Environmental changes in the biggest lagoon of Africa

Dep. of Plant and Environmental Sciences

On Tuesday, June 20, defended Ricky H. Taylor from South Africa his Doctoral Thesis “"Ecological responses to changes in the physical environment of the St Lucia estuary, South Africa” at Norwegian University of Life Sciences.

He gave first a public lecture with the title: "World Heritage sites in South Africa - conservation and socio-economic benefits" and then gave a presentation of his scientific work.

Abstract , R. Taylor

Ricky Taylor

The St Lucia estuary in Maputaland, northeast South Africa, is a shallow water body that experiences large fluctuations in salinity and water volume. These changes strongly control when and where the plants and animals occur in the system , and in what abundance they occur.

The purpose of this study has been to gain a holistic understanding of the St Lucia estuary at the ecosystem level, and to use this knowledge to predict the effects of future change on the system

St Lucia is a highly dynamic system driven by several different processes, each operating at different temporal and spatial scales. The closure and breaching of the estuary mouth has a large influence on the salinity and water levels within the St Lucia system. An understanding of which processes within the estuary occur during the open and closed phases has enabled the development of a conceptual model of the patterns of water level and salinity. The intermittent linkage of the Umfolozi River to the St Lucia estuary is a central factor which affects the freshwater-saltwater balance - especially during drought periods. This study provides an understanding of the impacts that the modification of the Umfolozi River has had on the mouth of St Lucia in the past 50 to 60 years, and the repercussions of these conditions on the whole St Lucia system.

The macrophyte vegetation is the component of the system that can best be used to indicate the state and trends of the ecosystem. This is because the plants are anchored, because the different species respond to changes over different time scales and because each plant species has a different range of environmental conditions it can tolerate. The estuarine vegetation, was described, classified and mapped and the determinants of the different vegetation communities were identified. Knowledge about which environmental factors that control each vegetation community assists us to predict how the vegetation will respond to changes.

To simplify the patterns shown by the vegetation we recognise four Primary Habitats – which are suites of habitats that, within each suite, have the same main determinants, but between suites are very different. One primary habitat, for instance, is dependant on the seepage of groundwater. The mapping of this habitat was used to show where groundwater seepage occurs. Then, a mathematical groundwater model was used to estimate the quantities of water discharged in these seeps, and to simulate the sustainability of the discharges for the duration of a drought that is more severe than any yet recorded.

Conceptually, changes in primary habitats were used to forecast ecosystem responses to open and closed mouth conditions. The hydrological changes controlled by the mouth state and the responses of the four primary habitats to these changes are described.

What is the extent and role of groundwater in the system? The St Lucia catchments have lost 20% of their Mean Annual Runoff, and the Umfolozi River is no longer linked to St Lucia. But the system has continued to be a functional estuary ecosystem for these past 50-60 years (albeit in an altered fashion). One recommended management action to protect the groundwater is to use fire to maintain the low-lying grasslands of the Eastern Shores groundwater catchment area. This is done to prevent encroachment of the deep-rooted woody plants that increase transpiration losses from the groundwater.

Global change will affect the St Lucia estuary. In this study we have predicted that over the next 100 years sea level will rise by 50 cm, there will be a warming of 2-3°C and a 5-10% increase in rainfall. The sediments of the mouth will accommodate the changes in the sealevel. The tidewater channel The Narrows has steep sides and consists of stable clay deposits. There will be little change of this channel other than being deeper and hence allowing more tidal exchange as there will be less resistance to the movement of water. The main basins will cover a larger area and will be deeper, as there is unlikely to be a significant increase in sediments entering the estuary. These changes will increase the volume of St Lucia by 65-70% and this will buffer changes in salinity.

The Primary Habitats are forecast to respond to these global changes. Based on the findings of this study, predicted ecosystem changes include:

- The lower and more stable salinity regime will lead to the development of much larger beds of the submerged water plants Potamogeton pectinatus and Ruppia cirrhosa than is currently the case;

- There will be an upward shift in the band of mangroves along the edge of the tidal portion of the estuary, but little change in abundance unless prolonged mouth closure associated with high water levels kills the mangroves;

- There will be decrease in saltmarsh habitat as the low-lying islands and other sedimentary deposits are inundated by the rising water.

However, the predicted impacts of global change on the St Lucia estuary over the next 100 years are likely to be small in comparison to the impacts that local changes that have had in the past 60 to 80 years (the period during which there has been extensive canalisation of the Umfolozi swamps and manipulations of the St Lucia mouth).

Ricky H. Taylor 2006. Ecological responses to changes in the physical environment of the St Lucia estuary. Norwegian University of Life Sciences. Doctor Scientiarum thesis 2006:7. ISSN: 0802-3220, ISBN: 82-575-0706-7



Updated: 27.03.07
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