Water quality evaluation for Eriocheir sinensis culture ponds with different aquatic plants

To investigate the effects of different aquatic plants on the water quality in Eriocheir sinensis aquaculture ponds with the zero-exchange water mode, we evaluated the water quality of aquaculture ponds by single factor evaluation method, average pollution index, comprehensive water quality index (CWQI) and principal component analysis. The aquatic plants planted were Elodea nuttallii (Class I group), Vallisneria natans (Class II group) and Hydrilla verticillate (Class III group), and the water source group is the control group (CK group). We observed the water quality monitoring indicators including pH, dissolved oxygen (DO), potassium permanganate index (COD_Mn), total nitrogen (TN), ammoniacal nitrogen (NH₄ ⁺-N), nitrate nitrogen (NO₃ ⁻-N), nitrite nitrogen (NO₂ ⁻-N), total phosphorus (TP) and phosphate (PO₄ ³⁻-P). The results show that the single factor evaluation for each group's water quality was Grade V. TN and TP were the primary contaminants. The average pollution index of Class I−III groups was 1.65, 1.59 and 2.97, respectively, while that of the control group was 2.00. The CWQI evaluation grade of water quality in Class I and II groups was better than that of the other two groups. The principal component scores of Class I−III groups were generally lower than those of CK group. In summary, compared with the external water source, the cultivation of E. nuttallii or V. natans with the zero-exchange water mode can improve water quality.