In estuarine ecosystems, large spatial and seasonal variations in δ13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton δ13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the δ13C of the terrestrial component, we here present a simple model to estimate the phytoplankton δ13C variations using an existing dataset on the δ13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India.