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

Nittala S Sarma; G Chiranjeevulu; Sudarsana Rao Pandi; Dokala Bhaskara Rao; V V S S Sarma

doi:10.1016/j.scitotenv.2023.167120

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

Sci Total Environ. 2023 Dec 20:905:167120. doi: 10.1016/j.scitotenv.2023.167120. Epub 2023 Sep 17.

Authors

Nittala S Sarma¹, G Chiranjeevulu², Sudarsana Rao Pandi², Dokala Bhaskara Rao², V V S S Sarma³

Affiliations

¹ Marine Chemistry Laboratory, Department of Chemistry, Andhra University, Visakhapatnam 530003, India. Electronic address: sarmanittala@gmail.com.
² Marine Chemistry Laboratory, Department of Chemistry, Andhra University, Visakhapatnam 530003, India.
³ CSIR-National Institute of Oceanography Regional Centre, 176 Lawsons Bay Colony, Visakhapatnam 530017, India.

PMID: 37717775
DOI: 10.1016/j.scitotenv.2023.167120

Abstract

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 estuaries 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 δ¹⁵N values, and decreased δ¹³C values, indicative of anthropogenic influence. CDOM properties, such as spectral slope (S_300-500) and spectral slope ratio (SR, S_275-295:S_350-400), align with these findings, with SE and NW estuaries displaying higher values. On average, CDOM contributes 110.5 μM (6.8 %) to DIC in NE, 390.7 μM (11 %) in SE, 24.4 μM (4.8 %) in SW, and 122.2 μM (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.

Keywords: Chromophoric/colored dissolved organic matter, CDOM; Dissolved inorganic carbon, DIC, CO(2)(aq) + HCO(3)(−) + CO(3)(2−); H(4)SiO(4); Indian Ocean; Ocean acidification, OA; Organic alkalinity, A(org).