Synergistic Remediation of Eutrophic Rural Pond Water Using Submerged Macrophytes and Daphnia magna

Eutrophication in rural ponds has become a widespread environmental concern, particularly in regions affected by agricultural irrigation. This study proposes an innovative Submerged Macrophytes-<i>Daphnia magna</i> combined remediation technology, aiming to synergistically improve water quality in naturally eutrophic ponds. Experimental water was sourced from rural ponds with preserved natural phytoplankton and bacterial communities. Treatments included low- and high-density <i>D. magna</i>, two submerged macrophyte species (<i>Myriophyllum aquaticum</i> and <i>Ceratophyllum demersum</i>), and their combinations. Results showed that combined treatments had no significant effect on pH but improved water transparency by up to 63.6% and significantly increased dissolved oxygen. Nutrient removal was notably enhanced in combined groups, with low-density <i>D. magna</i> + <i>M. aquaticum</i> achieving TN, TP, and NO₃^--N reductions of 56.1%, 63.2%, and 58.7%, respectively. Both macrophytes effectively mitigated NH₄⁺-N accumulation caused by <i>D. magna</i>, with <i>M. aquaticum</i> showing stronger inhibition. Furthermore, low-density <i>D. magna</i> reduced phytoplankton density, cyanobacteria density, chlorophyll-a, and microcystins by 74.8%, 80.3%, 68.9%, and 71.2%, respectively. This combined bioremediation approach demonstrates high ecological efficiency, scalability potential, and practical applicability for rural pond restoration.