A mechanistic and trait-based approach to investigating macroinvertebrates distribution and exposure to microplastics in riverine systems
- Authors: Owowenu, Enahoro Kennedy
- Date: 2024-10-11
- Subjects: Microplastics Environmental aspects , Water quality biological assessment , Hydrodynamics , Hydrogeomorphology , Biotope , Flow type
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466666 , vital:76765 , DOI https://doi.org/10.21504/10962/466666
- Description: Microplastics in rivers pose an ecological risk. Hydraulic biotopes form distinct flow patches that vary longitudinally along the river, potentially influencing the transport dynamics of microplastics. Macroinvertebrates exhibit adaptations to different hydraulic biotopes through their unique traits. These traits can mediate their exposure to microplastics, thereby imposing selective pressures on these organisms. Different taxa often demonstrate preferences for specific hydraulic biotopes characterized by distinct flow regimes. Understanding the transport dynamics of microplastics mediated by hydraulic biotopes and the potential exposure of macroinvertebrates at the hydraulic biotope scale is important for determining the fate of riverine microplastics and detecting species at risk. Both empirical and theoretical studies have highlighted the interconnectedness of hydrology, geomorphology, and microplastic transport in rivers, yet, there remains a gap in understanding how a hydro-geomorphological approach could enhance the understanding of the microplastic transport process. Little is known about the role of traits in driving macroinvertebrate exposure to microplastics at a scale relevant to ecological dynamics. This study addressed these gaps by applying a hydro-geomorphological approach to investigate the distribution of microplastics at the hydraulic biotope scale and assessed the potential exposure of macroinvertebrates using a trait-based approach. This study also explored the relationship between microplastic abundance and selected water physicochemical properties, as well as the influence of adjacent land use types. By integrating these aspects the research provided a comprehensive understanding of microplastics dynamics in river systems, shedding light on both environmental factors shaping their distribution and the potential impacts on aquatic organisms. The study was conducted over the wet and dry seasons (October 2021 – July 2022) at 10 sites located in the upper, middle, and lower reaches of the Swartkops and Buffalo River systems in the Eastern Cape Province of South Africa. The hydraulic biotopes (i.e., pools, runs, riffles) were grouped into two conceptualised forms, namely, sink and flush hydraulic zones and were characterized by hydraulic indices such as the Froude number and the Reynolds number. The flush hydraulic zone represents hydraulic biotopes where microplastics can potentially be remobilized quickly into suspension, and the sink represents biotopes where microplastics can potentially accumulate and remobilisation is far slower. Fast-to-moderate flowing hydraulic biotopes were conceptualised as microplastics flush zones while slow-flowing to still biotopes as microplastic sink zones. Samples were collected at different depths in each hydraulic zone to quantify suspended and settled forms of microplastics. Microplastics targeted in this study ranged in size from 0.063 mm to less than 5 mm. Classification was achieved through microscopic observation, and confirmation via Fourier Transform Infrared Spectroscopy (FTIR-ATR) was conducted for samples ranging from 0.5 mm to less than 5 mm. At the site level, settled microplastics showed statistically significant spatial and temporal variations between the sites, and between the seasons (P < 0.05). The suspended microplastic varied only spatially. Fibres and fragments were the dominant microplastic shape, while polyethylene and polypropylene were the dominant microplastic polymers. Suspended microplastics showed statistically significant variation between urban land cover and other land cover categories (industrial, agricultural, rural, and natural land cover). Microplastics abundance was associated with high levels of turbidity, total suspended solids, total inorganic nitrogen, higher temperatures and increasing electrical conductivity. At the hydraulic biotope scale, the mean occurrence of suspended microplastics (1.76 ± 1.44 items/L; mean + SD) in the flush hydraulic zone was higher than that in the sink zone (1.54 ± 1.46 items/L), while settled microplastics were more abundant in the sink hydraulic zone (1.82 ± 1.98 items/L) than the flush hydraulic zone (1.32 ± 1.49 items/L). This observation was in line with the prediction in this study. The mean suspended and settled microplastics concentrations were higher during the wet season across the flush and sink hydraulic zones than in the dry season. Global multivariate analysis of variance (MANOVA) and two-way analysis of variance (ANOVA) revealed significant spatial and temporal variations in settled microplastics abundances between the flush and sink hydraulic zones. The results indicated that geomorphologically defined units such as riffles and moderate to fast runs (flush) generally contained lower amounts of settled microplastics compared to pools and backwaters (sink). However, this distinction between the flush and sink microplastic zones was observed only for settled microplastics and not for suspended microplastics. Suspended and settled microplastics showed a statistically significant relationship with the Froude number index. The generalised additive model indicated that settled microplastics abundance distribution decreased significantly with increasing Froude number value in the flush zone. Suspended microplastics decreased at low Froude number values and showed an increasing trend at higher Froude number values of about 0.75. The results indicate the usefulness of the hydraulic biotope scale microplastic monitoring approach in detecting microplastic hotspots and explaining variations in microplastics abundances driven by instream hydraulics. Four traits and ecological preferences of macroinvertebrates including body size, gill type, feeding habit, and velocity preferences were selected and resolved into 17 trait attributes. The sink hydraulic zones such as pools were indicated to