Distribution, abundance, population structures, and potential impacts of the invasive snail, Tarebia granifera in aquatic ecosystems of north-eastern South Africa.

Aquatic ecosystems globally have been invaded by molluscs. Tarebia granifera is a highly successful invader, often becoming the dominant aquatic invertebrate species in an invaded ecosystem. Resultingly, it has been suggested that T. granifera may have severe negative impacts on these invaded ecosystems. Limited information is available regarding the population structures and densities of T. granifera, particularly in invaded countries such as South Africa, and information on this could assist in developing management and control strategies for this invasive species. The present study aimed to assess the current distribution, densities, and population structures of T. granifera in invaded habitats on the Limpopo and Phongolo River systems in South Africa. This was accomplished by collecting aquatic molluscs from sites across these systems. Water quality parameters were measured at each site and water samples were collected for chemical nutrient analyses. The density of snails was determined for each site and the population size and structure as well as birth rate was calculated for T. granifera. Tarebia granifera was found to be the dominant molluscan species in habitats where it was present and all size classes from newborn to mature adults were found throughout at some of the highest densities globally. Worryingly, native molluscan species, were often absent or in much lower densities than reported in literature at sites where T. granifera was present, suggesting a negative effect on the native molluscan density and diversity. Contrary to most previous studies, there were no significant correlations between T. granifera and the selected water quality parameters. Higher densities and newborn recruitment of T. granifera were observed in the spring than in autumn, likely in response to shifts in environmental conditions. This study provides crucial insights into the population structure, densities, and impacts of T. granifera in invaded habitats, particularly for relatively newly invaded regions such as southern Africa.

Effects of river regulation on aquatic invertebrate community composition: A comparative analysis in two southern African rivers.

While natural floods play a crucial role in shaping the composition of aquatic communities, the most rivers worldwide are regulated or dammed for anthropogenic purposes, resulting in alterations to the biological and chemical composition of these ecosystems. Studies have demonstrated various negative effects of river regulation on aquatic invertebrate communities globally. However, there is a scarcity of research in Africa, despite its vulnerability to anthropogenic impacts. This study aimed to compare aquatic invertebrate communities in the Phongolo River, an impacted regulated river, and the Usuthu River, a less impacted unregulated river, in South Africa. It further aimed to ascertain whether Lake Nyamithi, a naturally saline lake receiving water from both of the aforementioned systems, exhibited a stronger similarity to one of the two rivers in terms of its aquatic invertebrate composition. Aquatic invertebrate and water samples were collected from 2012 to 2018 over several surveys. The Usuthu River demonstrated a diverse and sensitive aquatic invertebrate community, emphasising its high conservation value. The Phongolo River demonstrated effects of anthropogenic impact, with taxa more resilient to changes in water quality and flow compared to the Usuthu River. Mismanagement and excessive water use may lead to the loss of any remaining sensitive aquatic invertebrate communities in this river. The presence of invasive molluscan in the Phongolo River and Lake Nyamithi also poses a threat to the native aquatic invertebrate communities. These invasive species are currently absent from the Usuthu River although other invasive species, such as the Australian redclaw crayfish, are found in both river systems. Lake Nyamithi displayed a unique aquatic invertebrate community, distinct from both rivers and their floodplains. This study provides important baseline information on the Usuthu River's aquatic invertebrates and emphasises the need to maintain adequate water flow in rivers and floodplains to protect biodiversity and sensitive species.

Modelling the historical distribution of schistosomiasis-transmitting snails in South Africa using ecological niche models.

