Phytoplankton diel rhythm in the waters of Quanzhou Bay in Fujian, China

Twenty-four hour observations with three hour interval on the phytoplankton diel-rhythm at three stations in the north opening of Quauzhou Bay of Fujian were conducted seasonally from autumn of November in 2001, winter of February, spring of May and summer of August in 2002.The results show that chlorophylla varied with tidal currents with higher biomass appeared at low tide period and lower biomass during the high tide period.Chlorophyll a was in general higher at the station in the side of inner bay than the outer ones, showing a high to low gradient of phytoplankt on biomass from the inner bay to the outer.In the other hand, higher biomass was mostly found at the bottom water layer other than the upper water layer, where the biomass had partly contributed by the suspended benthic microalga, such as Paralia sulcata.Further analyses indicated that phytoplankton community changed in its structure with lower biodiversity index in the daytime than in the night.The dissimilarity of the phyt oplankt on communities between day and night was as high as 9, exhibiting an evident diel rhythm by some species, such as Skele-tonema costatum that multiplies and dominates during the day.However, some benthic algae tended to have higher density during the night, suggesting the dispersion of alga cells by the currents at night after daytime growth.Cell multiplication, resuspension and dispersion process of phyt oplankt on should be considered as control factors in the diel rhythm though tidal currents could be the stronger factor.