Benthic macrofaunal changes in response to restoration efforts in the St Lucia estuarine lake, South Africa

Occurrence and persistence of water level/salinity states and the ecological impacts for St Lucia estuarine lake, South Africa

Abiotic characteristics and microalgal dynamics in South Africa's largest estuarine lake during a wet to dry transitional phase

Water beetles belonging to the suborder Polyphaga vary greatly in larval and adult ecologies, and fulfil important functional roles in shallow-water ecosystems by processing plant material, scavenging and through predation. This study investigates the species richness and composition of aquatic polyphagan assemblages in and around the St Lucia estuarine lake (South Africa), within the iSimangaliso Wetland Park, a UNESCO World Heritage Site. A total of 32 sites were sampled over three consecutive collection trips between 2013 and 2015. The sites encompassed a broad range of aquatic habitats, being representative of the variety of freshwater and estuarine environments present on the St Lucia coastal plain. Thirty-seven polyphagan taxa were recorded during the dedicated surveys of this study, in addition to seven species-level records from historical collections. Most beetles recorded are relatively widespread Afrotropical species and only three are endemic to South Africa. Samples were dominated by members of the Hydrophilidae (27 taxa), one of which was new to science (Hydrobiomorpha perissinottoi Bilton, 2016). Despite the fauna being dominated by relatively widespread taxa, five represent new records for South Africa, highlighting the poor state of knowledge on water beetle distribution patterns in the region. Wetlands within the dense woodland characterising the False Bay region of St Lucia supported a distinct assemblage of polyphagan beetles, whilst sites occurring on the Eastern and Western Shores of Lake St Lucia were very similar in their beetle composition. In line with the Afrotropical region as a whole, the aquatic Polyphaga of St Lucia appear to be less diverse than the Hydradephaga, for which 68 species were recorded during the same period. However, the results of the present study, in conjunction with those for Hydradephaga, show that the iSimangaliso Wetland Park contains a high beetle diversity. The ongoing and future ecological protection of not only the estuarine lake itself, but also surrounding freshwater wetlands, is imperative and should be taken into consideration during future management planning for the park.

The recent dry phase experienced by the St Lucia estuarine system has led to unprecedented desiccation and hypersaline conditions through most of its surface area. This has changed only recently, at the end of 2011, with the onset of a new wet phase that has already caused a major shift to oligo- and mesohaline conditions. The estuary mouth, however, remains closed to the ocean, making the weak connection recently established between the St Lucia and the Mfolozi estuaries the only conveyance for marine recruitment. As a result, only 10 indigenous and two alien aquatic gastropod species are currently found living in the St Lucia estuarine lake. This is out of a total of 37 species recorded within the system since the earliest survey undertaken in 1924, half of which have not been reported in the literature before. The tick shell, Nassarius kraussianus, which was consistently found in large abundance prior to the recent dry phase, appears to have temporarily disappeared from the system, probably as a result of the extinction of Zostera marine grasses inside the lake. Population explosions of the bubble shell Haminoea natalensis, with its distinct egg masses, were recorded seasonally until 2009, but the species has subsequently not been observed again. A molecular DNA analysis of the various populations previously reported as belonging to the same assimineid species, variably referred to as Assiminea capensis, A. ovata, or A. bifasciata, has revealed that the St Lucia assemblage actually comprises two very distinct taxa, A. cf. capensis and a species provisionally referred to here as “A.” aff. capensis or simply Assimineidae sp. In the mangroves, the climbing whelk Cerithidea decollata is still found in numbers, while ellobiids such as Cassidula labrella, Melampus semiaratus and M. parvulus are present in low abundances and all previously recorded littorinids have disappeared. A number of alien freshwater species have colonized areas of the system that have remained under low salinity. These include the invasive thiarid Tarebia granifera, which can be found in concentrations exceeding 5000 ind.m-2, the lymnaeid Pseudosuccinea columella and the physid Aplexa marmorata.

