Influence of environmental changes on picophytoplankton and bacteria in Daya Bay, South China Sea

Environmental changes driven by intense human disturbance and natural processes govern the abundance and distribution characteristics of two picophytoplankton groups (Synechococcus and picoeukaryotes). Picophytoplancton and high DNA (HDNA) and low DNA (LDNA) bacterial groups, identified by flow cytometry, were assessed during the autumn monsoon transition period in Daya Bay (South China Sea). The abundance of Synechococcus and picoeukaryotes ranged from 2.16 x 10(4) to 1.45 x 10(5) cell mL(-1) and from 0.78 x 10(3) to 7.95 x 10(3) cell mL(-1), respectively. The abundance of HDNA bacteria in surface water (mean: 5.58 x 10(5) cell mL(-1)) was greater than in bottom water (mean: 3.74 x 10(5) cell mL(-1)), with significant difference (n= 12, P = 0.05). The difference in LDNA abundances between surface (mean: 7.06 x 10(5) cell mL-1) and bottom (mean: 4.83 x 10(5) cell mL(-1)) waters was insignificant (n = 12, P > 0.05). The results of the principal component analysis showed that both picophytoplankton (Synechococcus and picoeukaryotes) and bacteria (HDNA and LDNA) were positively related to nutrients (NO3-N, NH4-N, and SiO3-Si). Three subsystems in the bay were identified as follows: the west and east parts, the central and northwest parts, and the mouth and central part.