UWCScholar :: Browsing by Author "Rajkaran, Anusha"

Browsing by Author "Rajkaran, Anusha"

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Biomass partitioning in an endemic southern African salt marsh species Salicornia tegetaria (Chenopodiaceae)
(Taylor and Francis Group, 2020) Brown, Catherine; Rajkaran, Anusha
The below ground biomass of salt marsh species accounts for more than half of the total plant biomass in salt marsh systems, yet no biomass data exist for salt marsh species in South Africa. The aims of the current study were to compare the biomass allocation of Salicornia tegetaria in six estuaries and relate findings to their environmental conditions. The current study measured the physico-chemical variables of the sediment (moisture content, organic matter content, electrical conductivity, pH) and pore water (temperature, salinity, pH, depth) at Olifants, Berg, Langebaan, Heuningnes, Nahoon and Kwelerha estuaries. Above and below ground biomass and stem height measurements were also collected. The below ground biomass (1.51 ± 0.24 kg m−2) and root/shoot ratio (1.36 ± 0.17) was the lowest at Heuningnes Estuary with no significant difference at the remaining estuaries, suggesting that factors, such as geomorphology and sedimentological processes, could have a stronger effect on the biomass allocation in this species. Important drivers of biomass allocation were sediment pH, redox potential and pore water depth. The current study provides baseline information for S. tegetaria, an endemic salt marsh species, for which there is a paucity of data. This species plays a major role in the ecology of the lower intertidal zone, which will be vulnerable to sea-level rise.
Changes in mangroves at their southernmost African distribution limit
(Elsevier, 2020) Rajkaran, Anusha; Adams, Janine B.
Mangroves in South Africa occur at one of the most southerly locations in the world, which provides a unique opportunity to study their dynamic responses to anthropogenic and natural perturbations. The exposed high-energy South African coastline restricts mangroves to 32 sheltered estuaries of which 18 (56%) are predominantly open to the sea. A large area of mangrove (47% of the country total) occurs in the uMhlathuze Estuary – a novel ecosystem formed by the creation of an artificial mouth. A Drivers-Pressures-State-Impacts-Response (DPSIR) framework was applied to understand factors of change and highlight governance and management responses. The largest mangrove area (440 ha) was lost during the construction of Durban Bay harbour. Mangroves (~7 ha) no longer occur in 10 small KwaZulu-Natal estuaries as a result of catchment and mouth disturbance. In the Eastern Cape, pressures are escalating in the form of harvesting for wood, cattle browsing and changes in mouth condition. Climate related warming and an increase in CO2 are positive conditions for mangroves to expand their distribution to higher latitudes but this will depend on propagule dispersal between estuaries and the availability of suitable habitats.
Dispersal and coastal geomorphology limit potential for mangrove range expansion under climate change
(Wiley, 2022) Raw, Jacqueline L.; Van der Stocken, Tom; Rajkaran, Anusha
Latitudinal range limits for mangroves on high-energy, wave-dominated coasts are controlled by geomorphological features and estuarine dynamics. Mangroves reach a southern global range limit along the South African coastline, but the distribution is patchy, with stands occurring in only 16% of the estuaries in the region. Yet, the persistence of forests planted >50 years ago beyond the natural distribution limit suggests that additional estuaries could support mangroves. Understanding regional drivers is necessary to inform global-scale estimates for how this important ecosystem is predicted to respond to climate change.
An estuary ecosystem classification that encompasses biogeography and a high diversity of types in support of protection and management
(Taylor & Francis, 2020) Rajkaran, Anusha; van Niekerk, Lara; Adams, Janine B.
