Do Microplastics Transfer from Lower to Higher Trophic levels in Rocky Tidal Pools at Nature’s Valley, South Africa?

Abstract

Microplastics (MPs) are pervasive pollutants in marine ecosystems, yet their transfer across trophic levels in rocky tidal pools (RTPs) remains understudied. We collected and analyzed 239 samples, including seawater, sand, seaweed, and invertebrates representing multiple trophic levels, from RTPs at Nature’s Valley (Indian Ocean). Microplastics were detected in 98% of samples across multiple trophic levels, with mean concentrations ranging from 9 to 56 particles per individual, indicating widespread MP contamination of RTP organisms. These MPs were quantified and categorized by morphotype, color, size, and polymer composition. A total of 7,075 MPs were counted in 160 invertebrates, 60 seaweed fronds, 10 sand samples, and 9 water samples. The polymer composition of 2,350 MPs was separately analyzed using Fourier transform infrared (FT-IR). The MPs were predominantly fibers (96%) composed of polyamide, polypropylene, and polyethylene. The most common colors (> 50%) were white and transparent. Morphotype, color, and size distributions show weak evidence of trophic transfer. However, MP polymer compositions showed no discardable correspondence between trophic levels. We therefore found very little evidence of trophic transfer between multiple trophic levels in the RTP organisms we studied. These findings indicate widespread microplastic contamination influenced by complex environmental exposure (such as bleaching and size reductions) and organismal interactions rather than clear trophic transfer pathways. Our study highlights the need for integrated approaches to elucidate MP dynamics in RTP food webs and informs future assessments of ecological risks associated with MP pollution in coastal habitats.

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