The role of mangrove primary production in the carbon cycle of mangrove ecosystems and in the coastal zone has long been an issue of debate. The present study investigates the organic matter dynamics in the mangrove ecosystem of the Coringa Wildlife Sanctuary located in the estuary of the Gautami Godavari, Andhra Pradesh, India, and makes extensive use of carbon and nitrogen stable isotopes as natural tracers of organic matter sources and their utilisation by different faunal communities.
The mangrove creeks are clearly identified as an active site of mineralization of organic matter and CO2 efflux, but our data also indicate that this is a rather localised feature. In contrast to the Godavari river where carbonate dissolution was found to be the main process influencing the inorganic carbon dynamics, variations in concentrations and stable isotope composition (delta13C) of the dissolved inorganic carbon pool (DIC) in the mangrove creeks were a result of the mineralization of organic matter, at least during the pre-monsoon season. The mangrove creeks and adjacent Kakinada bay showed distinct spatial and seasonal variations in delta13CDIC values, which are significantly more negative during and after the monsoon season. This pattern is hypothesized to be caused by the mineralization of the large amounts of terrestrial organic matter transported by the Godavari during monsoon, whereas mangrove litter is the main external source of organic carbon inputs during other seasons.
Particulate organic carbon (POC) was found to have a highly variable carbon stable isotope and elemental (POC/PN) composition, and our data show that the phytoplankton component has a seasonally and spatially variable delta13C signature which is masked by the terrestrial signal, but may at times fall in the same range as the delta13C of the allochtonous matter. Such variations are stressed to be important when using stable isotope data in evaluating the relative importance of different primary producers to aquatic faunal communities in such a dynamic ecosystem. Comparison of spatial and seasonal trends in delta13C signatures of zooplankton and suspended matter revealed a marked selectivity of the former for local aquatic primary production. Similarly, the markedly larger spatial variations in delta13C values of subtidal benthic invertebrates compared to the available carbon pools can be explained by the high selectivity of the benthic invertebrate community for pelagic and benthic microalgal food sources. Overall, mangrove-derived and other terrestrial carbon was found not to be a significant food source for zooplankton and benthic invertebrate communities in the aquatic environment.
Invertebrates in the mangrove-covered intertidal areas were found to display a wide range of stable isotope signatures, and our data overall show a fairly limited use of mangrove litter and a limited degree of (trophic) resource overlap. At least for the particular area studied, local and imported algae are a major source of carbon for intertidal benthic invertebrate communities. A compilation of stable isotope and elemental ratios from widely differing mangrove ecosystems showed that although organic carbon stocks in intertidal mangrove forests can be very high and almost entirely of mangrove origin, there are also systems in which deposited estuarine or marine suspended matter is the dominant source of organic carbon and nitrogen in these sediments. Such variations are expected to have a major impact on the carbon dynamics in mangrove ecosystems.