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Abstract
Particulate organic carbon (POC) plays an important role in the global carbon cycle. The POC in the Changjiang Estuary and adjacent coastal region of the East China Sea (ECS) is dominated by riverine input and marine production and is significantly influenced by the three gorges project (TGP). A coupled physical–biogeochemical model was used to evaluate TGP’s impact on POC. The results demonstrate that TGP regulates the area influenced by diluted water and POC through direct river and sediment discharge and affects the ecosystem. From the early to later TGP construction periods, the surface region with high-POC concentration (>40 μmol L−1) decreases by 20.5% in area and 11.5% in concentration. Meanwhile, POC in the whole water column decreases from 19.5 to 17.8 μmol L−1. By contrast, the concentrations of chlorophyll-a (Chl-a) and related nutrients increase. A three end-member mixing model based on quasi-conservative temperature and salinity is used to quantify relative contributions of different water sources to POC in our research area. We also estimate the biological POC production by the difference between the physical-biogeochemical model predicted POC and three end-member model mixing POC. The result demonstrate that under the regulation of TGP in the later period, the decrease of sediment load increases water transparency, which favors photosynthesis and oceanic biological produced POC. In addition, over 70% of the areas have C/Chl-a > 200 and high C/N ratios, which are circumstantial evidences that organic detritus and terrestrial input sources still dominate in the Changjiang Estuary and adjacent coastal ECS but are influenced by TGP’s regulation.
Highlights
The particulate organic carbon (POC), a small proportion of the total carbon [1,2], is an important component of the “biological pump,” which results in the capture and storage of carbon that sinks into the deep ocean [3,4,5,6]
The model results can be divided into three parts according to three gorges project (TGP) construction periods: 1) ES-TGP (1994–1997): early period, from the beginning of the three gorges dam construction to the damming of the Changjiang River; 2) MS-TGP (1998–2003): middle period, from the filling of TGP reservoir to the beginning of the power generation; and 3) LS-TGP (2004–2008): late period, from the time of water level raising and full power generation to the end of the construction
Historical observations and studies revealed that TGP regulates the river flux into the East China Sea (ECS), with a light but stable decrease, except for the LS-TGP, during which the sediment discharge presents a dramatical decrease of ~55% compared with the ES-TGP
Summary
The particulate organic carbon (POC), a small proportion of the total carbon [1,2], is an important component of the “biological pump,” which results in the capture and storage of carbon that sinks into the deep ocean [3,4,5,6]. It has been reported that ~0.4 Gt (1 Gt = 109 t) POC from the terrestrial ecosystem are discharged into oceans via rivers each year [7]. Biological productivity of phytoplankton is another important source of POC. Upwelling transports nutrients from the sediment to the euphotic layer as supply for the POC production [10]. The results showed that POC usually increases a few days after typhoon passage [13,14] due to sediment resuspension and the growth of phytoplankton [15,16]
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