of 1750m. The duration of the deployment was 12 months (from January 2001 to February 2002) with sampling intervals of approximately 15 days.

The results obtained from trap A at 500 m depth show that the total coccolithophore export production was generally low. The highest productivity season was recorded from January till late September with the highest flux values observed between late March to late June (maximum flux: 9 x 10⁸ coccoliths m^–2 day^–1 and 9.4 x10⁵ coccospheres m^–2 day^–1). During this period the pattern was characterized by three main phases that correlate well with the gradual increase in SST from January till September. The lowest fluxes were recorded between October 2001 and January 2002. The total calcareous dinocysts flux also followed the same pattern. The annual coccolith flux of 9.6 x 10¹⁰ m^–2 year^–1 at 500 m was one order of magnitude higher compared to that previously recorded in Bannock Basin at 3000 m water depth.

The maximum coccosphere flux from trap B at 1700 m depth (3,9x10⁵ m^–2 day^–1) occurred in late April, a month later compared to trap A. Apparently the magnitude of the coccosphere flux decreased with depth, but the overall seasonal pattern was preserved in the deeper environments. The annual coccosphere flux at 1700 m was 3,4x10⁷ m^–2 year^–1 and calcareous dinocysts displayed a similar trend with highest values occurring at mid April 2001. Both coccolith and coccosphere sinking assemblages at 500m were characterized by high abundances of Emiliania huxleyi, (average 65% and 46% respectively). Secondary species in coccolith fluxes were Florisphaera profunda (average 9%), Rhabdosphaera clavigera (average 6%), Syracosphaera spp. (average 5%). The calculated total coccolithophore and calcareous dinocyst carbonate flux is relatively low (between 0–26% of the total carbonate flux) attaining higher values during the high productivity period.

It is evident that the coccolithophore productivity pattern in the Cretan Sea (highest values at late March – late June) is the result of fertilization in the upper photic zone from the intrusion of the Transition Mediterranean Water (TMW) water masses from the east. Sea surface temperature (SST) and precipitation are environmental parameters directly associated to seasonality and therefore influence the observed coccolithophore productivity patterns.

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