Carbon and nitrogen isotopic composition of suspended particulate organic matter in Zuari Estuary, west coast of India

The spatio-temporal variations of the delta C-13 and delta N-15 of suspended particulate organic matter (SPOM), along with ancillary chemical and hydrographic parameters, have been examined monthly for one year (October 2010 to September 2011) at a total of seven stations to refine the understanding of SPOM sources and biogeochemical pathways in the tropical Zuari Estuary, west coast of India. The dynamic nature of C and N cycling in estuaries is reflected in the isotopic variation of SPOM as a function of space and time. The results exhibit marked seasonality, with the autochthonous component and the terrestrial component being the major SPOM sources during periods of low and high river discharges, respectively. Spatially, enrichments of 813C and 815N from the head (delta C-13 = 29.6 parts per thousand, delta N-15 = 1.78 parts per thousand, C/N = 7.09) to the mouth (delta C-13 = delta N-15 = 5.77 parts per thousand C/N = 6.12) of the estuary indicate downstream reduction of terrigenous influence. The mean C:N ratios varied between 4 and 9 over the study period with low values during the monsoon, in spite of large terrestrial input, which can be attributed to microbial modification of terrestrial SPOM. A significant positive correlation of delta C-13 with salinity suggests the reliability of delta C-13 as a biogeochemical tracer for organic matter provenance. Carbon: chlorophyll plots reveal suppressed autochthonous production during the southwest monsoon which may be attributed to the rapid flushing time, light-limitation due to cloud cover and high water-column turbidity. (C) 2014 Elsevier B.V. All rights reserved.