Distribution, sources and biogeochemistry of organic matter in a mangrove dominated estuarine system (Indian Sundarbans) during the pre-monsoon R. Ray a, *, T. Rixen a, A. Baum a, A. Malik b, G. Gleixner b, T.K. Jana c a Department of Biogeochemistry, Leibniz Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany b Department of Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Hans, Knoell Str. 10, 07745 Jena, Germany c Department of Marine Science, University of Calcutta, 35 B. C.Road, Kolkata 700019, IndiaEstuarine, Coastal and Shelf Science xxx (2015) 1-10, journal homepage: www.elsevier.com/locate/ecss, a r t i c l e i n f o Article history: Received 12 February 2015 Received in revised form 29 September 2015 Accepted 16 October 2015 Available online xxx Keywords: Organic and inorganic carbon Stable isotopes (C, N) Sundarbans a b s t r a c t The sources and distribution of dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in the Indian Sundarbans mangrove and Hooghly estuarine system were examined during the pre-monsoon (summer) 2014. DOC is the dominant form of organic matter (OM) in the studied estuarine waters and represents a mixture of mangrove and riverine sources. Microbial degradation of land derived OM results in a high pCO2 in the Hooghly estuarine waters while enrichment in d13C-DIC ascribes to CO2 uptake by phytoplankton in the Sundarbans water. Higher d15N in the particulate organic nitrogen (PON) of the mangrove and marine zone could be associated with enhanced phytoplankton production sustained by nitrate from mangrove derived OM decomposition and/or nitrate imported from the Bay of Bengal. Low organic carbon contents and elemental ratios (TN/TOC) indicate an intense mineralization and transformation of OM in the sediments, resulting insignificantly different OM compositions compared to those of the three major sources: land derived OM, mangrove leaf litter (Avicennia marina) and in situ phytoplankton production. © 2015 Elsevier Ltd. All rights reserved.