C-N-P driven changes to phytoplankton community structure and gross primary productivity in river-fed reservoir ecosystems on the Chinese Loess Plateau

Due to the fast dissolution kinetics of rich carbonate minerals on the Chinese Loess Plateau (CLP), the high pH values and high concentrations of Ca2+ and dissolved inorganic carbon (DIC) characterize the surface waters on the CLP, which represents the largest loess area in the world. The unique hydrochemical properties of aquatic ecosystems on the CLP may have substantial biological carbon pump (BCP) potential, a mechanism that changes the availability of C, N, and P, which affects phytoplankton structure or composition to change eutrophication and phytoplankton abundance to improve gross primary productivity (GPP). Therefore, understanding the C-N-P driven changes in river-fed reservoir ecosystems on the CLP is essential for improving watershed management, mitigating eutrophication, and optimizing carbon sequestration. Here, we investigated the phytoplankton community structure and GPP, the stable carbon isotope compositions of DIC, particulate organic carbon (POC) (δ13CDIC and δ13CPOC), and C-N-P nutrient levels in the Wuli River-Dongfeng and Hong River-Houjiahe river-fed reservoir ecosystems, in Xi'an city on the northwestern CLP, during the hydrological year from November 2020 to July 2021. Our results show that (1) phytoplankton photosynthesis was the major mechanism that drove the transformation of the DIC to organic carbon in the reservoirs, and the hydrodynamic effect can be enhanced by shortening the hydraulic retention time (HRT), which leads to increases in flow velocity, increases in water disturbances, and decreases in nutrient load of the water body, photosynthesis, and algal growth; (2) the GPP in the Dongfeng Reservoir, with high HCO3− concentrations but low CO2 (<10 μM) and low total nitrogen/total phosphorus (TN/TP) ratios (<4), was limited by N due to the predominance of Cyanophyta and Chlorophyta, and the GPP in the Houjiahe Reservoir, with low HCO3− concentrations but high CO2 (>10 μM) and high TN/TP ratios (>100), was limited by C due to the predominance of Bacillariophyta; (3) the availability of CO2 regulated the structure of the phytoplankton community in the reservoirs, and with an increase in dissolved CO2, the dominant species changed from Cyanophyta to Bacillariophyta. Therefore, it is inferred that by changing land use or adjusting the HRT of the reservoir, we may maintain the required C-N-P nutrient element ratios for promoting CO2 fertilization on the BCP in reservoirs on the CLP, which may simultaneously help increase carbon sequestration and alleviate eutrophication by changing the phytoplankton community structure from Cyanophyta to Bacillariophyta.