Aquaculture-induced pH deterioration in pond ecosystems: A case study in a subtropical headwater watershed, China

Deterioration of pH in pond ecosystems disrupts physiological processes and nutrient dynamics, leading to biodiversity decline and ecosystem imbalance. The increasing demand for aquatic productions has driven the expansion of fish farming ponds in agricultural watersheds. However, the role of fish farming ponds in aggravating water pH deterioration remains unclear. This study investigates the spatiotemporal variations of pH across 142 fish and non-fish farming ponds within a typical agricultural watershed in subtropical China and explores the complex interactions between pH and its determinants by piecewise structural equation modeling. From 2017 to 2020, fish farming ponds exhibited a higher average pH (7.77-8.21) compared to non-fish farming ponds (7.70-8.04), with more obvious spatial variations. Among properties of aquatic biotic activity indices and alkali metals, the correlation between pH and chlorophyll a content was particularly strong (0.5311-0.5125), while no significant correlations were found between pH and the concentrations or charges of alkali metals (P > 0.05). Both intrinsic changes and extrinsic factors together explained 59% of the variance in water pH, accounting for the random effects of aquaculture management. Water chemistry properties were found to have a direct positive influence on water pH, with a path coefficient of 0.475 (p < 0.01). In contrast, landscape composition and configuration indirectly affected pH by mediating water chemistry properties, with path coefficients of 0.371 (p < 0.05) and 0.474 (p < 0.01), respectively. Our findings reveal the contribution of fish farming ponds to water pH deterioration and offer implications for sustainable aquaculture and watershed management.