Coupling between chromophoric dissolved organic matter and dissolved inorganic carbon in Indian estuaries

This study investigates the coupling between Chromophoric Dissolved Organic Matter (CDOM) and Dissolved Inorganic Carbon (DIC) in eighteen Indian estuaries across salinity gradient of the east and west coasts during the monsoon season, characterized by significant river discharge. The hypothesis that humic acids (HA) and fulvic acids (FA), prominent in estuarine CDOM, closely correspond to the 'organic alkalinity' (A(org)) component of total alkalinity is examined. In most estuaries, specifically those along the northeast coast (NE) and southwest coast (SW), a significant linear relationship exists between DIC, CDOM abundance, and pH level. Notably, minor es-tuaries along the southeast coast (SE) and northwest coast (NW) exhibit elevated DIC levels beyond what this relationship predicts. These estuaries also reveal heightened ammonium levels, increased delta N-15 values, and decreased delta C-13 values, indicative of anthropogenic influence. CDOM properties, such as spectral slope (S300-500) and spectral slope ratio (SR, S275-295:S350-400), align with these findings, with SE and NW estuaries displaying higher values. On average, CDOM contributes 110.5 mu M (6.8 %) to DIC in NE, 390.7 mu M (11 %) in SE, 24.4 mu M (4.8 %) in SW, and 122.2 mu M (4 %) in NW estuaries. The relationship between total alkalinity minus DIC (TA-DIC) and pH25 suggests that CDOM, mediated by HA/FA, buffers the inorganic carbon system in estuaries. This buffering capacity weakens at elevated DIC levels, and this condition is marked by anomalous SR values compared to the baseline salinity -SR linear regression. This Study suggests that estuarine CDOM could largely represent "organic alkalinity" and could help monitor acidification in estuaries.