Effects of Artificial Reefs on Plankton Community Structure and Underlying Environmental Mechanisms: A Case Study from Qingdao Shique Beach Marine Ranching

Coastal ecosystem health is fundamental to marine biodiversity and fishery sustainability. Under increasing pressures from overfishing, pollution and habitat degradation, marine ranching with artificial reefs (AR) has become an important ecological restoration strategy. Although the responses of fish and macrobenthos to ARs have been widely documented, their impacts on plankton communities and the associated environmental drivers in temperate waters remain poorly understood. Because plankton form the base of marine food webs and respond rapidly to environmental change, clarifying their responses is essential for evaluating the ecological functions of marine ranching plankton. In this study, we investigated seasonal changes in plankton community structure and identified the key environmental factors shaping these changes in the Shique Beach marine ranching area, Qingdao, a representative temperate marine ranch in the Yellow Sea. Field surveys were conducted in January (winter) and June (summer) of 2022 at the reef area (R, n=11), the affected area (S, n=3), and the control area (C, n=3). In-situ and laboratory measurements covered water temperature, salinity, dissolved oxygen (DO), pH, chemical oxygen demand (COD), chlorophyll a (Chl-a), phosphate, inorganic nitrogen and silicate. Plankton samples were collected vertically with standard plankton nets, preserved and subsequently analyzed for species identification and quantification in the laboratory. The Shannon-Wiener diversity index (H′), Pielou's evenness index (J), and dominance index (Y) were calculated for community structure evaluation, species with Y≥0.02 were defined as dominant species. Non-metric multidimensional scaling (NMDS) was used to analyze β-diversity and community dissimilarity, and redundancy analysis (RDA) was applied to explore relationships between dominant plankton species and environmental variables. Pearson correlation analysis was conducted to reveal the associations between community characteristics and abiotic factors. The results showed that AR induced significant spatiotemporal variations in plankton communities. A total of 41 phytoplankton species (32 diatoms, 8 dinoflagellates, 1 haptophyte) were identified in winter and 42 species (31 diatoms, 11 dinoflagellates) in summer, 24 zooplankton species in winter and 27 species in summer. Crucially, species richness of both phytoplankton and zooplankton was consistently highest in the reef area throughout the two seasons. The average zooplankton abundance in the summer reef area reached 643 3.13 ind./L, which was significantly higher than that in the control area, indicating an obvious promotion of secondary production. NMDS combined with ANOSIM analysis revealed that the plankton community in the reef area was significantly distinct from that in the control area, especially for zooplankton (R=0.53, P=0.002). Obvious seasonal succession of dominant species was observed: the diatom Eucampia zodiacus dominated all zones in winter, whereas Guinardia flaccida became absolutely dominant in summer(Y>0.87). Diversity indices also differed seasonally: phytoplankton H′ was significantly higher in winter, while zooplankton H′ was higher in summer. RDA identified that water temperature, nutrient concentrations (particularly phosphate), and Chl-a were the key environmental drivers of plankton communities. Phosphate dynamics were crucial to plankton distribution, showing positive correlations with key copepod species in winter and with the summer dominance of G. flaccida. The N/P ratios demonstrated perennial phosphorus limitation across the study area (N/P>Redfield ratio of 16:1). In summer, the N/P ratio in the reef area decreased to 86.49 along with rising phosphate content. In conclusion, AR reshaped plankton community structure in this temperate marine ranch by increasing habitat heterogeneity and modifying local nutrient dynamics. These processes promoted plankton species richness, enhanced zooplankton production, and strengthened bottom-up ecological effects. The findings highlight the need to incorporate plankton indicators and nutrient regulation into ecological assessment and adaptive management of artificial reef-based marine ranching.