Microplastic Abundance, Composition and Distribution in the Endorheic Lake Naivasha, Kenya

Abstract

Plastic pollution has recently gained global concern due to the negative effect it presents on both the environment and human health. In the environment, plastics degrade yielding millions of micro- and nano-scale plastic particles. The microplastics have the ability to adsorb organic pollutants, heavy metals, and micro-organisms in either the aquatic or terrestrial environments. The microplastics size range and ubiquity in the environment provides an easy pathway into the food chain through ingestion, bio-accumulation and bio-magnification. This study was conducted with an aim of determining the abundance and composition of microplastic particles in Lake Naivasha surface waters and sediments. The study also entailed the evaluation of the possible physical chemical parameters that could affect the distribution of microplastic within the lake. Volume reduced and bulk sampling methods were used to collect samples of floating debris in surface waters and sediments respectively. The physical chemical parameters were measured in situ whereas the nitrogen and phosphorus nutrient levels were determined in the laboratory. Density separation using concentrated brine solution and wet peroxide oxidation methods were used for microplastics extraction. The recovered particles were analyzed using microscopy for physical characteristics (shape and color) while the microplastics chemical composition was determined using Fourier-transform infrared spectroscopy (FTIR). All the physical-chemical parameters in the sampled locations of Lake Naivasha exhibited significance differences (p<0.05) except salinity. These differences were attributed to the infestation by water hyacinth (lower dissolved oxygen and temperatures), originality of the lake (pH and total dissolved solids), and anthropogenic activities (higher turbidity, conductivity and nutrient levels). The average microplastic concentration was found to be 0.407±0.135 particles/m 2 and 177.3±87.4 particles/kg in surface waters and surface sediments respectively. The post-hoc Tukey Honest Significance Difference test exhibited significant differences (p<0.05) in the microplastics abundance in the studied locations of the lake. The highest microplastic levels were recovered in the lakes’ major inflow, River Malewa. The high variability in the microplastic densities exhibited between the sampled locations was attributed to human activities, water and wastewater intake through rivers and tributaries, and the presence of local wind patterns responsible for the general water circulation. Colored and non-colored microplastics, of shapes categories fragments, fibers and films were identified and were majorly composed of polypropylene, polyethylene, and polyester in surface waters, whereas polyethylene terephthalate, polyvinylchloride and nylon polymers were dominant in the surface sediments. The lakes’ dominating abundance of fibrous, fragmented and colored (83%) microplastics was an indication that secondary microplastics were the major source of pollutants in the lake. Moreover, correlational analysis done exhibited a strong positive existing association between microplastic quantities and turbidity, total nitrogen, and total phosphorus in Lake Naivasha surface waters. Thus, this study concludes that the microplastic pollution in Lake Naivasha is contributed by anthropogenic activities with the distribution drivers being related to the nutrients and turbidity levels of the lake. This study recommends on the improvement on microplastic waste management around rivers and lakes by local authorities and the National Environment Management Authority (NEMA), and increase in the public awareness and education.