Estimating spatial and seasonal phytoplankton delta C-13 variations in an estuarine mangrove ecosystem

In estuarine ecosystems, large spatial and seasonal variations in delta C-1 3 values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic leve ls. Obtaining clean phytoplankton samples for isotope analysis is usually i mpossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton delta C-13 variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestr ial detritus and phytoplankton) and the delta C-13 of the terrestrial compo nent, we here present a simple model to estimate the phytoplankton delta C- 13 variations using an existing dataset on the delta C-13 and elemental (C: N) composition of suspended organic matter from an estuarine mangrove ecosy stem in southeast India. These variations are related to the monthly rainfall pattern during the sam pling period. It is stressed that this method estimates approximate phytopl ankton delta C-13 values, which should not be used in e.g., mixing models. However. we propose that in cases where sufficiently large datasets are ava ilable, the described procedure could provide a valuable method to semi-qua ntitatively estimate the seasonal or spatial variations of the phytoplankto n delta C-13 signal.