As representative landscape water bodies, urban park ponds are typically shallow and hydrologically isolated, making them highly susceptible to algal blooms. This study focused on recurrent summer-autumn blooms of Euglena sanguinea in Hefei Binhu Forest Park. These blooms form thin, red, oil-slick-like surface scums that severely suppress aquatic photosynthesis. We investigated phytoplankton community succession and its drivers by collecting surface biofilm, mid-depth water, and bottom sediment samples from three representative ponds during the 2024 bloom season. Results revealed extensive E. sanguinea blooms in July-August, with surface cell density reaching 9.86 × 106 cells/L (42% of total) and biomass attaining 98.61 mg/L (94% of total). This bloom peak coincided with a 2.5-fold increase in surface dissolved total nitrogen (DTN) and phosphorus (DTP). Concurrently, the surface biofilm exhibited a peak extracellular polymeric substance (EPS) concentration of 43.92 mg/L and a film-forming rate of 90.73%, structurally supported by the predominance of large algal-bacterial aggregates (>64 μm), which accounted for nearly 80% of the particulate composition. The bounding EPS (BEPS), rich in tryptophan-like proteins, corresponded with peak biofilm hydrophobicity. Critically, this nutrient-enriched microenvironmental transformation selected for a low-diversity, high-dominance microbiome. Burkholderiaceae dominated the August biofilm (23%), contrasting sharply with sediment communities (dominated by Steroidobacteraceae, 7%) and post-bloom October biofilms (dominated by Sporichthyaceae, 21%). Mechanistic path analysis revealed that DTN and DTP stimulated bloom expansion not by directly promoting algae, but by enriching Burkholderiaceae and stimulating EPS production. These findings elucidate a microbially mediated pathway linking nutrient enrichment to E. sanguinea bloom formation, challenging the conventional direct nutrient-bloom paradigm. The study provides mechanistic blueprint for targeted, microbiome-informed management of urban landscape water blooms.