Siliceous plankton flux and radiolarian community structure in the highly oligotrophic Ionian Sea (Eastern Mediterranean) – Imprints of climate variability

Abstract

Based on sediment-trap material collected at 700 m depth from the oligotrophic southern Ionian Sea (Nestor site), this study explores for the first time the impact of seasonal and multiannual (2015–2017) climate variability on the siliceous plankton flux and radiolarian community structure at species level in the Eastern Mediterranean Sea. A total of 143 radiolarian taxa were identified, most of them at the species level. Juveniles represented 35–70 % of the total radiolarian flux. Thirty-nine diatom taxa were identified, including some freshwater species. Coinciding with organic carbon, opal and total mass fluxes, radiolarians and diatoms displayed rather similar pattern in seasonal flux variations, including two seasonal peaks. The first one was recorded in late winter (late February) 2015; it is regarded as the result of late winter intensification of water column mixing. The second peak in the siliceous plankton flux was recorded in late summer to early fall (August – September) of both years 2015 and 2017. This peak is interpreted to be the result of a local upwelling event following the reduced influence of the Pelops anticyclonic gyre. A third productivity event was recorded in April and May 2016, and is interpreted as being influenced by North African dust depositions. Despite the incompleteness of our sediment trap data series, our dataset is valuable as it is the first ever study conducted to the species level on polycystine radiolarian vertical export rates in the Mediterranean Sea, combined with insights into the temporal dynamics of siliceous zoo- and phytoplankton fluxes in the EMed.