Marine ecology-progress series vol:208 pages:79-92
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 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 phytoplankton 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 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 delta C-13 than did the suspended matter, with overall delta C-13 values between -30.14 and -16.45 parts per thousand. In addition, spatial differences in average delta C-13 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 delta C-13 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.