Atypical Seasonality of the Silicon Cycle in the Yellow River Estuary and Bohai Sea Revealed by Stable Silicon Isotopes

Biogeochemical Si cycle in coastal areas is of vital importance due to its close link with the carbon cycle. However, the coastal Si cycle has been heavily perturbated by human activities. In this study, we studied the spatiotemporal distribution of biogenic Si (BSi) and dissolved Si (DSi) combined with stable Si isotopes of DSi (delta Si-30(DSi)) in the Yellow River estuary and Bohai Sea, one of the most populated coastal areas in the world. Over an annual cycle, BSi and DSi concentrations varied from 0 to 43.5 mu mol L-1 and from 0.3 to 40 mu mol L-1, respectively. This was associated with large delta Si-30(DSi) variations from +0.49 +/- 0.22 parts per thousand (2sd) in spring to +2.92 +/- 0.14 parts per thousand in winter, which opposed to observations that summer delta Si-30(DSi) values were usually higher than those in winter. This atypical variation could be attributed to the water-sediment regulation on the Yellow River occurring every early summer, leading to a strong water mixing pattern and suppressing diatom production in summer. This mixing was further prolonged by extreme autumn rainfall on land. The pulse supply of nutrients subsequently enhanced primary productivity from autumn through winter. In spring, the resuspended seafloor sediments were likely an important DSi source with delta 30Si values of <-0.5 parts per thousand. Our findings suggest that natural Si seasonality has been greatly masked by human activities and climate events in the Bohai Sea. Our study serves as a reference of the Si cycle research endeavors worldwide for revealing the overlaying effect of anthropogenic consequences and natural variability. Plain Language Summary Three quarters of carbon in coastal areas are removed from the atmosphere through diatoms that utilize silicon as an essential nutrient. However, the biogeochemical silicon cycle in coastal areas has been strongly altered by human activities in recent decades. This study chooses one of the most perturbated coastal areas, the Yellow River estuary and Bohai Sea, to assess the status of today's silicon cycle using stable silicon isotopes. We find higher silicon consumption in winter than that in summer, which is opposite to natural patterns. This is attributed to the water-sediment regulation on the Yellow River occurring in early summer every year. Pulse inputs of water and particles lead to strong water mixing and increased water turbidity, hindering primary production in the river mouth and Bohai Sea in summer. Such effects can be further extended by heavy rainfall and water floods from land in early autumn. When the water calms down, diatom production emerges in autumn and winter. Our study demonstrates that human perturbations can superimpose natural patterns of the silicon cycle in coastal areas, which is further complicated by extreme climate events. This "artificial" phenomenon should be included in coastal nutrient budgets over seasonal to annual time scales.