Dissolved oxygen––a target indicator in determining use of the Gulf of Kachchh waters

Spatial and temporal variations of Dissolved Oxygen (DO) and Biochemical Oxygen Demand (BOD) in the Gulf of Kachchh (GoK), India are assessed based on data collected since 1976. DO concentrations in the interior GoK vary within a narrow range (6.05–6.86 mg l−1), whereas near-shore waters show wider variations (3.5–7.8 mg l−1). The DO concentration levels in the Gulf waters are close to saturation, varying from 75.4% to 108.6%, which is much higher than the 60% saturation level set for the SW-1 waters. BOD–DO simulation shows that when effluents containing a BOD concentration of 50.0 mg l−1 are introduced at three select locations, DO reduces from 5.1 mg l−1 to 3.1, 5.0 and 3.8 mg l−1, respectively, indicating a strong sensitivity to effluent load. Based on the DO analysis, BOD assimilation capacity of coastal waters around the Marine National Park (MNP) and Marine Sanctuary (MS) in the Gulf is determined. DO available for utilization for various categories of water use is illustrated, keeping in mind the ecology of MNP and MS areas. The significance of DO as a target indicator to zone the Gulf for different water use, is highlighted in this study.