Use of biogenic silica in sediment to record diatom density and their responses to environmental change in Sanggou Bay maricultural area over the past 200Â years

Two sediment cores (S1 and S2 from shellfish and kelp cultivation areas, respectively) were collected from the Sanggou Bay maricultural area for the analysis of biogenic silica (BSi), an ideal proxy for the reconstruction of diatom density. The results show that prior to the advent of mariculture, the maximum diatom density in the two cores were observed in the 1880s before the first industrial revolution, and the maximum densities were 6500 × 104 ind m−3 and 5300 × 104 ind m−3 for S1 and S2, respectively. The diatom density was primarily affected by natural factors, such as the sea surface temperature (SST), the East Asian Winter Monsoon Index (EAWM), and the wild shellfish feeding pressure. The diatom density was affected by different cultivation methods and maricultural density during the maricultivation period. The diatom density decreased to 2800 × 104 ind m−3 in the shellfish cultivation area (S1) in 1996, and the diatom density was only 3600 × 104 ind m−3 in the kelp cultivation area (S2). Subsequently, a shellfish-kelp polyculture approach was implemented and diatom density started to continuously increase. The total diatom densities in S1 and S2 were restored to 3700 × 104 ind m−3 and 4200 × 104 ind m−3 in 2006, respectively. Nonetheless, the total diatom density in core S1 was only 55% of the density in the 1880s. This observation indicates that additional efforts are required to promote modifications to improve the cultivation environment and establish an environmentally-friendly cultivation model.