Influence of alternate wetting and drying and duckweed on zooplankton abundance in rice paddy ecosystem

The escalating global demand for rice has transformed the traditional rice cultivation systems to alternate wetting and drying (AWD) irrigation as control irrigation (CI), particularly in dwindling water resources. Recently, the rice-duckweed system has shown positive outcomes in paddy habitats, but there is a gap in the literature regarding duckweed's impact on zooplankton with alternate AWD dry-wet cycles. Therefore, a field experiment was conducted to quantify the impact of flooding irrigation (FI) and control irrigation (AWD) with (FI + D, CI + D) and without (FI, CI) duckweed mats on zooplankton abundance at the early tillering (ET) and flowering (FL) stages, making a sequence before and after midseason drainage (MD) and successive AWD dry-rewet cycles, respectively. For both the ET and FL stages, CI + D significantly dominated other treatments in taxonomic richness and individuals/L. Post MD, a significant reduction in taxonomic richness was observed under FI compared to FI + D. At FL, the zooplankton individual/L were significantly reduced in the FI habitat, but there was no significant difference between FI + D and CI + D. FI resulted in low diversity after MD, indicating a disrupted habitat for zooplankton. PCA analysis showed that dissolved oxygen (mg/L) and pH at ET, and temperature (°C) at FL remained significant influential factors to governed zooplankton habitat. AWD dry-wet cycles control the outgrowth of duckweed and enrich the zooplankton habitat with decomposing duckweed. Our analysis showed that CI + D is a diversity-promoting and sustainable rice cultivation method, with further suggestions for long-term studies at the species level under the rice-duckweed system.