External influences on the eastern African ecoregion

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Connectivity within the Ecoregion

The monsoon winds of eastern Africa, with their characteristic seasonal reversal from the northeast (NE) to the southeast (SE), are a major influence on the climate and seas, affecting weather patterns, rainfall and human activities. Another feature of the eastern African ecoregion is the tides.
The coast from Chisimayu to Sodwana Bay, experiences a maximum tidal range of 3-4m, with a tidal regime roughly the same throughout the region, that is, time of low tide in Mombasa is almost the same as that in Maputo.

This almost uniform vertical movement of the coastal waters throughout the region increases the strength of tidal currents and the resulting mixing of inshore waters. Superimposed on the movement created by tidal currents, is the much larger scale movement of oceanic and long-shore currents that exist along these shores.

South Equatorial Current - the primary influence

Ocean currents are driven by the rotation of the earth and the prevailing winds. The main current that influences the region is the South Equatorial Current that travels west across the Indian Ocean reaching the African coastline at approximately the border between Tanzania and Mozambique.

There it splits into the north-flowing East Africa Coastal Current (EACC), and generally south-flowing Mozambique Current. The EACC travels steadily north at speeds of 2-5 knots (3-9km per hour) depending on the season. During the SE monsoon, the EACC is accelerated and continues north leaving the coast at Somalia.

During the NE monsoon season the current speed is reduced and the meeting with the southward- flowing Somali current off Kenya results in the east-flowing Equatorial Counter Current. The Mozambique current extends southwards in a twisting and curling manner along the Mozambique coast varying in extent from year to year.

The Agulhas Current

Towards the southern end of Mozambique, the Agulhas Current swings round from southern Madagascar and continues southwards along the KwaZulu-Natal coast of South Africa, eventually mixing with the cold waters off the southern tip of Africa.

Although the strength and extent of influence of the region's ocean currents varies from year to year, dictated by global environmental conditions that in turn affect the strength of the monsoon winds, the overall effect is a fairly consistent movement of offshore waters along the coastline.

Ideal for marine life

Combined with the daily ebb and flood of the tide, the resulting currents mix and distribute the inshore waters, its sediments, nutrients, plankton and other floating marine life. The minute floating plants and animals, the latter usually feeding on the former, and on each other, comprise the plankton community. This community, of which the familiar jellyfish is a member, includes the eggs and larval stages of much larger marine animals such as fish, lobsters, crabs, shrimps, oysters, coral, sponges, as well as the spores of seaweeds.

Transport of nutrients, plankton; movement of large marine animals

When a mature, female lobster (for example) casts adrift her half a million eggs, she is unlikely to ever see her young. The hatched larvae will join the plankton community, many will be eaten, and others will be washed out into the deep ocean never to find a suitable home. But a few, several weeks later, will finally metamorphose into miniature lobsters. The juveniles may be hundreds of kilometres away from the home reef of their parents.

Seeds of coconut trees, mangrove trees, seagrasses and many other coastal plants also rely on the currents as their means of dispersal. Other specialist swimmers use the currents to navigate and carry them throughout the region to reach feeding or breeding areas. Loggerhead turtles nesting in KwaZulu Natal in northern South Africa have been found in Zanzibar, and vast schools of tuna migrate each year through the eastern African waters to feed and breed.

This cohesion, provided by the coastal waters that bathe these shores, is vital to sustaining the biodiversity and productivity of the region.

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