Coccolithophore diversity and dynamics at a coastal site in the Gulf of Trieste (northern Adriatic Sea)

Two years-data (May 2011–February 2013) obtained from a monthly sampling carried out at the coastal long term Ecological Research station C1-LTER in the Gulf of Trieste (northern Adriatic Sea) were analysed to describe the seasonal dynamics and diversity of coccolithophore assemblages and to assess their relationship with environmental forcing. Coccolithophores represented 10.7% of the total Utermöhl phytoplankton that were mainly dominated by small (<10 μm) flagellates and diatoms (62.2% and 24.8% of total abundances, respectively). Coccolithophore abundances obtained by polarized light microscopy analysis ranged from 0.2 to 35.3 · 104 coccospheres L−1 with a mean value of 5.2 · 104 coccospheres L−1. A marked seasonal pattern was observed with a main peak in December–February (2.5–31.5 · 104 coccospheres L−1), in correspondence of the winter mixing, mainly due to Emiliania huxleyi, and a secondary peak in May–June (0.7–15.0 · 104 coccospheres L−1), coinciding with the increase of the light intensity and the beginning of the seasonal stratification, dominated by holococcolithophores and small Syracosphaera species. The most abundant taxa were E. huxleyi and holococcolithophores, followed by Acanthoica quattrospina, Syracosphaera species and other minor species. Statistical analyses recognized four distinct groups, corresponding to seasonal variations of environmental conditions. Considering the two years, some species displayed a recurrent seasonal pattern highlighting possible species-specific ecological requirements, while others showed an interannual variability probably due to local factors.