Isotopic composition of carbon and nitrogen of particulate organic matter in the Godavari estuary: Seasonality in sources and processes

Abstract

Rivers transport organic matter originating within the estuary and terrestrial sources to the coastal ocean. The seasonal variations in the magnitude of river discharge alter the sources of particulate organic matter (POM) in the estuary. In order to examine the dominant sources of POM in the estuary, monthly samples were collected and analyzed for the isotopic composition of carbon and nitrogen of POM along with hydrographic characteristics of water column data in the Godavari estuary, India. C/N and POC/Chl-a and isotopic composition of C and N indicated in situ production is the major source of POM during the dry (33 ± 69 m3 s−1) and moderate discharge (2580 ± 3034 m3 s−1) periods. The δ13C and δ15NPOM were close to that of sediment organic carbon (− 24.0 ± 1 and 1.0 ± 2‰ respectively) during the peak discharge period (16,016 ± 15,346 m3 s−1) indicating that sediment organic matter contributed dominantly to the POM pool as high flow mixes upper layer of sediments. The stable isotope fractionation model clearly indicated that POM during the peak discharge period is contributed by external sources and in situ production during other periods. The Bayesian stable isotope linear mixing model (stable isotope analysis in R, SIAR) suggested that both freshwater and marine algae contributed > 50% of the POM during the dry and moderate discharge periods whereas sediment organic matter during the peak discharge period. Since a significant fraction of the in situ organic matter is fluxed to the coastal ocean, its impact on the coastal ecosystem through microbial loop needs further investigation.