Monsoon-driven biogeochemical shifts and acidification risk in tropical estuarine ecosystems: A case study from the Indian coast.

Tropical estuaries serve as biogeochemical hotspots where the interactions between monsoon hydrology and human activities significantly impact ecosystem health. However, limited information exists on their carbonate chemistry, which is crucial for assessing climate vulnerability. This study provides the first seasonal assessment of hydrography, nutrients, and carbonate system dynamics in the Haripur estuary, Bay of Bengal. Seasonal evaluation revealed significant variations in pH, carbonate system indicators, and nutrients (p < 0.001). During the monsoon, pH declined to 7.12 ± 0.17, dissolved oxygen dropped to near-hypoxic levels (2.95 ± 0.35 mg L-1), and nutrient enrichment was observed with elevated dissolved inorganic nitrogen (6.07 ± 0.74 μM) and phosphate (1.61 ± 0.39 μM). Carbonate saturation states remained persistently corrosive, reaching minima of ΩAr (0.03 ± 0.01) and ΩCa = 0.04 ± 0.01) among the lowest reported for Indian estuaries. Multivariate analysis identified nutrient enrichment and carbonate imbalance as the dominant stressors, explaining 32.4 % of the total variance. These findings clearly indicate that the Haripur estuary functions as a regional hotspot of monsoon-driven acidification and a global outlier exhibiting year-round carbonate undersaturation. Urgent management interventions are recommended to mitigate hypoxia and acidification risks in this vulnerable tropical estuary through nutrient load reduction, enhanced tidal flushing, and ecosystem-based adaptation. The results further provide a valuable basis for developing best management practices in the context of regional and global climate change, thereby supporting the objectives of Sustainable Development Goal 14 (Life Below Water).