Variability of dissolved reactive phosphate and apparent oxygen utilization along the west coast of India

Abstract

This work focuses on the spatial variability of dissolved reactive phosphate along the west coast of India. In this study, samples of surface and bottom water was collected from each of the 27 sites during a single pre-monsoon transect along the west coast of India to study the variation of dissolved reactive phosphate. Phosphate showed an enrichment pattern with increase in depth of the water column. In addition to this, an offshore increase in phosphate concentrations was noticed for both surface and bottom stations. Almost all the surface stations of this study area were found to be characterized by negative apparent oxygen utilization (AOU) values, and bottom stations of the deeper offshore zone (200 m depth) displayed large positive AOU values. The bottom stations of shallow near coastal waters and the deeper offshore zone are characterized by an inverse relationship between phosphate and dissolved oxygen, whereas at these depths, a direct relation was observed between phosphate and AOU. AOU is calculated as the difference between oxygen saturation value at the in situ temperature and salinity and the actual measured concentration. This oxidative re-mineralization mechanism decreases the observed dissolved oxygen well below the expected theoretical values, which is concordant with the higher concentration of phosphate and high positive AOU values.

Keywords:

• Reactive phosphate
• Apparent oxygen utilization
• Dissolved oxygen
• Re-mineralization
• Shallow near coastal waters

Acknowledgements

The authors are grateful to Department of Ocean Development, Government of India and Dr. R. Damodaran, Professor, School of Marine Sciences, CUSAT for arranging the cruise.