Remote-Sensing-Based Estimation of Surface Nitrate and Its Variability in the Southern Peninsular Indian Waters

R K Sarangi

doi:10.1155/2011/172731

Abstract

A relationship between sea surface temperature (SST) and surface nitrate concentrations has been obtained for the first time based on in situ datasets retrieved from U.S. JGOFS (1991–96) and Indian cruises (2000–2006) in the Arabian Sea, Bay of Bengal and Indian Ocean region around the southern Indian tip. The dataset includes 1537 points. A sigmoid relationship obtained with value 0.912. NOAA-AVHRR pathfinder satellite monthly averaged SST data retrieved from the PODAAC/JPL/NASA archive during July 1999–June 2004. The datasets imported in the ERDAS-Imagine software and SST images generated on monthly and seasonal scales, for latitudes 5–12°N and longitudes 75–85°E. The ocean surface nitrate images retrieved based on the established sigmoid relationship with SST. The nitrate concentrations ranged between 0.01–3.0 μM and categorized into five ranges. The significant seasonal upwelling zone around the southwest coast of India (Kerala coast, Latitude 80.10–9.30°N and Longitude 75.60–76.20°E) was identified during July–September 1999–2004 with very high nitrate concentration (~1.00 μM). Low nitrate and nitrate-depleted zones observed during summer (March–May). In the Arabian Sea and northern Indian Ocean, high nitrate concentration (~0.50 μM) observed during the southwest monsoon (SWM), whereas the Bay of Bengal was marked with high nitrate (~0.50 μM) during the northeast monsoon (NEM). SST was high (~29°C) in the Bay of Bengal and low (~26°C) in the Arabian Sea and northern Indian Ocean during SWM and vice versa during the NEM. There is a clear inverse relationship between nitrate and SST in the study area during July 1999–June 2004.

Highlights

IntroductionIn the upper several hundred meters of the World Ocean, plant nutrient (nitrate, phosphate, and silicic acid) concentrations tend to increase with depth and temperature tends to decrease with depth [1]
In the upper several hundred meters of the World Ocean, plant nutrient concentrations tend to increase with depth and temperature tends to decrease with depth [1]
This inverse relationship between temperature and plant nutrient concentration occurs seasonally in surface water to varying degrees depending on geographic location, except at high latitudes where plant nutrients at the surface annually range from a minimal nonzero concentration to a seasonal maximum concentration related to large-scale geochemical processes [3, 4]

Summary

Introduction

In the upper several hundred meters of the World Ocean, plant nutrient (nitrate, phosphate, and silicic acid) concentrations tend to increase with depth and temperature tends to decrease with depth [1]. This inverse relationship between temperature and plant nutrient concentration occurs seasonally in surface water to varying degrees depending on geographic location, except at high latitudes where plant nutrients at the surface annually range from a minimal nonzero concentration (as determined by colorimetric methods [2]) to a seasonal maximum concentration related to large-scale geochemical processes [3, 4]. In the early phase of stratification, chlorophyll concentration is uniform throughout the mixed layer and a maximum develops at depth near the nitracline

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Conclusion