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.