The occurrence and distribution of microplastics in surficial sediments and fish in a South African reservoir

Abstract

Microplastics have become a major environmental concern globally due to their potential impacts on ecosystem function. They are known to be ubiquitously present, persistent and bio−accumulative, yet ecological impacts of microplastic remain poorly understood despite their ubiquity across all habitat types globally. The combined effects of seasonality and human population density on the extent of microplastics pollution are not well understood. To understand microplastics along human population gradient, I assessed sediment microplastics along a tropical reservoir shoreline across three seasons and seven sites. Multivariable analysis was used to assess relationships among substrate embeddedness, sediment organic matter, human population density, microplastics particle densities and microplastics characteristics. Subsequently, the functional response approach was developed and applied for quantifications of microplastics uptake by the fish across different environmental densities. Microplastics densities were relatively high during the hot-dry season (mean range 120–6417 particles−1 kg−1 dwt) while the hot-wet season had the lowest densities (mean range 5–94 particles−1 kg−1 dwt). Microplastics abundances positively correlated with population density, demonstrating the direct effects of human activity on microplastics contamination. Furthermore, I exposed a key species, the banded Tilapia sparrmanii (i.e. Smith 1940) to different concentrations of microplastics particles. Tilapia consumed microplastics even when relatively rare in their environment, and consumption rates related negatively to concentrations supplied, conducive with a saturating type II (i.e. inversely–density–dependent) functional response. Attack rate (i.e. search efficiency), handling time and maximum feeding rate estimates towards microplastic were estimated, providing key information on feeding behavior in relation to exposure concentrations. I propose the utility of functional response approaches for predictive quantifications of microplastic uptake rates. The sediment microplastics quantification results highlight the need to further explore microplastics distribution patterns in freshwater ecosystems within the global south. Further, my findings suggest particular risk for fauna during low rainfall periods through microplastics concentration effects. In turn, this can better-link laboratory exposure studies to environmental concentrations which are known to cause ecological impact, and provide a means of comparing uptakes among species and across environmental contexts.