Mangroves come from the east

Biologists give first insight into distribution of ecologically important mangroves in the West Indian Ocean

Mangrove forests are of great ecological and socio-economic importance. They have a permanent place on the international climate agenda because of their extensive carbon storage. Researchers from the Department of Biology at the Vrije Universiteit Brussel have now studied the genetic structure and diversity of mangrove forests in the Western Indian Ocean, a region where these aspects have been less studied until now. The study, published in , shows among other things that mangroves arrived and established earlier on the Seychelles, Madagascar and Aldabra, i.e. on the islands off the East African coast, than on the East African mainland coast itself.

Dr Tom Van der Stocken, lead author of the study: “This study provides a clear picture of the connections between mangrove populations in this region and illustrates the role of ocean surface currents therein. This knowledge is of fundamental ecological importance. Understanding how the seeds and fruits from these forests disperse, how populations are connected globally and which factors play a role in shaping dispersal and connectivity patterns is crucial also also for better understanding how these forests shift in response to climate change. However, the sheer spatial extent of this process makes this kind of research particularly challenging. The seeds and fruits of mangroves are distributed via rivers, tides, as well as coastal and open-ocean currents. While most are spread locally within the forests, over short distances, a generally smaller a generally smaller but important proportion is carried with the currents over tens, hundreds and sometimes even thousands of kilometres.’

The researchers used genetic methods for the study. Prof Ludwig Triest: “The genetic structure of mangrove forests can be seen as a kind of blueprint of the connectivity between populations and therefore provides insight into the patterns of dispersal, past and present. We investigated the genetic structure and diversity of a large number of mangrove populations in the Western Indian Ocean region, specifically from Kenya to southern Mozambique on the East African mainland coast and on islands such as Madagascar, Aldabra and the Seychelles.’

The researchers found that the genetic structure of the studied mangrove populations largely corresponds to the patterns of ocean surface circulation in the region. Prof Triest: “The splitting of the Indian South Equatorial Current off the African mainland coast, for example, creates a clear genetic break between mangrove forests on either side of this circulation pattern. We find a similar result for the Delagoa Bight region in southern Mozambique, where the combination of a strong southward jet stream and a cyclonic eddy impedes the exchange of seeds and fruits between two nearby populations. In turn, the general southward flow and the anti-cyclonic circulation in the Mozambique Channel ensure good dispersal and connectivity between populations in this region.’

A notable result from the study is that mangroves established earlier on the Seychelles, Madagascar and Aldabra than on the East African mainland coast. In general, the establishment and expansion of mangroves in the Western Indian Ocean occurred later than in the Eastern Indian Ocean. Prof Triest: “Presumably there was a supply of seeds and fruits from the east that was maintained by the strong Indian South Equatorial Current. The splitting of this sea current into a northward and a southward flow near the African mainland may also explain the high genetic diversity of mangrove populations along the coast of Kenya and Tanzania.?

The results of the genetic study thus clearly show how ocean surface currents can connect populations at great distances, but also how ocean surface circulation patters can prevent connectivity between populations in close proximity. So nearby, geographically, does not necessarily mean better connected,- says Dr Van der Stocken.


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