Assessing the effects of grass carp excretion and herbivory of submerged macrophytes on water quality and zooplankton communities

Grass carp (Ctenopharyngodon idella) play an important role in the ecological restoration of water bodies, and it is crucial to understand the mechanism behind this. An experiment was performed in tanks consisting of three treatments: 1) without fish (control treatment, CON), 2) with fish unable to feed on submerged macrophytes (excretion treatment, EXCR), and 3) with fish swimming free (herbivory plus excretion treatment, HERB-EXCR). Treatments were conducted with varying macrophytic compositions (Vallisneria natans (Lour.) Hara, Ceratophyllum demersum L., and both species) and carp densities (low, medium, and high: 0.15, 0.30, and 0.45 g·L⁻¹ respectively, in EXCR and HERB-EXCR). Results indicated that in EXCR and HERB-EXCR, water quality was better with a lower density of grass carp. In EXCR, the water quality in tanks with V. natans was worse than in other tanks, and water quality in C. demersum tanks was better under the HERB-EXCR treament. Compared to EXCR, grass carp in HERB-EXCR significantly increased concentrations of NH₄⁺-N, NO₂⁻-N, and chlorophyll a. The effects on biomasses of protozoa, copepods, and total zooplankton in HERB-EXCR were greater than in the other treatments. Integrated analysis showed that grass carp herbivory on submerged macrophytes could be the central mechanism accounting for the changes in water quality and zooplankton communities.