Schistosomiasis is a vector-borne disease transmitted by freshwater snails and is prevalent in rural areas with poor sanitation and no access to tap water. Three snail species are known to transmit schistosomiasis in South Africa (SA), namely Biomphalaria pfeifferi, Bulinus globosus and Bulinus africanus. In 2003, a predicted prevalence of 70% was reported in tropical climates in SA. Temperature and rainfall variability can alter schistosomiasis-transmitting snails' development by increasing or decreasing their abundance and geographical distribution. This study aimed to map the historical distribution of schistosomiasis from 1950 to 2006 in SA. The snail sampling data were obtained from the historical National Snail Freshwater Collection (NFSC). Bioclimatic variables were extracted using ERA 5 reanalysis data provided by the Copernicus Climate Change Service. In this study, we used 19 bioclimatic and four soil variables. The temporal aggregation was the mean climatological period pre-calculated over the 40-year reference period with a spatial resolution of 0.5° x 0.5°. Multicollinearity was reduced by calculating the Variance Inflation Factor Core (VIF), and highly correlated variables (> 0.85) were excluded. To obtain an "ensemble" and avoid the integration of weak models, we averaged predictions using the True Skill Statistical (TSS) method. Results showed that the ensemble model achieved the highest Area Under the Curve (AUC) scores (0.99). For B. africanus, precipitation-related variables contributed to determining the suitability for schistosomiasis. Temperature and precipitation-related variables influenced the distribution of B. globosus in all three models. Biomphalaria pfeifferi showed that Temperature Seasonality (bio4) contributed the most (47%) in all three models. According to the models, suitable areas for transmitting schistosomiasis were in the eastern regions of South Africa. Temperature and rainfall can impact the transmission and distribution of schistosomiasis in SA. The results will enable us to develop future projections for Schistosoma in SA based on climate scenarios.

A stable isotope analysis of the dietary patterns of the aquatic apex predator, the African tigerfish (Hydrocynus vittatus).

Stable isotope analyses, specifically δ13 C and δ15 N, are useful tools increasingly used to understand ecosystem function, food web structures, and consumer diets. Although the iconic tigerfish Hydrocynus vittatus is regarded as an apex predator in southern African freshwater systems, little information is available regarding their feeding behavior and how this may change with growth or differ between ecosystems, with most information stemming from stomach content analyses (SCA). The aim of the present study was to address this lack of information through a baseline study of the diet of large and small tigerfish in various lentic and lotic ecosystems in South Africa using stable isotope methods. Fish and various food web components and food sources were collected from two river and two lake ecosystems in South Africa. The δ13 C and δ15 N values for all samples were determined and multivariate analyses and Bayesian analytical techniques applied to determine the feeding ecology of H. vittatus and how this may differ with size and habitat type. Analyses revealed a substantial difference in the type and abundance of food sources contributing to the diet of H. vittatus between ecosystems, most prominently between the lotic systems, where less dietary specialization was observed, and lentic systems where more specialization was observed. Furthermore, there was a distinct difference in diet between small and large tigerfish, especially in the lotic system, indicating an ontogenetic diet shift as tigerfish grow and further supporting previous SCA studies. This is the first study of its kind on the African continent for H. vittatus and the findings illustrate the value of stable isotope analysis in providing in-depth information into the feeding ecology of consumers and how this may differ between size classes and habitat types.

Drought altered trophic dynamics of an important natural saline lake: A stable isotope approach.

Climate change and associated droughts threaten the ecology and resilience of natural saline lakes globally. There is a distinct lack of research regarding their ecological response to climatic events in the Global South. This region is predicted to experience climatic events such as El Niño-Southern Oscillation (ENSO) more often and with greater severity with the potential to alter the structure and functioning of aquatic ecosystems significantly. From 2015 to 2016 South Africa experienced one of the most severe country-wide droughts as a result of a strong ENSO event. Our study aimed to investigate the effect of this supra-seasonal drought on the trophic structure of fish communities in a naturally saline shallow lake of a Ramsar wetland using stable isotope techniques. Fishes and potential basal sources were collected from the lake, during predrought conditions in 2010 and after severe drought (recovery phase; 2017). The δ13C and δ15N values of food web elements were determined and analysed using Bayesian mixing models and Bayesian Laymen metrics to establish the proportional contribution of C3 and C4 basal sources to the fish (consumer) diets, and examine the fish community in terms of isotopic niche and trophic structure, respectively. Fish consumers relied predominantly on C3 basal sources in the predrought and shifted to greater reliance on C4 basal sources, decreased isotopic niche space use and a reduction in trophic length in the recovery phase. Drought altered the type and abundance of the basal sources available by limiting sources to those that are more drought-tolerant, reducing the trophic pathways of the food web with no significant alterations in the fish community. These results demonstrate the resilience and biological plasticity of Lake Nyamithi and its aquatic fauna, highlighting the importance of freshwater inflow to saline lakes with alterations thereof posing a significant threat to their continued functioning.