Browsing by Author "Atkinson, Lara"

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    Distribution of epifauna in offshore benthic environments along the west and south coast of South Africa
    (2018) Shah, Aliya; Atkinson, Lara; Sink, Kerry; Reed, Cecile
    Marine unconsolidated sediments, such as sand, gravel and muds, constitute the most extensive benthic ecosystems globally. Biological data for these ecosystems are frequently sparse which can hinder the success and implementation of marine management strategies for benthic ecosystems. There are limited studies in South Africa on benthic epifauna. This study investigates the composition and distribution of epibenthic invertebrate assemblages along the west and south coast of South Africa (sampled using depth-stratified demersal trawls) to inform marine environmental management. Sample depth varied from 36m to 899m. Multivariate tools (PRIMER and PERMANOVA+) were used to analyse spatial (west vs south coast) and temporal (2011 vs 2017) patterns in epifauna. This study also investigated an overlap region between the west and south coast. A group average linkage cluster analysis defined biotopes using significant branching (p< 0.05). Biotopes were compared against the 2012 National Biodiversity Assessment (NBA) benthic habitat map to investigate whether epifaunal biotopes identified, align with the existing classification. A significant difference among epifauna between region and depth was found, where the west coast had a higher average number of individuals and species per station. Sympagarus dimorphus and Pelagia noctiluca were characteristic species for west and south coast respectively. Epifauna was found to be significantly different between 2011 and 2017, with a notable increase in the abundance of Crossaster penicillatus in 2017. The majority of the biotopes aligned with the current NBA classification, in particular the Agulhas Sandy Shelf Edge ecosystem type on the south coast and South Atlantic Upper Bathyal and Namaqua Muddy Inner Shelf ecosystem types on the west coast. This thesis contributes to the mapping and description of offshore ecosystem types to inform marine environmental impact assessments, marine spatial planning and marine protected area expansion.
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    Marine ecosystem classification and conservation targets within the Agulhas ecoregion, South Africa
    (2022) Nefdt, Leila; Karenyi, Natasha; Griffiths, Charles; Sink, Kerry; Atkinson, Lara
    Deep-sea benthic ecosystems remain poorly studied in South Africa, limiting understanding of community biodiversity patterns and their environmental drivers. This is one of the first studies to (i) visually investigate marine epifaunal community patterns and their environmental drivers along the Agulhas ecoregion outer shelf, shelf edge and upper slope to support marine ecosystem classification and mapping, and (ii) to determine the conservation targets for selected national marine ecosystem types to inform improved management of the marine environment, through Marine Spatial Planning processes. Visual surveys of the seabed were conducted to quantify epifauna during the ACEP Deep Secrets Cruise in 2016, using a towed benthic camera system. Twenty-nine sites were sampled, ranging from 120-700 m in depth and spanning the shelf-slope transition from the western edge of the Agulhas Bank to offshore of the Kei River mouth. A total of 855 seabed images were processed, and 173 benthic taxa quantified. Corresponding environmental variables were used to determine potential drivers of observed biodiversity patterns. Data were analysed using multivariate analyses, including CLUSTER, MDS and DistLM, in PRIMER v6 with PERMANOVA. Ten different epifaunal communities were classified and described with key characteristic taxa identified. Communities found in habitats that comprised mostly hard rocky substrata generally exhibited higher in species richness and were most commonly characterized by stalked crinoids, various corals and bryozoans, whereas communities found in habitats comprising unconsolidated sediment were lower in species richness and commonly characterized by polychaetes, cerianthids and brittle stars. Communities found in habitats comprising both hard and soft substrata had a mix of the above-mentioned epifauna. The distribution of these communities was mostly influenced by substratum type, longitude, trawling intensity, depth, and presence of visible particulate organic matter. The combined interactions of topography, substratum and the unique hydrodynamic conditions along the Agulhas ecoregion shelf-slope transition are likely responsible for the observed patterns. The observed community patterns were also compared to the existing classification of marine ecosystem types from the 2018 National Biodiversity Assessment. Fine-scale heterogeneity was revealed within the examined marine ecosystem types, particularly with substratum type and associated community variability and should be recognized and incorporated into future iterations of the national marine ecosystem classification and map. Species-area curves were used to calculate conservation targets for three ecosystem types, defined by the 2018 National Biodiversity Assessment, namely the Agulhas Coarse Sediment Shelf Edge, South West Indian Upper Slope, and the Agulhas Rocky Shelf Edge. Considering the epifaunal species richness (using the bootstrap estimator) and area, per image and per ecosystem type, the rate of accumulation of species was calculated and used to estimate the percentage of species expected to be represented by any given percentage of protected ecosystem type area. Between 20 and 30% of the area within these ecosystem types will need to be protected to represent 80% of the species. This study has shown that an integration of environmental parameters together with biodiversity measures to better understand and classify offshore benthic ecosystems has worked well. However, to improve the resolution of the national marine ecosystem classification and map, there needs to be greater input of fine-scale biological and environmental sampling and mapping of substratum types across the Agulhas ecoregion shelf-slope transition zone. This work is contributing to improvements in the national marine ecosystem classification and map and hence the spatial assessment and planning processes that rely on these products.
