3.1.5. Why the GoM is a marine biological paradise?
The Indian Continental shelf of GoM is significantly narrow (< 20 km cross-shore expand)1,8. Being a tropical region, the GoM has a warm climate with less change in sea surface temperature over seasons (26.5 to 30.3°C)8. The sea surface salinity in the GoM varies seasonally (32-35.4), but remains high due to its continuity to the Arabian Sea and the lack of any significant rivers in the vicinity8,11. The GoM gets partially renewed during the Southwest Monsoon (May - September) associated with the large scale ocean circulation around India and Sri Lanka8,9,11. The surface currents in the GoM during the Southwest Monsoon (May to September) enables the flow of high saline Arabian Sea waters into the PB. The surface currents reverse during the Northeast Monsoon (November - February) and bring the coastal low saline Bay of Bengal waters into the GoM (Table 1). This circulation pattern enables (a) GoM to receive a high amount of plankton stock from the Arabian Sea during the Southwest Monsoon and from the Bay of Bengal during the Northeast Monsoon8,10,11 and, (b) Palk Bay to efficiently trap the suspended sediments coming from the Bay of Bengal during the Northeast Monsoon10,11. As a result, the bottom sediment in the GoM is mostly sandy and well-aerated conducive for the life of diverse corals and sensitive fauna1,4.
The nearshore characteristics of the Indian southwestern shelf and the GoM evidenced the oceanographic mechanisms that make the GoM evolve into a biological resource reserve. Primarily, during the Southwest Monsoon, the entire western shelf waters of India are under the influence of the coastal upwelling and the incursion of the oxygen-deficient waters27-29. This situation has a profound influence on organisms living in the nearshore region along the southwest coast of India2-4,28,29,33. In contrast, the adjacent GoM is devoid of any such negative impacts of low oxygenated waters on fauna. The benthic habitats in the shelf waters are the feeding grounds for most of the demersal finfishes and shellfishes and provide a link between the water column and fisheries2,33-36. Mobile consumers of benthic macro-invertebrates usually emigrate/escape from areas where dissolved oxygen concentrations reach hypoxic levels2,33-37. Demersal finfishes and shellfishes disappear seasonally from the outer shelf of the southwest coast of India and often aggregate in oxygenated estuarine waters2,29,38. If low-oxygen conditions are relatively mild, some of these demersal consumers may remain in the vicinity for a short period, exploiting stunned benthic prey, which is not customarily available39-41. As dissolved oxygen concentrations rise after the hypoxic event, mobile demersal consumers generally return to their original habitat. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia is likely to limit higher trophic - level transfer via the inhibition of macrobenthic production42,43.
Oxygen deficiency is the primary cause of the dead zones in the world ocean41-44. Dissolved oxygen is the prime physiological driver of respiration, photosynthesis, and calcification of coral communities44,45. Crustaceans and echinoderms are typically more sensitive to hypoxia, and at <2 mg L−1 dissolved oxygen, crustacean dies within 119 h of exposure, is the reason for piles of dead crabs or lobsters a common feature in hypoxic zones34-41. Naturally, the lack of seasonal hypoxia in the GoM avoids the seasonal wipe out of sensitive fauna there, facilitating increased faunal richness and diversifications, which is advantageous especially to the native sessile and sedentary organisms having longer life span. Sedentary organisms like molluscs, echinoderms and clams may undertake seasonal migration from the Indian southwestern shelf to the GoM as a response to escape from low oxygenation during the Southwest Monsoon2,4. So here we propose that the hypoxia avoidance migration of the mobile fauna from the nearby southwestern Indian shelf may also be contributing to the precious resource potential of arthropods, molluscs, bivalves and echinoderms inhabiting in the GoM1. Thus GoM may also be acting as a refuge for marine fauna while oxygen-deficient waters spread over the Indian southwestern shelf (Supplementary Figure 8). Availability of primary (plankton) food is another essential requirement in sustaining a rich and diverse fauna in any aquatic ecosystem. In the GoM, this gets possible chiefly through the seasonal surface circulation along the Indian shelf waters. The GoM receives the positive effect of upwelling (as plankton-rich advected surface waters) during the Southwest Monsoon from the southwest coast of India. Relatively low saline (plankton-rich) coastal waters from the Bay of Bengal complement the plankton availability in the GoM during the Northeast Monsoon. Thus the adequate amount of plankton stock (primary food) occurs in the GoM almost throughout the year. All these positive factors facilitate an advantageous ecological setting in the GoM with very minimal seasonal variations in major environmental parameters (temperature, salinity, dissolved oxygen, and plankton stock). This situation helps to sustain high species diversity (k-selection) and build up their population sizes to the maximum of the carrying capacity of the environment. We revealed the GoM as a naturally evolved invaluable resource reserve in the region, especially during the upwelling time (Southwest Monsoon) when seasonal wipe out of sensitive fauna occurs along the southwestern Indian shelf. Indeed, the sheltered geographical positioning between India and Sri Lanka made the GoM a naturally gifted faunal resource reserve in the region.