Seasonal variations of Aulacoseira granulata population abundance in the Pearl River Estuary

Seasonal variations of the abundance of Aulacoseira granulata populations were studied in the Pearl River Estuary from August 2007 to May 2009. The average abundance of A. granulata reached 134.14 � 10 3 cells dm � 3 (10.66 � 10 3 filaments dm � 3 ), contributing 9.1% to the total phytoplankton. Compared with freshwater areas, A. granulata abundance in the Pearl River Estuary was one or more orders lower. Among the four variant forms of A. granulata, the dominance sequence was A. granulata var. granulata > var. angustissima > var. curvata > var. angustissima f. spiralis. The dominance of A. granulata var. granulata and A.g. var. angustissima benefited from their linear shape. A. granulata abundance was higher in the warm seasons and lower in the cold seasons. The fluctuations of Si concentrations could also influence contrasting results as in May 2008 and May 2009. The spatial distribution of A. granulata was mainly influenced by the water runoff of the Pearl River at different outflow channel. We found that the relative abundance of var. angustissima to A. granulata var. granulata was much higher in winter than in other seasons, which maybe because high cell surface/volume area (A/V) benefited var. angustissima by ensuring more effective uptake of nutrients and CO 2 and a slower sinking velocity than var. granulata.

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