Lake St. Lucia is the largest estuarine lake in South Africa and is a UNESCO World Heritage Site. The St. Lucia Estuary resides in a poor state due to drought and human impacts which have resulted in the closure of the mouth to the sea since 2002 and changes in freshwater inflow particularly from the Mfolozi River. This provides a unique opportunity to study the responses of the mangroves that usually grow in an intertidal environment. High water levels and inundation caused mangroves to die in The Narrows in 2013/2014 which caused concern. The aim of this study was to investigate the response of the mangroves to fluctuating environmental conditions. This was done by measuring the population structure and sediment characteristics at four sites in 2010, 2013 and 2014. The reconnection of the Mfolozi River to St. Lucia in 2013 via the Beach Canal and Back Channel as well as good rains led to an increase in water level which caused permanent inundation of pneumatophores (aerial roots of the white mangrove Avicennia marina) and anoxic conditions resulting in the mortality of mangroves fringing the main estuary channel (Site 3 on The Narrows). Self-thinning is a natural process that has occurred at Sites 1 and 2 while Site 4, which was previously considered a dying population, had a recruitment of a new seedling cohort in 2014. The dieback of mangroves along the water's edge is part of the natural dynamics because this would have occurred under the natural state for this system when the mouth closed to the sea, water level was high and the estuary was fresher. This study has shown that site-specific differences will allow for recruitment and overall survival of mangroves at St. Lucia Estuary.

Anthropogenic impacts on the water and salt budgets of St Lucia estuarine lake in South Africa

Opportunistic observations made in the St Lucia estuarine lake, mainly during the period 1991 to 2000, show that the rhizostomatid scyphozoan Crambionella stuhlmanni often forms aggregations with densities in excess of 1 ind.m-3. Its salinity tolerance varies in the range of 12–65 ‰, with the highest jellyfish concentrations often observed at hypersaline levels of 40–65 ‰. Considering that the species has persisted in the system under the present closed mouth conditions, it is suggested that it is capable of completing its life cycle within the estuarine lake. Its role in the ecosystem needs to be investigated, with emphasis on its potential ability to remove suspended sediment particles from the water-column.

Fiddler crabs occur globally across tropical and subtropical coastal habitats including mangrove swamps, which are in decline worldwide. The genus has been well-studied in the Americas, Australia and Asia, whereas information on African species is scarce. This review summarises the existing literature on fiddler crabs and highlights gaps in research for species at their southernmost distribution ranges in South Africa. Biological and ecological aspects including life cycle and reproduction, feeding ecology, biotic interactions and tolerance to environmental fluctuations and pollution are discussed. The systematics and distribution of the five fiddler crab species that occur in South Africa is provided. The St Lucia estuarine lake is presented as a case study for the effects of environmental change on Uca populations. Future research should focus on less-studied Uca populations, such as those found in the Indo-Pacific region. The effects of climate change and habitat modification on Uca populations should also be more widely investigated.

Wind-driven circulation patterns in a shallow estuarine lake: St Lucia, South Africa

In situ feeding rates and grazing impact of Mesopodopsis africana O. Tattersall in the St Lucia Estuary, South Africa

Turbidity effects on feeding and mortality of the copepod Acartiella natalensis (Connell and Grindley, 1974) in the St Lucia Estuary, South Africa

The Mfolozi–Msunduzi estuarine system historically shared a common mouth with the St Lucia estuarine system. In 1952, a separate mouth was created, 1.5 km south of St Lucia mouth, to prevent silt carried by the Mfolozi River from entering Lake St Lucia. Despite its proximity to the comparatively well-studied St Lucia Estuary, there is very little information on the Mfolozi– Msunduzi estuarine system in general and no information on its benthos. In the present study, 17 taxa were recorded from biannual quantitative sampling in 2007 and 2008. Results indicated that the system was dominated by the polychaetes Ceratonereis sp., Dendronereis arborifera and Capitella capitata, the crab Paratylodiplax blephariskios and the tanaid Apseudes digitalis. The main factors influencing the distribution of the benthos were oxygen concentration, temperature, the open or closed state of the mouth, and salinity, with particle size and organic content of the substratum also being important in determining community structure. Although the dominant taxa were previously recorded as abundant in St Lucia, numbers present in the Mfolozi–Msunduzi system were generally lower than those recorded in St Lucia, suggesting that periodic flooding and the unstable nature of the sediments in the Mfolozi–Msunduzi prevented its benthos from attaining the densities recorded in the adjacent St Lucia Estuary. While the Mfolozi–Msunduzi system is classified as a river mouth, its benthos was more similar in composition to that of the Mhlathuze and Mlalazi estuaries, which are classified as an estuarine bay and a permanently open estuary, respectively. It is suggested that reclassification of the Mfolozi–Msunduzi system as a permanently open estuary would be more appropriate in terms of its benthos.

