Background Carbon (C), nitrogen (N), and phosphorus (P) in water are the main limiting elements that affect the growth of algae and the community structure of phytoplankton. The study of the dissolved inorganic carbon/total phosphorus ratio (DIC/TP) provides an important way to alleviate the eutrophication problem of reservoirs and lakes, which provides a new restoration solution of it.

Objective The study aims to analyze the nutritional status, phytoplankton community characteristics and carbon-phosphorus ratio of 11 reservoirs (Mianhuatan Reservoir, Chitan Reservoir, Shanmei Reservoir, Dongzhen Reservoir, Dongzhang Reservoir, Shanzai Reservoir, Dongxi Reservoir, Shidou Reservoir, Guanchang Reservoir, Tangban Reservoir, and Bantou Reservoir) in different areas of Fujian Province in the summer and autumn of 2023.

Methods In the summer and autumn of 2023, 11 reservoirs in different areas of Fujian Province were used as research objects to conduct on-site sampling and monitoring of water quality, phytoplankton and environmental factors.

Results It was shown that the comprehensive trophic level index (TLI) of 11 reservoirs ranged from 35.9 to 54.4, while Guanchang Reservior was at the mild eutrophication level, and the other 10 reservoirs were at the mesotrophic level. A total of 5 phytoplankton were detected in the 11 reservoirs. There were 36 genera in the phylum, of which Chlorophyta had the most with 15 genus, Cyanophyta had 11 genus, Bacillariophyta had 7 genus, Pyrrophyta had 2 genus, and Cryptophyta had 1 genus. The density of phytoplankton ranged from 1.13×10⁶ to 5.77×10⁸ cells·L⁻¹, and Cyanophyta was the dominant category in 8 reservoirs (Mianhuatan Reservoir, Chitan Reservoir, Dongzhen Reservoir, Dongzhang Reservoir, Shanzai Reservoir, Dongxi Reservoir, Guanchang Reservoir, and Tangban Reservoir). The DIC/TP value of water bodies in 11 reservoirs ranged from 17.4 to 402.6, with the lowest DIC/TP value was observed in Guanchang Reservoir in Ningde City, and the highest DIC/TP was observed in Chitan Reservoir in Sanming City. Through correlation analysis, it was found that there was a significant negative correlation between DIC/TP, nutritional status index and phytoplankton density.

Conclusion The carbon-phosphorus ratio has a significant impact on the phytoplankton community structure and eutrophication, and is of great value in exploring simple and low-cost ways to alleviate eutrophication.

Significance The innovation of DIC/TP index shows that the difference in limitations of biological utilization of CO₂ and P can change the structure and composition of planktonic-submersed plant communities by adjusting the concentration of DIC, TP and DIC/TP in the water body, thereby mitigating eutrophication process in the water bodies. Therefore, based on the carbon and phosphorus limiting effect of aquatic ecosystems, considering the mitigation effect of CO₂ on eutrophication and the application of DIC/TP indicators is a win-win situation for protecting water quality safety and promoting water body carbon sinks, as well as research on serving and managing ecosystems in the future.