Bay of Bengal

Category: Marine and Oceanic Biomes.

Geographic Location: South Asia.

Summary: This major lobe of the Indian Ocean, along with its coral reef, estuarine, and coastal habitats, supports great numbers and diversity of species.

The major northeastern lobe of the Indian Ocean, the Bay of Bengal encompasses some 850,000 square miles (2.2 million square kilometers) of ocean fed by such major rivers as the Mahanadi-Brahmani, Godavari, Krishna, and Kaveri from India in the west; the Ganges-Brahmaputra-Meghna system from India and Bangladesh in the north; and the Irrawaddy from Burma (Myanmar) in the east. Arcing from Sri Lanka and the east coast of India and across the Ganges Delta to Burma and the Andaman-Nicobar archipelago, the Bay of Bengal has an average depth of 8,500 feet (2,600 meters), a maximum depth of 17,260 feet (5,260 meters), and water volume of some 1.3 million cubic miles (5.6 million cubic kilometers). It is considered the largest bay on Earth.

The floor of the Bay of Bengal is almost entirely made up of the Bengal Fan, a structure first created in the ancient tectonic collision of the Indian Plate and Eurasian Plate, then eroded over eons into a series of underwater canyons by the massive inflow of sediment from the Himalayan headwaters through the Ganges-Brahmaputra-Meghna river system.

Annual continental runoff of freshwater into the bay, estimated at more than 710 cubic miles (3,000 cubic kilometers) is considerable, and peaks during the monsoon season, June through September. The rivers transport minerals and nutrients into the bay, as well as effluents and other pollutants from the cities, towns, and farms they pass through. Total sediment discharge into the Bay of Bengal is estimated at 24 billion tons annually.

Ecology and Species

The Bay of Bengal biome comprises estuaries and wetlands, coral reefs, and such unique zones as the world's largest salt-tolerant tidal mangrove forest—in the Sundarbans, where river and marine environments merge across the Ganges Delta. Upwelling along many of the near-shore areas contributes to the relative ease with which populations of phytoplankton and zooplankton thrive in these seas. Atoll, fringing, and barrier reefs all occur within the Bay of Bengal coral matrix, supporting substantial communities of sponges, mollusks, crustaceans, echinoderms, and fish.

Key fish species in the Bay of Bengal include the commercially important tuna, including striped bonito (Sarda orientalis), eastern little tuna (Euthynnus affinis), bullet tuna (Auxis rochei), and big eye tuna (Thunnus obesus); Indopacific king mackerel (Scomberomorus guttatus), Spanish mackerel (Scomberomorus commerson), and Indian mackerel (Rastrelliger kanagurta); Indian salmon (Eleutheronema tetradactylum); at least 15 species of anchovy, four of sardine, and two herring; and tiger prawn (Penaeus monodon).

Sea turtles include the leatherback (Dermochelys coriacea)—particularly numerous along the island chains of the Andamans and Nicobars, a prime egg-laying region—and the endangered olive ridley sea turtle (Lepidochelys olivacea). The olive ridley has been known to mass on the beaches of the eastern Indian state of Orissa in numbers of more than a half-million during its nesting season. The Gahirmatha Marine Wildlife Sanctuary, with some mangrove wetlands areas first set aside in 1975, protects this overfished reptile and also helps shield such at-risk fauna as the saltwater crocodile (Crocodylus porosus); eight types of at-risk kingfisher, including the black-capped kingfisher (Halcyon pileata) and pied kingfisher (Ceryle rudis); and resident herons such as Asian open bill (Anastomus oscitans), night heron (Nycticorax nycticorax), and white ibis (Threskiornis melanocephalus).

Mammals thriving in the Bay of Bengal include the dugong (Dugong dugon), which favors the seagrass beds found along the island chains; the Irrawaddy River dolphin (Orcaella brevirostris), preferring estuary zones and the Sundarbans; and cetaceans like Bryde's whale (Balaenoptera edeni), which inhabit such areas as the submarine canyon known as the Swatch of No Ground.

Crowning the biological richness of the Bay of Bengal in its northern reaches is the Sundarbans mangrove: Its National Park area in India is a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site, and its entire Indian expanse is a UNESCO Global Biosphere Reserve, while the Sundarbans Reserve Forest in Bangladesh is a Ramsar Wetland of International Importance spanning 2,323 square miles (601,700 hectares). The Sundarbans is vital for migratory fowl. Such apex predators as the greater spotted eagle (Aquila clanga) and Pallas's fish-eagle (Haliaeetus leucoryphus) winter here and summer in north-central Asia. Indian skimmer (Rynchops albicollis), spotted greenshank (Tringa guttifer), and spoon-billed sandpiper (Eurynorhynchus pygmeus) also rely on the Sundarbans.

Species as diverse as the Bengal tiger (Panthera tigris) and the rare Ganges shark (Glyphis gangeticus) depend on this vast mangrove, with its teeming food supply, prime breeding grounds, and protection from cyclones. Subsistence fishermen here sometimes train the smooth-coated otter (Lutrogale perspicillata) to drive fish into their nets.

Balancing Human Activities

Despite the protected areas within this vast region, much of the coast habitat is under threat from energy industrial and general urban development, the mangrove timber industry, wet rice cultivation, bird hunting, and aquaculture in the form of shrimp pond farms. Often the reach of these threats extends outward across the Bay of Bengal. For example, transmission of aquaculture effluents or escaped cultivars that can carry toxins and parasites is likely to be a factor in widespread disease impacts such as the as-yet unknown pathogen that in 2009 began affecting fish, mollusks, and crustaceans including such commercially vital species as the tiger prawn and white-leg shrimp (Litopenaeus vannamei). The Network of Agriculture Centres in Asia-Pacific is one of the organizations attempting to move its members toward more sustainable economic pursuits that build habitat protection into their methods and practices.

About 450 million people live in the coastal zone around the Bay of Bengal; annual fishery production here is on the order of three million tons, with a value of approximately $4 billion. Poverty and population pressures, insufficient governmental planning and control, increased tourism, and insufficient attention to waste management are all threats to the ecology of the region. Due to the low-lying nature of much of the land bordering the bay, the region is also extremely sensitive to changes in ocean level; there is growing concern for how global warming may affect the climate and habitats of the region. Salt intrusion, invasive species, and phytoplankton population vector alterations are among the catalog of risks being examined.

One cooperative international group focused on these issues is the Bay of Bengal Large Marine Ecosystem Project (BOBLME), an effort by Bangladesh, India, Indonesia, Malaysia, the Maldives, Myanmar, Sri Lanka, and Thailand. BOBLME aims to halt the ecological decline of the bay, improve the lives of people living on the coastline, prepare for changes expected from climate change, regulate growth and development, and better manage the fishing industry within this biome. The organization is only in the very early stages of formulating ways to combat the effects of global warming across the Bay of Bengal.

Bibliography

Kumar, M. Dileep. Biogeochemistry of the North Indian Ocean. New Delhi: Indian National Science Academy, 2006.

Quader, O. Coastal and Marine Biodiversity of Bangladesh (Bay of Bengal). Dhaka, India: Space Research and Remote Sensing Organization, 2010.

Sengupta, Debasis, Bharath Raj, and S. S. C. Shenoi. “Surface Freshwater From Bay of Bengal Runoff and Indonesian Throughflow in the Tropical Indian Ocean.” Geophysical Research Letters 33, no. 1 (2006).