Dynamics of the macrophyte vegetation of the Mgobezeleni floodplain and estuary, Northern KwaZulu-Natal

Abstract

This empirical study describes the vegetation responses to natural and artificial perturbations that have affected the Mgobezeleni Estuary and floodplain in recent decades. Typically the floodplain and estuary are affected by the changing morphology of the beach berm and the constancy of groundwater-fed baseflow entering the estuary from the catchment. Over the past several decades there have been two major perturbations to the mangroves. A road-bridge was built across the estuary in the early 1970s that raised the water level in the mangrove area. This killed many of the mangroves before the bridge was demolished and then rebuilt to rectify the situation. The second event occurred in 2007 when extremely high seas flushed saline marine water into the lower portion of the estuarine floodplain. This killed swamp vegetation that was unable to tolerate the saline water. Decaying vegetation was flushed into the main part of the estuary and the ensuing de-oxygenated water flooded the remaining living mangroves killing all except for a handful. There has been a slow and constant expansion of Ficus trichopoda dominated swamp forest into the estuarine floodplain, displacing sedge swamp. This study highlights the importance of the hydrological regime and alerts us to the sensitivity of this system to future recruitment of mangroves after an extreme perturbation.