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Structure and environmental drivers of target phytoplanktonic resting stage assemblages in the central Mediterranean Sea

Silvia Casabianca*, Samuela Capellacci, Fabio Ricci, Francesca Andreoni, Tommaso Russo, Michele Scardi, Antonella Penna

*Corresponding author:

ABSTRACT: Phytoplankton species can produce resting stages that persist in the sediments over long periods and accomplish important ecological functions. With the aim of investigating the assemblage structure in relation to environmental drivers and human pressures, resting stages of diatoms and dinoflagellates including HAB target taxa were analyzed in surface sediments of three Mediterranean regional areas. Abundance of resting stages were determined by molecular qPCR. Multivariate data analysis confirmed that the abundance of resting stage assemblages seemed related to depth. Regional differences in the composition of the resting stage assemblages were evident and environmental drivers were correlated with those regional differences. Three main groups of samples were defined according to SST and depth thresholds. Samples from the northern and central Adriatic Sea (average SST < 18°C) formed the richest assemblages, both in terms of abundance and species richness, while deep samples from all other basins (depth > 368 m) were poorer and less diverse than those from shallower sites. Resting stage taxa contributed differently in the three groups. Diatom spores and dinoflagellate cysts were the most abundant taxa, but Alexandrium minutum cysts and Ditylum brightwellii spores also accounted for a large share of the overall inter-group distance. The structure of resting stage assemblages can be regarded as the time- and space-integrated response of a subset of phytoplankton species to the environmental conditions including the physical oceanographic dynamics that favored or prevented sedimentation of resting stages.