A mechanistic and trait-based approach to investigating macroinvertebrates distribution and exposure to microplastics in riverine systems
Authors:   , Owowenu Enahoro Kennedy 
Published: December 2024

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.