<p>Marine ranching is an ecosystem-based management practice that uses engineered habitats and adaptive management to restore coastal habitats and support sustainable fisheries. It has become an important restoration strategy to counter declining fishery resources and habitat degradation. Artificial reefs, as key engineering components, can modify local hydrodynamics and nutrient conditions, thereby influencing plankton communities and water quality. Here, we examined plankton community structure and environmental parameters in an artificial reef area and a nearby control site of the Haizhidu Marine Ranching System (Bohai Sea, China) based on a single summer survey (July 2023). Plankton abundance was markedly higher in the reef area, with phytoplankton and zooplankton reaching 3.2 and 9.2 times the levels at the control site, respectively. Phytoplankton diversity (<i>H′</i> and <i>D</i>) was higher in the reef area, whereas zooplankton showed slightly lower evenness and diversity, indicating stronger dominance by a few taxa. Reef-associated waters had higher dissolved oxygen (DO) and phosphate phosphorus (PO₄–P) concentrations and lower ammonium nitrogen (NH₄–N) levels than the control site, consistent with improved local water quality. Overall, this case study suggests that artificial reefs can enhance plankton abundance, reshape community structure, and be associated with better water quality in a marine ranching system.</p>