For nearly three decades, the Whitfield (1992) characterisation scheme served as a reference framework to type South African estuaries. We outline a revised ecosystem classification scheme that incorporates biogeographical zonation and introduces new types. Coastal outlets were re-categorised as estuaries or micro-systems. For functional estuaries, the Estuarine Lakes, Estuarine Bays and Predominantly Open Estuary types were largely retained. New types are Estuarine Lagoons and Arid Predominantly Closed Estuaries. The numerically dominant, temporarily open/closed category was subdivided into Small and Large Temporarily Closed Estuaries, with a total habitat area of 15 ha, serving as threshold separating these two subdivisions. River mouths were renamed Fluvially Dominated Estuaries and divided into large and small size categories to reflect dissimilar catchment influences. Micro-systems were separated into micro-estuaries, micro-outlets, and waterfalls. South Africa’s 290 estuaries were classified into 22 estuarine ecosystem categories arising from nine estuary types occurring across four biogeographical zones.
Food resources available to juvenile fish (particularly rhabdosargus holubi) in seagrass beds of the swartkops etuary
(University of the Western Cape, 2023) Brown, Kylen Leigh; Rajkaran, Anusha
Estuarine habitats are critical nursery areas for many species of marine fishes during their early life stages in terms of food availability and structural complexity, which increases survival and growth rates. The Cape stumpnose Rhabdosargus holubi is one of the most abundant marine fish species found in estuaries along the south-east coast and depends on estuaries as exclusive nursery habitats. This study aimed to identify the epiphytic diatom and macrofauna food resources available to R. holubi in the seagrass Zostera capensis habitat of the Swartkops Estuary. This habitat has been identified as a core nursery area for this species through a combination of greater abundance, higher growth and survival of juveniles. This study compared what was found in the seagrass habitat and assessed whether this habitat provided for the prey resources most commonly observed in the diet of R. holub.
Global typologies of coastal wetland status to inform conservation and management
(Ecological Indicators, 2021) Rajkaran, Anusha; Sievers, Michael; Brown, Christopher J.
Global-scale conservation initiatives and policy instruments rely on ecosystem indicators to track progress towards targets and objectives. A deeper understanding of indicator interrelationships would benefit these efforts and help characterize ecosystem status. We study interrelationships among 34 indicators for mangroves, saltmarsh, and seagrass ecosystems, and develop data-driven, spatially explicit typologies of coastal wetland status at a global scale. After accounting for environmental covariates and gap-filling missing data, we obtained two levels of clustering at 5 and 18 typologies, providing outputs at different scales for different end users. We generated 2,845 cells (1� (lat) � 1� (long)) globally, of which 29.7% were characterized by high land- and marine-based impacts and a high proportion of threatened species, 13.5% by high climate-based impacts, and 9.6% were refuges with lower impacts, high fish density and a low proportion of threatened species. We identify instances where specific actions could have positive outcomes for coastal wetlands across regions facing similar issues. For example, land- and marine-based threats to coastal wetlands were associated with ecological structure and function indicators, suggesting that reducing these threats may reduce habitat degradation and threats to species persistence. However, several interdimensional relationships might be affected by temporal or spatial mismatches in data. Weak relationships mean that global biodiversity maps that categorize areas by single indicators (such as threats or trends in habitat size) may not be representative of changes in other indicators (e.g., ecosystem function). By simplifying the complex global mosaic of coastal wetland status and identifying regions with similar issues that could benefit from knowledge exchange across national boundaries, we help set the scene for globally and regionally coordinated conservation.
Impacts of plastic debris on biota and implications for human health: A South African perspective
(South African Assn. For The Advancement Of Science, 2020) Naidoo, Trishan; Rajkaran, Anusha; Sershen, Naidoo
Entanglement and ingestion of plastics are the main ecological impacts of marine plastic debris on marine biota, but indirect effects such as the transport of alien species and benthic smothering are also important to note. Entanglement of invertebrates, sharks, turtles, birds and marine mammals is mainly caused by macroplastics (>5 mm), and leads to reduced mobility, ineffective foraging and subsequent mortality. The main plastic types associated with entanglement are improperly discarded fishing nets, lines, ropes and straps.
