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

Abstract

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 × 10⁴ to 1.45 × 10⁵ cell mL^–1 and from 0.78 × 10³ to 7.95 × 10³ cell mL^–1, respectively. The abundance of HDNA bacteria in surface water (mean: 5.58 × 10⁵ cell mL^–1) was greater than in bottom water (mean: 3.74 × 10⁵ cell mL^–1), with significant difference (n = 12, P = 0.05). The difference in LDNA abundances between surface (mean: 7.06 × 10⁵ cell mL^–1) and bottom (mean: 4.83 × 10⁵ 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.