Impacts of Cape Town stormwater outlets and marine sewage outfall on microplastic distribution and coastal biodiversity

Abstract

Microplastics (MPs) are contaminants that have raised a concern about the health of coastal ecosystems. The exact sources of MPs brought into coastal waters are relatively unknown, however, runoff from stormwater outlets is said to be one of the main general inland sources that contribute to MPs in coastal ecosystems. Stormwater runoff in many coastal cities is poorly managed and contributes approximately 80% of plastics found in coastal ecosystems. This study aims to identify if stormwater outlets are indeed a source of introducing MPs into coastal environments via runoff and whether MPs introduced into these locations are causing a risk within the environment and biota. The study compared impacted stormwater sites at Camps Bay, Mouille Point and Three Anchor Bay (Cape Town, South Africa) with a referenced control site. It was found the stormwater impact sites were indeed a source to contribute a higher abundance of MPs. The study found that the total abundance of MPs in samples during summer had a significantly higher concentration of MPs with a total abundance of 4.96 ± 0.39 (standard error) particles per unit, while winter had an abundance of 3.95 ± 0.78 particles per unit. It should, however, be noted that water and sediment were found to have a significantly higher concentration during winter, while biota (mussels, sea urchins, whelks) were significantly higher during summer. Using four risk indices (Contamination factor (CF), Pollution load index (PLI), Pollution risk index (PRI), Polymer risk index (H) and a condition index, MPs around stormwater outlets were assessed to gain an understanding of the risk MPs are having on the environment and biota. Risk assessments done on water, sediment and biota found that all sites were at a high risk for CF and moderate for PLI. The H and PRI calculations found that only Camps Bay was at a high and very high risk due to the types and amounts of polymers found at this location. The condition of mussels was also found to be negatively influenced by stormwater outlets as their condition decreased with the increase of MPs. To conclude, stormwater outlets are indeed a source of transporting MPs into the coastal environment and pose risks to the health of organisms like mussels.