Mangrove colonisation of the Mlalazi Estuary, South Africa: a response to artificial breaching

Abstract

The Mlalazi Estuary has 40 ha of mangroves, but before the 1930s there were none. The purpose of this study is to understand why this change  occurred, and how the understanding thus gained can provide ecological information that will help understand the dynamics of similar estuaries in the region. The approach was to accumulate evidence gathered from historical records, published literature and scientific investigations. In the past 6 000 years, the Mlalazi Estuary evolved from an open bay to a permanently open estuary and then to its current state of a temporarily closed estuary. There is evidence that the mangroves that occurred in the system were lost when the estuary mouth started closing for extended periods. Since 1896, Mlalazi has been artificially breached whenever closure resulted in the backing up of water. This mouth-breaching regime resulted in physical conditions suitable for mangrove growth. Mangrove propagules would have dispersed from adjacent estuaries; taking more than 30 years for Avicennia marina and Bruguiera gymnorrhiza to colonise, and approximately 100 years for Rhizophora mucronata. Once established the mangrove population expanded and there is evidence that available habitat is becoming limiting. Any alteration to the current mouth management strategy is likely to affect the mangroves and must not be implemented without a full understanding of its consequences. The Mlalazi Estuary is a unique estuary in South Africa in that mouth manipulation has led to the colonization of mangroves not previously current in the system.

Keywords: conceptual understanding, historical ecology, geological evolution, human impacts, estuary mouth management, population  establishment