Persistent organic pollutants (POPs) present in water may be bioconcentrated in phytoplankton and further transferred into higher trophic levels. In the present study, seawater, sediment, phytoplankton and macroalgae (Ulva lactuca L.) samples were collected from two estuarine bays in South China and analyzed for 24 polybrominated diphenyl ethers (PBDEs) and 22 organochlorine pesticides (OCPs). The concentrations of PBDE congeners except BDE-209 were low in both phytoplankton and Ulva. BDE-209 was the predominant congener in phytoplankton and Ulva, accounting for 89.5% and 86.6% of the total average concentrations of PBDEs (48.5 and 4.1 ng g(-1) dw), respectively. The average concentrations of DDTs, HCHs and 1-chloro-2,2-bis(4-chlorophenyl) ethane (p,p'-DDMU) in phytoplankton were 398, 241 and 11.3 ng g(-1) dw, respectively, while those of DDTs and HCHs in Ulva were 8.4 and 33.1 ng g(-1) dw. The levels of both PBDEs and OCPs were an order of magnitude higher in phytoplankton than in Ulva, indicating that phytoplankton with larger surface areas have higher uptake efficiency for POPs than Ulva. Bioconcentration factors (BCFs) of DDT and PBDE in phytoplankton from the two bays were in the range of 10(5)-10(6), suggesting that bioconcentration may be one of the key sources of POPs and algae can be an important route for POPs to move toward higher trophic levels. (C) 2016 Elsevier B.V. All rights reserved.