Stable isotopes of carbon and nitrogen in suspended matter and sediments from the Godavari estuary

Abstract

Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic matter. The mean isotopic ratios (d 13 C sed -25.1 ± 0.9, d 13 C sus -24.9 ± 1, d 15 N sed 8.0 ± 2 and d 15 N sus 6.5 ± 0.9%) and elemental concentrations (C sed 0.45 ± 0.2%, C sus 0.9 ± 0.7%, N sed 0.07 ± 0.05% and N sus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of d 13 C from the upper to lower estuary in both suspended (-27.5 and -24.3%, respectively) and sedimentary (-26.2 and -24.9%, respectively) phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive relationship exists between the d 13 C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton. But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier.

Isotope‐based Fluvial Organic Carbon (ISOFLOC) Model: Model formulation, sensitivity, and evaluation

“…Measuring and modeling of d 13 C of carbon pools provides an extra set of source and transformation equations to water quality studies and thus carbon stable isotopes have been extensively applied in recent years, primarily within data-driven approaches. d 13 C measurements of FPOC have been used to apportion sources of carbon in aquatic systems ranging from small streams to coastal waterbodies [Fox and Papanicolaou, 2007;Fox, 2009;Kendall et al, 2010;Schindler Wildhaber et al, 2012;Sarma et al, 2012]. Source apportionment studies have placed heavy emphasis on the ability of d 13 C to differentiate soil organic carbon (SOC) sources derived from C3 and C4 plants due to their significantly different d 13 C signatures of 224 to 229& and 210 to 214&, respectively [Smith and Epstein, 1971;Onstad et al, 2000;Palmer et al, 2001;Fox and Papanicolaou, 2007;Fox, 2009;Jacinthe et al, 2009;Brunet et al, 2011].…”

Environmental controls on the speciation and distribution of mercury in surface sediments of a tropical estuary, India

“…Relatively high Hg loading and low SPM concentrations (as reported by Sarma et al, 2012) in the overlying water column at Kotipalli station increased the concentrations of Hg as water soluble, exchangeable and carbonate complexes in the sediments. Similar observation was found in the sediment collected at Yanam.…”

“…High Hg loading in the sediments collected in the middle of estuary increased the relative concentrations of Hg in the fraction 1. However, lowest Hg loading, and more SPM concentrations (as reported by Sarma et al, 2012) and high salinity in the overlying water column at Bhairavapalem probably responsible for the lowest concentrations of Hg as Fr. 1 in the sediments.…”

“…However, very low Hg to chloride ratio in this station, variation in salinity is not expected to play significant impact on Hg distribution. The high concentration of SPM (Sarma et al, 2012) in this region was probably responsible decreasing Hg association with organic phases in the sediments.…”

“…The increasing salinity of the overlying water column (mixing of freshwater and brackish water) might have increased coagulation processes of Hg associated SPM and Hg-humate complexes and increased the total Hg content in the sediments collected at Yanam (middle of the estuary) than Bhairavapalem (downstream). High concentration of SPM in water column at the Bhairavapalem (downstream) in the Godavari estuary (Sarma et al, 2012) might have enhanced metal exchange reaction between sediment and SPM. Mixing of metal-rich fluvial sediments and metal-poor marine sediments was probably another reason for the decrease of Hg content in sediment at the Bhairavapalem station.…”