- Dona Paula, Goa, India.
- +91-0832- 2450327
- iiosc2020[at]nio[dot]org
Abstract Submission No. | ABS-2022-05-0261 |
Title of Abstract | Why is seasonal anoxia confined only to the central west coast of India? Upwelling vs anthropogenic effects |
Authors | Gupta GVM* |
Organisation | Centre for Marine Living Resources & Ecology |
Address | Centre for Marine Living Resources & Ecology (CMLRE), Ministry of Earth Sciences Kochi, Kerala, India Pincode: 682508 Mobile: 9446021967 E-mail: gvmgupta@gmail.com |
Country | India |
Presentation | Oral |
Abstract | The growing number of oxygen-deficient coastal zones around the world and their impacts on marine life has always been a controversial issue as their development is largely attributed to anthropogenic activities which can be mitigated by human actions. The west coast of India houses the world's largest seasonal coastal hypoxic-anoxic zone and despite several studies in the recent past have addressed its biogeochemical effects, it is still debated the exact mechanism(s), i.e. natural vs terrestrial, behind its formation. The new coherent datasets collected from estuaries to coastal to offshore regions during early, peak and late summer monsoon (SM) established, for the first time, that this largest coastal hypoxic-anoxic zone along the west coast of India (WCI) is not driven by the anthropogenic inputs but formed naturally through upwelling of oxygen minimum zone (OMZ) waters. Coastal oxygen deficiency during SM upwelling gradually intensified from hypoxic during early SM to suboxic/anoxic (<5 µM) by late SM, with the latter confined to the central shelf between 11ᵿ and 18ᵿN, which is equivalent to almost half of the WCI. This anoxic central zone (where famous Goa and Mangalore are occupied) is located away from receiving significant anthropogenic inputs unlike in the south (Kochi) and north (Mumbai), where the two largest coastal cities receive substantial allochthonous inputs, yet they remained at hypoxia during entire SM. The two-fold lower concentrations of nutrients in the estuaries of central region during SM, where coastal anoxia is confined, than those in the northern and southern regions infer that the increased anthropogenic activities have not impaired these coastal waters. This is true as the coastal hypoxia in the south remain unchanged compared to five decades ago and the monsoonal estuaries of India are acting as heavy sink zones and export only <10% of anthropogenic nutrients to the coastal seas. The stable isotopic composition of particulate organic matter showed confinement of significant anthropogenic signals only to few pockets of nearshore regions and the paleoceanographic records did not show significant impact of anthropogenically derived eutrophication, inferring that this coastal deoxygenation is not an human induced effect. Rather, the extent of coastal deoxygenation follows the corresponding spatio-temporal changes in the distribution of dissolved oxygen concentrations in the offshore OMZ. The eastern Arabian Sea has a perennial OMZ with its southern boundary at 12ᵿN and its core OMZ (10 µM of oxygen) has a northern boundary at 18ᵿN. The formation of cyclonic eddies in the south and central regions respectively during peak and late phases of SM leads to a shift in the upliftment of oxycline from outside the core OMZ to within core OMZ during the progression of SM. Accordingly, the development of anoxia over the central WCI (11°-18°N) is caused by the upwelling of suboxic waters sourced from the core OMZ, while those of the south and north are from hypoxic waters (~20 µM) outside the core OMZ. The formation of weak or no coastal anoxia during IOD and ENSO years further supports of it being driven naturally. The upwelling driven coastal biogeochemistry further intensifies this oxygen deficiency. Notably, the spatio-temporal variation of oxycline depth also governed by the interaction between equatorward spread of Arabian Sea high saline waters and the upwelling is a significant finding as large anoxic volumes have greatly reduced the habitat for higher pelagic organisms. The fish mortalities due to such situations have been often occurring over the central anoxic coast (Goa). Nevertheless, considering the natural origin of the world's largest zone of coastal hypoxia-anoxia over the WCI it is likely that not all the coastal dead zones around the world are anthropogenically derived and emerging studies are evidently supporting this argument |