The Great Mosque at Kilwa Kisiwani

I met a South African divemaster in Kilwa last week.  He was in Tanzania restoring boats.  I had asked him how the diving was around Kilwa, and he said that he was surprised how dead the reef was.  The water is clear.  There’s not much pollution.  Where are the live reefs?

Coral reefs are sensitive to many factors:  pollution, nutrients, temperature, pH, light (including UV), and harvesting of both the reef fish and the coral itself.  Looking around many parts of the Tanzanian coast, coral harvesting is apparent everywhere.   Stones cut from the layers and layers of old coral make up the bulk of the traditional building materials. The stones are not cut from live reefs, but are quarried from dry land, pretty much like any other building stone. Stone Town, Zanzibar is named for them.  I don’t know the geology of this archipelago, but I presume the islands were once completely covered by coral reefs that gradually uplifted.  Snorkeling around Zanzibar, you can still see live coral, but is it as healthy as it could be?

The other main building material traditionally used is mangrove trees.  The wood is extremely hard and contains chemicals that repel insects.  Our guide believes the repellant properties comes from the salt the mangroves are exposed to continuously (I haven’t figured out the mechanism, though–osmolytes or just good old resistance to herbivory?).  The wood from mangroves makes strong, long-lasting poles for walls and roofs.

The ruins of Kilwa Kisiwani tell a story through coral and mangroves since the 11th century.  Trade swelled and then subsided, and so did the community.  The old, grand city of 12,000 with a palace and about 90 or so mosques now has about 800 inhabitants.  Many of the current houses are made from stones recovered from the rubble and the Great Mosque still inspires awe and prayer. UNESCO is restoring some of the historical buildings.

Both of these materials can be harvested sustainably.  But with population growth, the demand for building materials increases.  Add this to the pressures of coastal development, pollution, global climate change, and overfishing–corals and mangroves all over the world are threatened.

Both coral reefs and mangroves provide critical habitat and perform essential ecosystem functions.  They act as a nursery for countless fish and crustaceans (some of which are delicious), they buffer the coastline from wave action, and they can process excess nutrients and other pollutants (within reason–they can be overwhelmed).  Just like the palace and mosques they built, coral and mangrove ecosystems will collapse if they are not maintained.


The coral stone that built the coast

Solid beams of mangrove