Abstract
Microplastic pollution has emerged as a pervasive environmental issue, affecting diverse
ecosystems and organisms globally. Despite increasing recognition of its widespread presence, the
impacts of microplastics on freshwater ecosystems remain inadequately understood. This study
aimed to explore the spatial and temporal dynamics of microplastics in two streams (Maretlwane
and Sterkstroom), which together constitute the Gwathle catchment draining the Magaliesberg
mountains in the North West province, South Africa. Microplastic concentrations were quantified
in sediment, water and club-tailed dragonfly larvae (Odonata, Gomphidae), which represent a
typical and abundant secondary consumer organism in the sediment biotope. Thirteen sites were
sampled across the two streams during both the wet summer and dry winter seasons (March and
September 2022, respectively). Sites were sampled in the pristine headwaters of each stream (i.e.
nature reserve land) as well as from a variety of landscapes that the streams encounter downstream,
which included agricultural, mining and urban-impacted landscapes. In addition to the comparison
of microplastic concentrations among different land uses, various hydrological parameters were
measured at each site in order to relate these to microplastic concentrations. Microplastic
abundance across the sediment, water and biotic samples was quantified under a stereomicroscope
using a hot needle test and characterisation of the polymer composition of the dominant particle
types was performed using Raman spectroscopy. Building on previous research on microplastic
ingestion by primary consumers in the same streams, this study examines whether there is trophic
transfer of microplastics to secondary consumers. Laboratory analyses revealed sediment
microplastic concentrations averaging 72.82 particles/kg and water concentrations at 0.08
particles/L during the dry season, both higher than in the wet season. Dragonfly larvae exhibited
higher mean concentrations in the wet season, averaging 0.79 particles/g. Microplastic abundance
in sediment and water samples (but not dragonfly larvae) was significantly elevated at sites outside
nature reserves compared to those inside nature reserves. The effects of individual land uses
(urban, agriculture and mining) on stream microplastic abundance were not significant and showed
mixed trends, with microplastic abundance being highest in urban landscapes for sediment and
water, but for dragonfly larvae microplastics were highest in agricultural areas. Whilst positive
linear trends between microplastic abundance and hydrological variables (stream velocity, depth
and discharge) were detected, these were not significant. A clear trend for both streams was a
strong positive relationship between water and sediment microplastic abundance and distance
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downstream, indicating an accumulation of microplastics down the length of the Gwathle
catchment. This is the first study in a southern African context to provide evidence indicating the
trophic transfer of microplastics in stream ecosystems, highlighting their pervasive nature and
stressing the need for further research on their effects on ecosystem health and function.
Additionally, it underscores the importance of further research investigating land-use impacts and
in-stream dynamics on microplastic abundances.