Coupling plankton - sediment trap - surface sediment coccolithophore regime in the North Aegean Sea (NE Mediterranean)

Abstract

Abstract Quantitative coccolithophore analyses were performed on data of 2011–2015 dataset derived from the examination of plankton samples (until 100\u202fm water depth), the sinking particulate matter (collected by a time series sediment trap) and surface and core sediments of the North Aegean Sea sampling site M2 (Athos Basin, NE Mediterranean). The aim was to achieve a better understanding of the potential modifications of the coccolithophore assemblage between the surface waters (plankton), the water column (sidocoenosis) and the underlying sediment (thanatocoenosis). Sediment trap calcareous nannoplankton multiannual mean fluxes (20\u202f×\u202f108 coccoliths m2\u202fday−1) documented similar values to the accumulation rates recorded in the surface sediment (23.6\u202f×\u202f108 coccoliths m−2\u202fday−1), whereas within the last 500\u202fyears the rates have ranged from 2.19\u202f×\u202f108 to 5.4\u202f×\u202f108 coccoliths m−2\u202fday−1. The dominant species in all sampling types was Emiliania huxleyi, reaching in some cases striking relative abundances of 85–90%. Morphometric analyses on E. huxleyi coccoliths documented the presence of a lightly calcified morphotype in the water column and sediment trap samples in addition to the seasonal occurrence of heavier calcified E. huxleyi coccoliths, indicating enhanced Black Sea water inflows during May 2011, February 2015 and October 2015. However, such a signal was not preserved in the surface sediment assemblage mostly due to processes of diagenetic calcite overgrowth. Notably, Florisphaera profunda was not included in the upper water column plankton assemblage but in the sediment traps and surface sediments. Thus, it is presumed to flourish in nutrient-enriched layers below the sampled 100\u202fm water column depth. Several fragile Syracosphaeraceae and holococcolithophore species and the delicate Algirosphaera robusta were not present in the sinking assemblage or on the seafloor; however, the main features of the living assemblages were generally preserved. The recorded high relative abundances of E. huxleyi (~60%) in the sediment were in accordance with positive North Atlantic Oscillation (NAO) shifts during recent production of dense water masses in the North Aegean.