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

Abstract

Between January 1995 and August 1996, suspended matter and zooplankton were sam- pled 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 sta- tions, and was found to have a highly variable carbon stable isotope composition, with δ 13 C 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 δ 13 C sig- nature, which is surpressed by the terrestrial signal but may at times fall in the same range as the δ 13 C of the allochtonous matter. It is argued that the phytoplankton δ 13 C decreases after the onset of the monsoon rains, most likely due to the 13 C-depletion of the DIC pool caused by the microbial respira- tion of the allochtonous organic matter. At each of the 4 sites selected for concurrent zooplankton sampling, the zooplankton showed a much wider range of δ 13 C than did the suspended matter, with overall δ 13 C values between -30.14 and -16.45‰. In addition, spatial differences in average δ 13 C 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 δ 13 C 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.