Investigating phytoplankton fluctuations and drum filter effectiveness on an abalone farm in Hermanus, South Africa.
(University of the Western Cape, 2021) Ponton, Timothy John; Rajkaran, Anusha
Aquaculture is a growing industry in South Africa, with the production of abalone, Haliotis midae, at the forefront. The Western Cape Province hosts 12 of the 18 abalone farms in South Africa, with a concentration of farms in the Walker Bay region of Hermanus and Gansbaai. Walker Bay is situated on the western side of the Agulhas bank, which allows warm water from the Agulhas current, as well as cold water from the Benguela current to mix. This area experiences seasonal upwelling pulses in summer, which provide the environment with a high nutrient load. This encourages the prevalence of harmful algal blooms (HAB) that can consist of toxin-producing dinoflagellate species. These species have the ability to kill organisms in the nearshore. This poses a problem for aquaculture farms situated in the area, where HAB events have caused the death of millions of abalone and has decreased productivity of farms in previous years. Farms therefore need to implement stringent phytoplankton monitoring schedules, as well as develop better filtering methods to reduce the density of phytoplankton that may flow into the farm. This study aimed to understand the phytoplankton community assemblages that may be pumped into an abalone farm (Abagold Ltd) over a 16 month period. This was achieved by investigating how phytoplankton community metrics such as abundance, species diversity, richness and evenness fluctuated over a 16 month time period. The frequency of HABs were investigated, comparing the peaks of blooms and how they differed between seasons and the subsequent impact on monitoring activities by the farm until the bloom passes. Secondly, a study was done to determine the efficiency of drum filters to reduce the density of phytoplankton cells from the water that is sourced from the ocean and pumped through the farm. Phytoplankton community assemblages were sampled and identified to genus level, and species level when possible, once a day for 16 months, from September 2018 to December 2019. As the risk of potential HAB formation rises, the number of sample collections increased to assist in the decision making process of the operational manager of the abalone farm to mitigate negative impacts originating from HAB events. The species richness, Shannon-Weiner diversity index and Pielou’s index of evenness were calculated. The number of phytoplankton samples collected each day were tallied to understand the change in monitoring frequency with regard to HAB abundance. Phytoplankton community samples were collected before and after a 15 μm drum filter during bloom events, after which the densities were then compared. The peak mean monthly cell density occurred in late early autumn of 2019 (March: 721 179 ± 226 473 cells/l). During this time, the diversity (Shannon-Weiner Index) of species was lower than that of mid spring and this trend is supported by literature, where a decrease in diversity occurs with an increase in HAB density. The relative abundance of species was calculated to quantify the dominant species present
Opportunities for improving recognition of coastal wetlands in global ecosystem assessment frameworks
(Elsevier, 2021) Brown, Christopher J.; Adame, Maria F.; Rajkaran, Anusha
Vegetated coastal wetlands, including seagrass, saltmarsh and mangroves, are threatened globally, yet the need to avert these losses is poorly recognized in international policy, such as in the Convention on Biological Di-versity and the United Nations (UN) Sustainable Development Goals. Identifying the impact of overlooking coastal wetlands in ecosystem assessment frameworks could help prioritize research efforts to fill these gaps. Here, we examine gaps in the recognition of coastal wetlands in globally applicable ecosystem assessments. We address both shortfalls in assessment frameworks when it comes to assessing wetlands, and gaps in data that limit widespread application of assessments. We examine five assessment frameworks that track fisheries, greenhouse gas emissions, ecosystem threats, and ecosystem services. We found that these assessments inform management decisions, but that the functions provided by coastal wetlands are incompletely represented. Most frameworks had sufficient complexity to measure wetland status, but limitations in data meant they were incompletely informed about wetland functions and services. Incomplete representation of coastal wetlands may lead to them being overlooked by research and management. Improving the coverage of coastal wetlands in ecosystem as-sessments requires improving global scale mapping of wetland trends, developing global-scale indicators of wetland function and synthesis to quantitatively link animal population dynamics to wetland trends. Filling these gaps will help ensure coastal wetland conservation is properly informed to manage them for the outstanding benefits they bring humanity.
