Tuesday, March 25, 2008

Coral Ecosystems of India's West Coast

I came across a small news report in this Sunday's Times of India of the recent discovery of a thriving coral ecosystem about 80 nautical miles from Panjim in Goa. This coral reef system has developed atop a shallow 40 km long bank known as Angria Bank. The sea floor surrounding this bank is about few 100 feet or so deep and suddenly rises to just 20 meters or so. The image below shows the location of Angria Bank.

The bathymetric surface is also seen in the image and although generalized you can still see the topography of the sea-floor on a regional scale. The dotted line marks the edge of the continental shelf of India. The shelf is the drowned extension of the continental crust of India. When you say that plate tectonic movements caused India to split away from Africa, the split did not occur at the position of the present coastline but roughly at the edge of the continental shelf. Back during the Pleistocene glaciation much of this shelf may have been exposed as sea-levels periodically dropped by a few hundred feet worldwide. As the glaciation ended around 12,000 years ago, sea-levels rose up to the present coastline, drowning large parts of the shelf. The Angria Bank as you can see is at the edge of the shelf. At the point marked Angria Bank you can see couple of elongated humps on the sea floor. That is the raised shallow bank. To the west the sea floor suddenly becomes deeper along a steep slope that grades into the deep sea plains. This marks the transition from the continental crust of the shelf to the ocean crust of the deep sea plains. The banks topographic high above the surrounding sea bed is probably structural in nature. The west coast shelf of India is riddled with coast parallel faults which originated with the tectonic breakup of India first with Africa and then with Madagascar. Vertical movements along these faults produce horst and graben structures, elongate ridges and depressions. The Angria Bank is likely such a horst. The coral community there must have started developing after the Holocene sea-level rise few thousand years ago and coral growth continues today. I am not sure what the composition of the foundation of this reef is. It could be basalt rock, the submerged continuation of the continental flood basalts that are exposed all over Maharashtra or it could be older Cenozoic sediment or even Pleistocene reefs developed during the interglacial phases of the Pleistocene glaciation when sea-level was high.

Such isolated shallows as ideal environments for coral growth. Far from continental rivers, the water are clear and devoid of terrigenous sediment which can make water turbid. Such turbidity can prevent sunshine from penetrating the water, inhibiting the symbiotic algae that lives within corals from efficient photosynthesis. Too much suspended sediment may also clog up the coral animals feeding mechanism. Many of the world's major coral ecosystems such as the Bahamas, Bermuda, Lakshadweep, Maldives are situated in such isolated shallows, some distance away from major continental river mouths and deltas.

The newspaper reports that the Maharashtra tourism department hopes to make this into a hot eco-tourism spot. I searched for Angria Bank and found several private tourist operators already offering a "four hour ride on speed boat" from Panjim for world class diving to the Angria Bank. Exploring coral ecosystems is one of the great experiences. My graduate adviser and I used to teach a class in carbonate depositional systems and took students to study the Florida Keys. I can tell you no matter how many nature documentaries you have seen, nothing prepares you for your first encounter with coral reefs. That world on the sea floor is something else. Besides just catering to tourists this newly found ecosystems needs to be studied in detail. Reef Watch a Mumbai based organization has already started a research program to document and study the biodiversity. And I hope the government steps in and works towards making the Angria Bank a marine bio-reserve. Better tourists with a camera than fishing trawlers.

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