Presence of microplastics in benthic macroinvertebrates along the Kenyan coast

Abstract

Microplastics (MPs) are plastics less than 5 mm in diameter. Their small size renders them invisible to deposit- and filter-feeding fauna, leading to unintentional ingestion. This study investigated the presence of MPs in an oyster (Saccostrea cuccullata) and three species of brachyuran crabs (Tubuca dussumieri, Cranuca inversa and Gelasimus vocans) along the Kenyan coast. Sampling was carried out at eight stations distributed between three sites: Tudor, Port Reitz and Mida creeks, in January and February 2018, during low spring tide. The sample comprised 206 crabs and 70 oysters. Samples were digested using 10% KOH at 60 °C for 24 hours and then passed through 38-µm sieves. Sieved products (<38 µm) were filtered through Whatman filter membranes (0.8 µm) and viewed under a dissecting microscope for MPs. The study identified mainly MP fibres, which were of different colours: red, yellow, black, pink, orange, purple, green, blue and colourless. Colourless fibres were the most prevalent, comprising at least 60% of the total MPs. Mean lengths of MPs fibres of different colours were between 0.1 and 4.2 mm. The mean concentration of MPs (MPs g−1 wet tissue) was 0.65 (SE 0.13) in crabs and 3.36 (SE 0.53) in oysters, and the difference between the two taxa was significant (independent two-sample t-test: t = 5.61, df = 14, p = 0.01). The higher mean concentration in oysters was attributed mainly to their filter-feeding habit. This study exposes MP pollution along the Kenyan coast and its uptake by marine fauna, and thus strengthens the case for better control of plastic wastes in the ocean.