Open AccessIntra-annual variability of carbon and nitrogen stable isotopes in suspended organic matter in waters of the western continental shelf of India
Author(s) -
Maribel Maya,
Supriya G. Karapurkar,
Hema Naik,
Rajdeep Roy,
Damodar M. Shenoy,
S.W.A. Naqvi
Publication year - 2011
Publication title -
biogeosciences discussions
Language(s) - English
Resource type - Journals
ISSN - 1810-6285
DOI - 10.5194/bgd-8-3923-2011
Subject(s) - oceanography , phytoplankton , δ15n , water column , isotopes of nitrogen , monsoon , organic matter , environmental science , δ13c , total organic carbon , denitrification , sedimentary organic matter , plankton , continental shelf , estuary , nitrogen , environmental chemistry , stable isotope ratio , geology , chemistry , ecology , nutrient , biology , physics , organic chemistry , quantum mechanics
The δ13C and δ15N of water-column suspended particulate organic matter (SPOM), elemental carbon and nitrogen concentrations, and C/N ratios in SPOM, along with ancillary chemical and biological variables including phytoplankton pigment abundance, were determined every month, with the partial exception of the southwest (SW) monsoon period, from March 2007 to September 2008 at a fixed site located off Goa (central west coast of India). The results reveal significant shifts in isotopic signatures, especially δ15N, of SPOM before and after the onset of the SW monsoon. Very low δ15N values, reaching a minimum of −4.17‰, are found during the pre-monsoon period. Although the average δ15N values for the SW monsoon (6.55‰) and post-monsoon (6.19‰) are substantially higher, these values are lower than expected from a region that experiences intense water-column denitrification, as well as those reported previously from the open Arabian Sea. Our results provide the first direct evidence for the addition of substantial amounts of isotopically light nitrogen by the diazotrophs, especially Trichodesmium, in the region. The δ15N of SPOM is generally lower than the mean value (7.38‰) for surficial sediments in the region, presumably because of diagenetic enrichment. The results support the notion that sedimentary δ15N may not necessarily reflect denitrification intensity in the overlying waters due to diverse sources of nitrogen and variability of its isotopic composition. The observed intra-annual variability of δ13C of SPOM is small (seasonal averages: pre-monsoon: −21.40‰, SW monsoon: −20.41‰ and post-monsoon: −22.15‰). Phytoplankton production and probably species composition could drive the observed changes. Occasional shifts in δ13C toward more negative values are suggestive of terrestrial inputs, but by and large the SPOM in the region seems to be of marine origin with relatively low and constant C/N ratios (5.8–6.5) occurring throughout the study period