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Phytoplankton seasonality and composition along the coastal NE Adriatic Sea during the extremely low Po River discharge in 2006

Phytoplankton composition and abundance were analyzed in the period January – September 2006, along the NE coastal Adriatic. In 2006, the Po River discharge was twice as low as the 90 year average. The maximum riverine water inflow did not appear in spring but in September, resulting in a shift of phytoplankton maximum from spring to autumn. Diatoms provided most abundant phytoplankton with an annual maximum of 6 x 10^5 cells/L in the west Istrian coastal region (in September-October). The annual average abundance of diatoms was low (5.1 x 10^4 cells/L), but bigger than that of dinoflagellates (28x), coccolithophorids (13x), cryptophytes (6x) and green nanoflagellates (9x), respectively. A comparison was made between phytoplankton seasonality in the dry year 2006, and the year 2001 characterized by greater freshwater inputs. The drought period was characterised by the small cell size diatoms Cyclotella and Thalassiosira. Po River discharge from the west, the north-westerly ingoing current (Eastern Adriatic current) and oligotrophic karstic waters discharging along the eastern coast, strongly influence phytoplankton dynamics in the NE coastal Adriatic Sea.

Phytoplankton; seasonality; abundance; Po River; discharge; Adriatic Sea

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