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Changes in the biogeochemical cycling of mercury (Hg) and Hg species were investigated in a typical marine aquaculture area located at Zhoushan island, Zhejiang province, east China. Mercury species were analyzed in different environmental samples collected during a field survey and a simulation batch experiment. The field work comprised both summer and winter collection of water and sediment samples from marine aquaculture sites (MAS) in a field survey and from corresponding reference sites (CRS) located 2500 m from the MAS. THg concentrations in water were 91.3 ± 70.3 and 115 ± 22.6 pmol L−1 in summer and winter, respectively. Particulate Hg accounting for ˃60% of THg and positively correlated with total suspended solid content in water. Dissolved organic carbon in water was positively correlated with dissolved Hg. Significantly higher (p < 0.001, F = 102) total methylmercury (TMeHg) contents were observed in MAS (0.31 ± 0.26 ng g−1) than in CRS (0.06 ± 0.03 ng g−1) in the sediment solid phase. Moreover, MeHg formation rate in MAS was clearly higher than in CRS in the simulation experiment. Both the field survey and the simulation experiment highlighted the readier formation of MeHg in MAS than in CRS. TMeHg levels in blackhead seabream (Acanthopagrus schlegelii), red snapper (Lutjanus campechanus) and perch (Perca fluviatilis) were only 52.7 ± 5.74, 23.7 ± 2.51 and 24.3 ± 3.86 ng g−1, values significantly lower than the safety guideline (1000 ng g−1) established by the World Health Organization. Unlabelled Image • THg concentration in water was 91.3 pmol·L−1 and PHg account for 63.4% of THg. • Fish farming increased THg accumulation and MeHg formation in sediment. • Suspended solid influence the Hg distribution and sedimentation in water. • MeHg concentration in fish tissues was significantly lower than WHO guidelines. [ABSTRACT FROM AUTHOR]