Summary of Abstract Submission

Abstract Submission No. IO50-02-0018PresentationOral


Hema Naik*1, Damodar Shenoy1, Amal Jayakumar2, Amit Sarkar3, Anil Pratihary1, Gayatree Narvenkar1, Mangesh Gauns1, Siby Kurian1, P.V. Narvekar1, S.W.A. Naqvi 1

1 CSIR-National Institute of Oceanography, Dona Paula. Goa., India
2 Princeton University, Princeton, New Jersey., U.S.A.
3 National Centre for Antarctic Ocean Research, Vasco., India


The western continental shelf of India houses the world┐s largest naturally-formed coastal oxygen-deficient zone. Early observations initiated in 1997 at a fixed site (Candolim Time Series, CaTS) off Goa at a water depth of ~28 m indicated an intensification of oxygen deficiency, as inferred by maximal hydrogen sulphide (H2S) accumulation in upwelled near-bottom waters. Prior to 2007, sulphidic conditions prevailed seasonally during all years except two. However, such conditions were recorded only twice subsequently. The large inter-annual variability in the intensity of oxygen deficiency appears to be controlled in part by the southwest monsoonal rainfall, indicating that weaker stratification might allow greater entrainment of nutrients into the euphotic zone, thereby supporting higher primary production, in turn leading to enhanced oxygen demand at depth. In general, stronger oxygen-deficiency occurred in years when the warm surface layer was thinner. The data also suggest a substantial long-term warming of the near-bottom upwelled waters. Overall, the observed general weakening of anoxia in recent years as compared to the early part of this millennium is consistent with modelling results, supporting the view that offshore forcing may have a greater modulating effect than nutrient inputs by river runoff on seasonal oxygen deficiency. However, the exact forcing is not clear as the extent of oxygen-deficiency does not exhibit any clear association with either the Indian Ocean Dipole or El-Nino events.