Predicting Shifts in the geographical distribution of two estuarine plant species from the subtropical and temperate regions of South Africa
(Springer, 2019-11-21) Veldkornet, Dimitri Allastair; Rajkaran, Anusha
Climate suitability maps are useful to determine changes in the distribution of species. The aim of this study was to predict the future distribution of two estuarine species (Bassia diffusa and Hibiscus tiliaceus) from two biogeographical regions under climate change. The Bioclim and Maxent modelling algorithms were used to calculate current climate suitability and were projected onto future climate data to develop climate suitability maps. The trained area under a receiver operating characteristic curve (AUC) values ranged from 0.723 to 0.936 for both species under the current climatic conditions, and from 0.695 to 0.933 for future conditions. Distribution range contraction for Bassia diffusa is predicted along its eastern distribution limit (South-East coast) in South Africa, suggesting extirpation in 42 out of 62 estuaries. This may allow mangroves to expand into areas where B. diffusa will be lost. Low suitability was predicted for H. tiliaceus in warm temperate estuaries which may limit further expansion. Future climate change scenarios in this region predict that temperature will not increase above the minimum temperature tolerance of H. tiliaceus. This approach can be applied to determine the potential range shifts of other estuarine species, supporting the development of proactive strategies to reduce climate change impacts on biodiversity.
A tale of two habitats: preliminary comparison of fish abundance and diversity between saltmarsh- and mangrove-dominated creeks in the Nahoon Estuary, South Africa
(University of the Western Cape, 2019) Rajkaran, Anusha; Keur, R
A preliminary assessment of abundance and richness of fishes utilising saltmarsh and mangrove habitats in the Nahoon Estuary, situated on the eastern coast of South Africa, was undertaken during July 2017. The structural composition and complexity of the two habitats were assessed and underwater cameras were used to provide preliminary insight into the use of the different habitats by fish taxa. The saltmarsh habitat had a higher fractal dimension and density of stems than the mangrove habitat. The species richness of fishes was slightly higher in the mangroves than in the saltmarsh, with nine and seven taxa recorded in the two habitats, respectively. The mean relative abundance of fishes was higher in the saltmarsh than in the mangroves and this was mainly due to the dominance of shoaling estuarine zooplanktivores in the saltmarsh. The mean relative abundance of estuary- associated marine fish species was, however, higher in the mangroves, which may be indicative of greater nursery use of this habitat by marine fishes. Although this is the first study to simultaneously assess the role of both saltmarsh and mangroves as fish habitat in estuaries of temperate South Africa, the findings are preliminary, and further study on seasonal differences in species assemblages is recommended.
The phylogeography, biomass allocation and phenology of Salicornia tegetaria (S. Steffen, Mucina & G. Kadereit) Piirainen & G. Kadereit, an endemic salt marsh species in South Africa
(University of the Western Cape, 2018) Brown, Catherine Eileen; Rajkaran, Anusha
Salicornia tegetaria is an endemic salt marsh macrophyte that is widely distributed in estuaries along the South African coast. The aims of the study were to understand the phylogeography of the species, compare the biomass allocation in two regions and to determine phenological patterns of S. tegetaria between the warm and cool temperate biogeographical regions. The phylogeography of S. tegetaria was studied using the noncoding chloroplast DNA region rpS16 and nuclear rDNA ITS region. Five samples each were collected from eighteen estuaries stretching from Orange River in the Northern Cape to Mngazana Estuary in the Eastern Cape. Above- and belowground biomass was collected and physico-chemical conditions measured at Olifants, Berg and Langebaan Estuaries in the cool temperate, and Heuningnes, Nahoon and Kwelera Estuaries in the warm temperate biogeographical regions. The growth and flowering phenology of S. tegetaria in relation to environmental conditions was investigated in the cool temperate Langebaan Estuarine Embayment and compared to findings in the warm temperate, permanently open Kowie Estuary. The physico-chemical gradient found between the cool and warm temperate biogeographical regions may be useful to study climate change effects on plant species. The comparison of similar habitats in each region may provide insight into how different climate regimes may affect biomass allocation and phenology.
