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 Gaut ami Godavari estuary and adjacent Kakinada Bay (Andhra Pradesh, India). Sus pended matter was sampled at 13 different stations, and was found to have a highly variable carbon stable isotope composition, with delta C-13 values ranging overall between -30.94 and -19.18 parts per thousand, and a highly variable elemental (C:N) composition. Our data suggest that the phytoplankt on component has a seasonally and spatially variable delta C-13 signature, which is surpressed by the terrestrial signal but may at times fall in the same range as the delta C-13 of the allochtonous matter. It is argued that the phytoplankton delta C-13 decreases after the onset of the monsoon rains , most Likely due to the C-13-depletion of the DIC pool caused by the micro bial respiration of the allochtonous organic matter. At each of the 4 sites selected for concurrent zooplankton sampling, the zooplankton showed a muc h wider range of delta C-13 than did the suspended matter, with overall del ta C-13 values between -30.14 and -16.45 parts per thousand. In addition, s patial differences in average delta C-13 were much more pronounced for zoop lankton than for total suspended matter. These data indicate that zooplankt on feed on a component of the suspended matter pool, which has more pronoun ced seasonal and spatial delta C-13 variations than the total suspended mat ter. Thus, despite the large amounts of terrestrial and mangrove detritus p resent in the water column, the locally produced phytoplankton appears to b e a more important carbon source for the zooplankton.