Estimating Spatial and Seasonal Phytoplankton δ13C Variations in an Estuarine Mangrove Ecosystem

Abstract

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.

These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton δ¹³C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton δ¹³C signal.