Spatio-temporal variability of aquatic productivity and carbon dynamics in a tropical estuary: Potential influence of heavy metal contamination

Rising atmospheric CO2 is a major driver of global warming, and aquatic systems play a critical role in regulating carbon exchange through biological and biogeochemical processes. In estuarine environments, phytoplankton mediated photosynthesis converts dissolved CO2 into organic matter, thereby contributing to biological carbon sequestration and supporting estuarine carbon cycling. But still, increasing anthropogenic inputs, including nutrient enrichment and heavy metal contamination, which can disrupt this natural buffering capacity. This study investigates the combined influence of nutrient loading and heavy metal intrusion on seasonal CO2 dynamics in a tropical estuarine system. Physicochemical parameters, nutrient concentrations, and a composite heavy metal toxicity index were analyzed across pre-monsoon and monsoon seasons to evaluate their impact on phytoplankton productivity and carbon cycling. Results indicated a significant seasonal shift, with monsoon conditions characterized by elevated nutrient inputs (nitrate, phosphate, Fe and Mn) along with increased chlorophyll concentration driven by surface runoff. Despite enhanced nutrient availability, high turbidity and suspended sediment loads during the monsoon reduced light penetration, suppressing photosynthetic CO2 uptake. Consequently, increased organic matter inputs intensified remineralization processes, leading to elevated pCO2 levels and widespread CO2 supersaturation. In contrast, pre-monsoon conditions supported lower pCO2 and dominant CO2 sink behavior, particularly in marine influenced regions. The transition from CO2 sink to source highlights the strong influence of hydrological forcing, sediment resuspension, and anthropogenic contamination on estuarine carbon dynamics. These findings suggest that seasonal changes in nutrient availability and elevated heavy metal concentration may contribute, together with hydrological processes, to shifts in the balance between primary production and respiration. Effective management of these inputs is therefore essential to sustain ecosystem function and climate resilience.