Sources of suspended organic matter and selective feeding by zooplankton in an estuarine mangrove ecosystem as traced by stable isotopes

Between January 1995 and August 1996, suspended matter and zooplankton were sampled at different locations in a mangrove ecosystem located in the Gautami Godavari estuary and adjacent Kakinada Bay (Andhra Pradesh, India). Suspended matter was sampled at 13 different stations, and was found to have a highly variable carbon stable isotope composition, with δ13C values ranging overall between –30.94 and –19.18‰, and a highly variable elemental (C:N) composition. Our data suggest that the phytoplankton component has a seasonally and spatially variable δ13C signature, which is surpressed by the terrestrial signal but may at times fall in the same range as the δ13C of the allochtonous matter. It is argued that the phytoplankton δ13C decreases after the onset of the monsoon rains, most likely due to the 13C-depletion of the DIC pool caused by the microbial respiration of the allochtonous organic matter. At each of the 4 sites selected for concurrent zooplankton sampling, the zooplankton showed a much wider range of δ13C than did the suspended matter, with overall δ13C values between –30.14 and –16.45‰. In addition, spatial differences in average δ13C were much more pronounced for zooplankton than for total suspended matter. These data indicate that zooplankton feed on a component of the suspended matter pool, which has more pronounced seasonal and spatial δ13C variations than the total suspended matter. Thus, despite the large amounts of terrestrial and mangrove detritus present in the water column, the locally produced phytoplankton appears to be a more important carbon source for the zooplankton.