Fiddler crabs (Uca spp.) undergo the zoeal stage of development in open-ocean waters, where they experience stable salinity levels, low turbidity and reduced predation. The St Lucia estuarine system has undergone many geomorphological changes, both natural and anthropogenic, and the estuary mouth has been closed since the early 2000s. Despite recent attempts to improve marine connectivity, it remains limited, occurring primarily on the flood tide through channels connected to the adjacent Mfolozi River. Larval export from the St Lucia Estuary is therefore almost non-existent. A laboratory study was undertaken to examine the silt and salinity tolerance of Uca annulipes first stage zoeae, to investigate whether survival in the closed-estuary conditions would be possible. Salinity tolerance was narrow, with zoeae displaying 100% mortality at salinities <20 and >35 after five days. Zoeae were widely tolerant to silt loading and did not display a significant decrease in survival over a range of 0–1 000 NTU. A limited salinity tolerance is in accordance with the life-history strategy of fiddler crabs, and a high tolerance to turbid waters can be advantageous to small-bodied merozooplankton. Given the stenohaline nature of the zoeae, marine connectivity is therefore essential for the persistence of U. annulipes in this estuarine habitat.

The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment management, leading to large quantities of suspended particulate matter (SPM) entering the estuary from the Mfolozi River. This paper quantities some of the solute and sediment dynamics in the Mfolozi Estuary where the main documented environmental concern is the periodic input of SPM from the Mfolozi Estuary to the St. Lucia system, causing reduction of light penetration and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach mean bed shear stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities dominate the sediment transport processes in the estuary when fluvial input in the Mfolozi River is of the order of 15–20 m3 s–1. Observed and predicted flood tide velocities are too low (<0.35 m s–1) to suspend and transport significant amounts of SPM. Observed results indicate that although the SPM load entering the estuary is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estuary's salinity and velocity fields. It is calculated that the Mfolozi Estuary would fill with sediment in 1.3 years if it was cut off from the sea. The major fluvial flood events help maintain the estuary by periodically pushing sediment seawards (spit progrades seawards 5 m yr–1) and scouring and maintaining the main flow channel in the estuary. During low fluvial flow conditions, tidal flow velocities will become the dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi estuaries under present conditions is complicated by the strong transverse velocity shear between the two systems at their combined mouth. This is creating a salinity-maintained axial convergence front that suppresses mixing of solutes and SPM between the systems for up to 10 h of the tidal cycle during observed conditions.

During the last century, water abstractions, eutrophication and other factors have severely impacted the natural biophysical functioning of estuaries on a global scale (see e.g. Livingston, 2001; McLusky and Elliott, 2004; Wolanski, 2007). The St Lucia estuarine lake is an example of an important system that has been severely impacted by human interventions, despite its status as a UNESCO World Heritage Site and Ramsar Wetland of International Importance (e.g. Van Vuuren, 2009; Whitfield and Taylor, 2009; Cyrus et al., 2010a; Lawrie and Stretch, 2011a, 2011b). Similar issues are faced by other large and important systems worldwide, such as the Coorong Estuary in Australia and Chilika Lake in India (Ghosh et al., 2006; Webster, 2010).In this chapter the term ‘hydrodynamics’ is used in a broad sense to refer to all processes concerning the availability and movement of water through the system – for example catchment hydrology (refer to Chapter 5) is integral since it characterizes the inflows to the system. We focus mainly on the water and salt budget of the lake system, and the impact of various management interventions that have been implemented in the past (refer to Chapter 2), and that may be used in the future. Hydrodynamics, in the broad sense used here, is the primary driver of estuarine functioning.