Objective In order to explore the ecological characteristics of planktonic organisms in the rice-crab-carp symbiotic system, we released Cyprinus carpio var. longke-12 mirrors of different densities in the rice field to provide a basis for optimizing the rice-crab model.

Methods Four groups of C. carpio var. longke-12 mirrors with different densities were polycultured in rice fields with Eriocheir sinensis from June to September 2022, and the ecological community structure of plankton was analyzed.

Results A total of 72 species of phytoplankton belonging to 7 phyla and 40 genera were detected in the water, among which Bacillariophyta and Chlorophyta were the most, with 19 species, accounting for 26.4% . A total of 28 species belonging to 22 genera and 4 phyla of zooplankton were detected, with 16 species of rotifers accounting for 57.1%. The abundance of phytoplankton was 2.45×10⁶~4.36×10⁶ cells/L, with an average of 3.35×10⁶ cells/L; the abundance of zooplankton ranged from 194.3~808.3 ind/L, with an average of 347.1 ind/L. The average Shannon-Wiener diversity index (H’) of phytoplankton was 0.75 (range 0.92~2.12), and the average H’ value of zooplankton was 1.36 (range 0.96~1.91). The average Pielou evenness index (J) of phytoplankton was 0.72 (range 0.32~0.96), and the average J value of zooplankton was 0.62 (range 0.43~0.86). The average Margalef species richness index (D) of phytoplankton was 0.92 (range 0.67~1.14), and the average D value of zooplankton was 1.30 (range 0.98~1.56). There were no significant differences in the three indexes (H’, J, D) of phytoplankton and zooplankton among groups (P>0.05). RDA sequencing analysis showed that Chrysophyta was significantly positively correlated with total nitrogen (TN), Cyanobacteria, Bacillariophyta and Chlorophyta were significantly positively correlated with ammonia nitrogen (NH₄⁺-N), total phosphorus (TP) and density of carp (D_crap), Pyrrophyta were significantly positively correlated with pH and permanganate index (COD_Mn), Cryptophyta and Euglenophyta were negatively correlated with D_crap. Rotifers were significantly positively correlated with NH₄⁺-N and D_crap, while Cladocera were significantly negatively correlated with NH₄⁺-N and D_crap.

Conclusion The plankton community structure is greatly affected by environmental factors such as pH, NH₄⁺- N, TN, TP, COD_Mn and D_crap. With the increase of C. carpio var. longke-12 mirrors density, the abundance of diatom increased, and the abundance of Cryptophyta and gymnophyta decreased. The results of the study provide theoretical support for the healthy ecological breeding model of rice-crab-carp.