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    The influence of sampling method on detecting benthic biodiversity patterns at the ecoregion scale on the South African west coast
    (2022) Brandt, Silke; Karenyi, Natasha; Atkinson, Lara
    Long-term monitoring of marine benthic communities provides data which are essential for effective ocean management. However, long-term monitoring is limited by the difficulty and expense of sampling deep seafloor areas comprehensively enough to represent the whole benthic community. This has led to the development of a wide array of seafloor sampling methods. Consequently, the integration and prioritisation of data collected using different methods remains an area of concern. Demersal research trawling and grab sampling are two methods employed to sample the marine benthos in South Africa, targeting different habitats and fauna, at different scales and with different sampling efficiencies. Both datasets inform the national marine ecosystem classification, yet the consistency between biodiversity patterns detected by each sampling method has not yet been explored. The aim of this thesis is to determine the influence of sampling method on the detection of benthic biodiversity patterns. This was explored using demersal trawl and grab datasets collected from twenty-four pairs of stations within the Southern Benguela Shelf ecoregion on the west coast of South Africa (70 – 600 m) between the years 2009 and 2020. Differences in benthic structure, univariate diversity indices, and multivariate assemblage structure were compared between demersal trawl and grab datasets using both taxonomic and trait-based approaches. This study utilised the rarely applied co-correspondence analysis (CoCA) to test for congruency in multivariate assemblage patterns sampled by trawls and grabs. Furthermore, the use of Biological Traits Analysis (BTA) allowed for the assessment of functional diversity patterns which is often a missing link when measuring biodiversity relative to ecosystem functioning. The epifaunal community was dominated by Asteroidea, Decapoda and Gastropoda, whereas Polychaeta, Amphipoda and Bivalvia dominated infaunal communities. BTA found trawl samples to be dominated by epifauna exhibiting large sizes, dorso-ventrally flattened body forms, free-living and surface-crawling life habits, moderate mobility, predatory feeding, planktotrophic larval development and medium-long lifespans. Grab samples were dominated by infauna with small sizes, vermiform or laterally flattened body forms, low mobility, surface deposit feeding strategies, planktotrophic larval development and short lifespans. Demersal trawl and grab sampling detected significantly similar patterns in species abundance, species richness, species diversity, and functional richness values across the west coast. Species evenness, functional evenness, functional diversity, and functional redundancy gave no evidence of a significant relationship between the two sampling methods. CoCA found infaunal assemblage patterns to be highly correlated with epifaunal assemblage patterns across the study area using both taxonomic and trait-based approaches. Environmental and spatial gradients, including depth, longitude, and sediment characteristics, played key roles in structuring broad scale biodiversity patterns. The results of this thesis have implications for how biological datasets from trawl and grab surveys should be prioritised or weighted at different ecological scales when incorporated in the South African marine ecosystem classification and mapping process. This is the first step in transitioning the current ecosystem classification from a data informed, expert driven approach to an expert informed, data driven approach through the use of quantitative multivariate statistical techniques. Furthermore, multi-method biodiversity studies are crucial to represent the entire benthic community and understanding the extent to which the choice of sampling method affects the biodiversity patterns detected is an integral component of accurate ecosystem delineation. The findings from this thesis can be applied to future assessments of South Africa's marine ecosystem classification, increasing its accuracy, and therefore contributing to improved ecosystem-based sea-use management.