Gc Bate

A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system

In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15–25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure (5–15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.

A review of the ecology and management of temporarily open/closed estuaries in South Africa, with particular emphasis on river flow and mouth state as primary drivers of these systems

Research in South African temporarily open/closed estuaries that includes studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fishes and birds is used as a basis to review the ecology and management of this estuary type on the subcontinent. Particular attention is given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by riverine and marine events, as well as anthropogenic influences. In addition, the wider implications of these research findings for the management of temporarily open/closed estuaries in terms of freshwater supply are explored, together with the role of government legislation in maintaining the ecological integrity of these important wetland systems.

Relinkage of the Mfolozi River to the St. Lucia estuarine system – urgent imperative for the long-term management of a Ramsar and World Heritage Site

The St. Lucia Estuary was separated from the Mfolozi River in the early 1950s following canalisation of the Mfolozi floodplain swamp that resulted in the deposition of large amounts of sediment in the estuary. A separate Mfolozi River mouth has been maintained artificially since then and the deleterious consequences in terms of freshwater supply to the St. Lucia system are now becoming fully apparent, especially following the latest drought. Fish and penaeid prawn stocks in Lake St. Lucia have collapsed and the loss of this major southern African estuarine nursery area for these faunal components has had a significant impact on fish and prawn catches in adjacent coastal waters. It is therefore imperative that a re-linkage of the Mfolozi River to the St. Lucia system occurs so that the health and conservation status of the St. Lucia ecosystem, which is a Ramsar and World Heritage Site, can be assured. This review of the major findings presented at a recent scientific workshop explores the need for, and consequences of, a reconnection between the Mfolozi River and St. Lucia Estuary and makes some preliminary suggestions towards the achievement of that goal. Foremost amongst these proposals is to use the subsiding Mfolozi floodplain as a sink for excess sediment carried by the Mfolozi River before it enters the St. Lucia system, as well as various strategies in terms of reconnection options.

Response of the microalgae to changes in freshwater inflow in the Berg Estuary, southern Africa

The Berg Estuary is permanently open in the Mediterranean-type climate of the Western Cape Province, South Africa. Construction of the Berg River Dam was completed in 2007 and is expected to influence the quantity and quality of river water entering the estuary at the head. This study examined the distribution of phytoplanktonic and benthic microalgal communities throughout the estuary and the results are used to predict changes in the microalgal biomass in response to reduced flow. Salinity profiles measured in August and November 2005 indicated distinct differences in flow between winter and summer. River water was the greatest source of nutrients in August and previous research has shown that the source switches to marine water during periods of low flow. Low residence time, as a result of strong river and tidal flow, appeared to be the major factor preventing phytoplankton chlorophyll a (chl a) from exceeding 8 μg/1. Benthic ch1 a was significantly higher in August compared to November and the pattern of distribution was dissimilar to other South African estuaries. Flagellates were the dominant group of phytoplankton at most times during the study. Particularly high densities of phytoplankton were present 10.1 km from the mouth in August, dominated by flagellates (77% of total cell numbers) and 28 km from the mouth in November, dominated by diatoms (86% of total cell numbers). The predicted response of microalgae to reduced river inputs is discussed in detail. Two sampling sessions, coinciding with low- and high-flow conditions as outlined in the intermediate Resource Directed Measures (RDM) procedure of the Department of Water Affairs and Forestry (DWAF), were insufficient to accurately determine changes in microalgal biomass and community structure. Instead, predictions had to be made based on data from other South African estuaries.

Micro-estuary – a new estuary type recognised for South African conditions

Ezemvelo KwaZulu-Natal Wildlife embarked on a programme to identify all the estuaries within the KwaZulu-Natal (KZN) province. Following an aerial survey along the coastline in 2012, images of all the visible inlets and estuaries were captured. Among these are a number of areas where flows of freshwater to the sea appeared to be present but which were not among the estuaries that had previously been named. There are already well defined and recognised estuary types being managed in South Africa in terms of existing environmental legislation. Following the initial survey and the observation that there are coastal inlets/freshwater outlets that are potentially small estuaries that had not been included among them prompted a further investigation into whether some of these inlets were in fact features that should be classified in some way or other. Based on a rapid qualitative assessment, 181 sites were classified as either a “micro-estuary”, “freshwater seep”, “hard-structure freshwater outflow point” or as sites that had been considerably modified by engineered structures. A total of 61 of these 181 sites are included in a new classification as micro-estuaries. Most of these systems are in a degraded condition and require intensive rehabilitation to restore them to their historical ecological function. This study emphasises the urgency of accepting “micro-estuary” as a new estuarine type to provide formal protection against further deterioration. This is the first study to provide a description and spatially define these smaller systems present along the KZN coast.