favour exposure to and ingestion of microplastics compared to the flush zones such as riffles and fast runs. Large body size macroinvertebrates were associated with the sink zone. Taxa with a very small body size had a higher likelihood for microplastics ingestion than taxa with other body sizes. Collectorgathering macroinvertebrates taxa that have operculate gills with small body sizes were more prone to exposure to microplastics in hydraulic biotopes with slow to very slow velocities. Fibres were the most abundant plastic ingested by macroinvertebrates preferring the flush zone while fibres and fragments were mostly ingested by those preferring the sink zones. The binomial logistic model revealed a highly significant result for the likelihood of operculate gill shape to clog in the sink hydraulic zone. The result of the binomial logistic regression indicates the usefulness of the trait-based approach for predicting exposure to microplastics. Overall, the study reveals the influences of hydro-geomorphological features on the transport dynamics of microplastics and the usefulness of the trait-based approach in the ecological study of microplastics in riverine systems. , Thesis (PhD) -- Faculty of Science, Institute for Water Research, 2024
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- Date Issued: 2024-10-11
Grain size analysis, coastal hydrodynamics and erosion protection: a case study from Knysna and Plettenberg Bay, South Africa
- Authors: Pezisa, Ayabulela Raymond
- Date: 2022-12
- Subjects: Grain -- Analysis , Hydrodynamics , Shore protection
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/27698 , vital:69394
- Description: The modern beach sands and Cretaceous Knysna Formation distributed along the coast of Plettenberg Bay and Knysna coast in the Western Cape Province of South Africa, have been examined and studied in the field outcrops and laborataries via grain-size analysis, XRD, SEM, EDX and microcope petrography analyses. This study looked into the coastal hydrodynamics and environment protection. The project aims to investigate the sediment distribution, grain-size variation, sedimentary structures, coast erosion and mitigation in the Plettenberg and Knysna coast to address environmental issues in the south coast of South Africa. The research will provide new insight onto coastal sedimentation, hydrodynamic condition, coastline erosion and the safety of the coastal environment and human property. The study will promote government attention on the sea level change, which caused flooding and environmental disaster along the south coastal area of South Africa. The rock sequence in the inland side of the study areas belongs to Cretaceous Knysna Formation, which comprises seven upward fining sequences. The stratigraphic sequence is underlain by the Table Mountain quartzite of the Cape Supergroup, and is overlain by Tertiary sediments and modern coastal dune sands. The Knysna Formation at the research area consists of massive conglomerate, sandstone and minor mudstone of mainly fluvial dominated sediments. The grain size analysis reveals that the modern fluvial channels at Plettenberg Bay and Knysna areas are of dominant coarse sands with minor silt and mud, which defines the sediments were deposited by moderate to high energy currents. Whilst the beach zones in Plettenberg Bay are predominated by fine to medium-grained marine sands. Grain-size analyses of beach sands show well-sorted, fine to coarse skewed in grain size distribution, indicating a relative lower to medium uniform energy condition during transportation and deposition. The bivariate plots of grain-size distribution demonstrate of the shallow agitated marine environment with the influence of tide and aeolian processes. Hydrodynamic condition in the beach area was more persistant and less variation compared to the river environment. The mineralogy and petrology studies revealed that in Plettenberg Bay and Knysna sediments are predominantly consisted of minerals quartz, feldspar, calcite, muscovite, aragonite, clay minerals, and salts (halite). Skeletal carbonate minerals (shell and coral fragments) are more than chemical precipitated carbonate minerals. The microtextures detected on the surface of the fluvial and marine sand grais involve V-shaped pits, upturn pits, dissolution pits and secondary mineral precipitation that were created by chemical and mechanical processes formed via sea-water dissolution, corrosion, and transport crashing. Whereas the boring holes and burrows created by activity of microorganisms boing into the surface of the grains. These microtextures of the river and beach sands exhibit a shallow marine and fluvial environments with medium to high energy conditions and active organic activities. Several sedimentary structures were detected in the coastal environments, including various types of ripple marks and dunes, burst bubble-hole, swash line, rill marks, rhomboid marks, burrows, boring and bioturbation, planar lamination and gravel pavement. In addition, sedimentary structures were also identified in the Cretaceous Knysna Formation such as air/water escape hole, convolute bedding, lenticular bedding, tabular cross-bedding and load cast. The sedimetnary structures closely linked with hydrodynamic conditions and therefore can be used as indicators for depositional environments. Flooding and erosion had become a coastal disaster that results in sediment redistribution throughout the coastal system and therefore caused landscape reform like coastal cliffs and sharpened dunes in erosive areas. Particularly, coastal hazards become more and more serious in recent years due to climate and sea leavel changes. Thus, to recognise coastal erosion and disaster and make a management strategy is of significant importance to compete against coastline retreat and to protect infrastructure and human safety in the coast area. The author had proposed a number of mitigation methods for environmental protection and for combating coastal erosion, including breakwaters, groins, jetties, vertical walls, rock armour, vegetation, boundary hardening, and revetment etc, which are the effective ways for protection of coast retreat, property damage and human safety. , Thesis (MSci) -- Faculty of Science and Agriculture, 2022
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- Date Issued: 2022-12