Towards characterising microplastic abundance, typology and retention in mangrove-dominated estuaries
(MPDI, 2020) Naidoo, Trishan; Rajkaran, Anusha; Govender, Joelene
Plastic and, particularly, microplastic (MP) pollution is a growing research theme, dedicated largely to marine systems. Occurring at the land–sea interface, estuarine habitats such as mangroves are at risk of plastic pollution. This study compared MP pollution (level, morphotype, polymer composition, size and colour) across four South African estuaries, in relation to the built and natural environment. Mouth status, surrounding human population densities and land-use practices influenced the level and type of MP pollution. Systems that were most at risk were predominantly open estuaries surrounded by high population densities and diverse land use types. Microplastic levels and the diversity of types detected increased with increasing levels of anthropogenic disturbance. Overall, microfibres dominated in estuarine water (69%) and mangrove sediment (51%). Polyethylene (43%) and polypropylene (23%) were the dominant polymers overall. Weathered fishing gear, weathered packaging items and run-off from urban/industrial centres are probable sources of MP pollution.
Understanding mangrove dynamics in range edge populations in South Africa
(University of the Western Cape, 2023) ZIde, Ayanda; Rajkaran, Anusha
Mangrove distribution along the East African region occurs from Somalia to South Africa, including Madagascar and other islands. It comprises approximately 10 species, including Avicennia marina (Forsk.) Vierh, Bruguiera gymnorrhiza (L.) Lam. and Rhizophora mucronata Lam. that are core mangrove species and common in the region. Within the Indo-West Pacific (IWP) Province, studies have determined the genetic variation of mangrove species, but only a limited number of studies have included B. gymnorrhiza and R. mucronata populations occurring in the East African region, while none have included those occurring in South Africa. Genetic variation and environmental conditions affect plant performance and the longevity of mangrove populations.
Understanding the creek dynamics and environmental characteristics that determine the distribution of mangrove and salt marsh communities at Nahoon Estuary
(Elsevier, 2016) Geldenhuys, Chanel; Cotiyane, Phumlile; Rajkaran, Anusha
The southern distributional limit for mangroves on the east coast of Africa is thought to be at the planted mangrove forest at Nahoon Estuary (33° S) in the Eastern Cape, South Africa. This study investigated the influence of a tidal creek on the intertidal zone and the physical and biological differences between the salt marsh and mangrove forest communities at Nahoon Estuary. Three transects were established across the tidal creek and one transect in each of the following habitats mangrove, mangrove–salt marsh, and the salt marsh area. The tidal creek introduced oxygenated (~6 mg.l−1) and saline water with high levels of total suspended solids (120–424 g.l−1) into the intertidal zone. In areas where tidal water was retained, algal mats formed over pneumatophores during summer. The vegetation distribution in the mangrove–salt marsh community was significantly affected by elevation, ammonium concentration, and porewater temperature while the salt marsh vegetation distribution was influenced by porewater salinity, sediment, pH and the percentage of sand content. Porewater nitrogen was mostly present as ammonium, and phosphate concentrations were moderate ranging from 1.3 μM in the salt marsh to 3.7 μM in the mangrove community. Mangrove and salt marsh communities are clearly constrained by the physical characteristics of the intertidal area (elevation) and this will ensure that both communities will be maintained at Nahoon Estuary. However with climate change and sea level rise, this may change in the long term with mangroves expanding into elevated areas.

Browsing by Author "Rajkaran, Anusha"

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