Overview and status of estuarine microphytobenthos ecological research in South Africa

This article presents a historical overview of estuarine microphytobenthos (MPB) research in South Africa published over the period 1950 to the present, highlighting major milestones, challenges and estuarine management problems, as well as future research needs within the South African context. The studies that were covered comprise peer-reviewed books and journal articles on relevant research conducted in any estuarine environment during the period reviewed. There was a general increase in MPB research outputs over the decades, from only two publications in the 1950s, to over 20 outputs between 2010 and the present. Whereas the MPB studies in South Africa cover a broad spectrum of themes, the research priorities in these works have changed from taxonomically biased studies to those of ecologically based research. Research in the 1950s and 1960s was exclusively taxonomic in nature, with the first ecological investigation incorporating MPB being produced in the 1970s. By the 1980s, ecological studies dominated the research outputs and this trend has persisted to the present. The 1990s and 2000s saw the rise of research into the role of fresh water as a driver of MPB dynamics, whereas the period 2010 to the present saw an increase in more diverse ecological themes, ranging from an autecological investigation to food-web studies and the assessment of multiple drivers of MPB dynamics. However, the majority of studies have focused on either diatoms or estimates of overall MPB biomass. Moreover, there is a regional underrepresentation that runs broadly along biogeographic lines, with the bulk of the work having been conducted in the warm-temperate and subtropical zones of South Africa. Challenges and future research needs for the region are outlined, as is the need to expand MPB research to include other aspects of the biology and ecology of this flora.

Blackwater in South African estuaries with emphasis on the Mgobezeleni Estuary in northern KwaZulu-Natal

The blackwater Mgobezeleni Estuary is a small system below a peat swamp. From the air the estuary is completely black, but the water revealed only a light tea colouration with a Secchi reading of >1.7  m. The sediment is covered with black low-density fragments overlying light coloured sand. When the estuary is open the colouration of the sea is blacker than might be expected from the light tea-coloured estuary water. The results of X-ray spectroscopy showed the black fragments have a high carbon content that is most likely peat. The dissolved organic carbon content of the Mgobezeleni Estuary water is lower than that in Swartvlei in the South Western Cape, which has very dark water. The presence of very large deposits of peat at the head of the estuary together with spectroscopy data indicate that the colouration of the water emanates from leached leaves of swamp vegetation. The penetration of light through the water is not reflected from the light sand due to the peat covering, giving the appearance of very dark water. The literature suggests that the cause of blackwater is due to humic acids, peat and tannins leached from fallen leaves. The results of this study point to peat fragments not reflecting sunlight rather than peat extract and humic acids as the main cause. The slight colouration of the water is likely due to the small amount of fresh leaf litter in the catchment for the release of tannins that colour the Mgobezeleni Estuary water.

Ecology and Conservation of Estuarine Ecosystems: Acknowledgements

List of contributors Foreword Alan Whitfield Preface 1. South Africas first World Heritage Site Roger N. Porter 2. Management history Ricky H. Taylor 3. Geological history Greg A. Botha, Sylvi Haldorsen and Naomi Porat 4. The marine environment Allan D. Connell and Sean N. Porter 5. Catchment hydrology Derek D. Stretch and Andrew Z. Maro 6. The Wetlands William N. Ellery, Suzanne E. Grenfell, Michael C. Grenfell, Marc S. Humphries and Kirsten B. Barnes 7. Estuary and lake hydrodynamics Derek D. Stretch, Clint P. Chrystal, Robynne A. Chrystal, Christopher Maine and Justin J. Pringle 8. Groundwater hydrology Bruce E. Kelbe, Ricky H. Taylor and Sylvi Haldorsen 9. Physico-chemical environment Renzo Perissinotto, Nicola K. Carrasco and Ricky H. Taylor 10. Microalgae Renzo Perissinotto, Guy C. Bate and David G. Muir 11. Macrophytes Janine B. Adams, Sibulele Nondoda and Ricky H. Taylor 12. Benthic invertebrates Deena Pillay, Sarah J. Bownes and Holly A. Nel 13. Zooplankton Nicola K. Carrasco, Renzo Perissinotto and Hendrik L. Jerling 14. Penaeid prawns Anthony T. Forbes and Nicolette T. Forbes 15. Fish and fisheries Digby Cyrus 16. Birds Jane Turpie, Ricky H. Taylor, Meyrick Bowker and Caroline Fox 17. Crocodiles Xander Combrink, Jonathan Warner and Colleen T. Downs 18. Hippopotamuses Ricky H. Taylor 19. Alien and invasive species Nelson A. F. Miranda and Janine B. Adams 20. Food webs and ecosystem functioning Ursula M. Scharler and Fiona C. MacKay 21. Climate change impacts Andrew A. Mather, Derek D. Stretch and Andrew Z. Maro References Appendix: web page database, 1900-2010 Nuette Gordon Index.