Why does the Water in a Natural Pool from Transparent Turn into Pumpkin Soup Color?

Lakes, reservoirs, and ponds are crucial inland water bodies that provide essential water resources and deliver significant ecological, social, and cultural services. As a key indicator of water quality, identifying the causes of water color changes is of paramount importance. The periodic reddish-brown "pumpkin soup" phenomenon observed in the Clean Pool (CP) of Heilong Pool (HP) in southwestern China has raised concerns about water quality and ecosystem health. We used analytical approaches including, nutrient elements, heavy metal concentrations, dissolved substances, algal community composition, and δD-δ¹⁸O isotope analytical models to investigate the ecological and geochemical mechanisms underlying this phenomenon. The results indicate that, despite Bacillariophyta dominating the algal community in HP, they are not the deciding factor of water color changes. Instead, Fe(OH)₃ colloidal particles, which originate from groundwater-surface water interactions and are influenced by redox environment fluctuations, are identified as the key factor causing the reddish-brown discoloration. Hydrological analysis reveals that atmospheric precipitation and groundwater dynamics significantly affect the formation and transport of Fe(OH)₃ particles. The distinct physical and biological characteristics of the Clear and Turbid Pools further accentuate the landscape contrast between the two water column. This study challenges the conventional assumption that algal blooms are the sole cause of water color anomalies, emphasizing the critical role of hydrogeochemical processes in karst landscapes. Simultaneously, this findings provide new insights into the evolution and regulation of special habitats in karst regions and offer a theoretical framework for managing similar water bodies. Additionally, the study underscores the importance of integrating hydrological, geochemical, and ecological perspectives to address complex environmental phenomena in extreme terrain condition.