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First evidence of an intensive bloom of the coccolithophore Syracosphaera halldalii in a highly variable estuarine environment (Krka River, Adriatic Sea)

In this paper, we provide the first evidence of a bloom formation of coccolithophore Syracosphaera halldalii in a salt‐wedge estuary of Krka River (Mediterranean Sea), during stratified conditions. The increase of coccolithophore abundance was firstly noticed in August 2017 and occurred again in October 2017. The increased abundance appeared over wide salinity (16.9–35.3) and temperature ranges (16.4–23.3ºC). On these two occasions, the highest abundance of S. halldalii confirmed by SEM counts reached 2.51 × 106 cells L−1 at halocline in October. The contribution of S. halldalii to total coccolithophore assemblage was more than 97%. Vertical distribution of total coccolithophores abundance varied over time and followed the temperature curve, with the highest abundances co‐occurring with maximum temperature in the water column. A positive association of coccolithophore abundances with nitrates was found, but pulses of both organic nitrogen and nitrites seem to be important for sustaining the bloom after the initial proliferation. During the entire study period, S. halldalii was found only in the heterococcolith‐bearing phase, and the absence of the holococcolith‐bearing phase could be related to increased turbulence in the estuary. SEM analysis of cell morphology was in concordance with earlier descriptions, with apical coccoliths and body coccoliths of approximately equal size and both having distinct protruding teeth along the inner rim. A single exothecal coccolith was found in association with two S. halldalii cells indicating possible dithecatism of this species.

Syracosphaera halldalii, coccolithophore, bloom, Adriatic Sea

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