Deciphering the algal successional trajectories and ecological regulation mechanisms during the cooling phase: A case study of a eutrophic urban lake

Abstract

Global rise in eutrophication has brought out various types of algal blooms. However, research on eutrophic lakes has predominantly focused on the risks and drivers of harmful algal blooms (HABs), while spatial and temporal data on algal community dynamics under low-temperature conditions remain limited. This data gap has hindered a comprehensive understanding of algal succession and its ecological effects. Through high-frequency, in-situ, multi-site vertical observations in a typical eutrophic urban lake, this study provides the first systematic analysis of algal community succession across three key dimensions: total algal biomass, community structure, and vertical distribution. The results show a significant decline in Cyanobacteria with decreasing water temperature (WT). However, total average algal concentration (TAC) does not continuously decrease. Instead, it peaks at 47.45 μg/L at 6 °C, due to a relatively high Fv/Fm (0.4–0.6) and a notable increase in Cryptophyta relative abundance (3.88 %–87.20 %). In Stage A (Gradual cooling), a diverse algal community comprising multiple phyla coexists, primarily concentrated in the surface layer (0.4 m), exhibiting clear vertical niche differentiation. As succession progresses (Rapid cooling and Gradual warming), Cryptophyta gradually replace Cyanobacteria, Chlorophyta, and Diatom, occupying their vacated ecological niches and shifting the peak of algal biomass to the mid-water layer (0.8 m). Mantel tests identified synergistic drivers, including WT, algal nutrient availability (TN, TP, COD), and algal competition. Notably, Cryptophyta succession contributed to increased SD and DO levels SD and DO levels, underscoring the role of interspecific competition and succession in maintaining ecosystem stability during WT shifts. Our findings enhance the understanding of algal community succession and their potential role in supporting long-term ecosystem stability, offering insights for aquatic ecosystem restoration and algal bloom management.