Microplastic pollution in the ocean and coral reefs of the Western Indian Ocean

Abstract

Plastics, a century old answer to everyday problems, are now the cause of environmental and health problems. The concern of plastics in the natural environment is causing global distress. Never was the intention by creating plastics to be destructive to the natural environment, ecosystems, and human health. The threats of microplastics (MPs, <5 mm) are poorly understood, but with time this pollutant is becoming a growing problem. MPs are inadvertently released into the environment as primary MPs, but can also form from larger plastic items that undergoes weathering and degradation creating secondary MPs. Quantification of MPs are important as it provides pollution baselines and allows the assessment of MPs’ threats to human and environmental health. In this dissertation, I quantified and assessed MP concentrations and characteristics of MPs in seawater and coral from the Western Indian Ocean (WIO). MP concentrations were quantified along the Tanzanian coast and Comoros Islands in samples from two major currents, the East African Coastal Current (EACC) and the Mozambique Current (MC), at 20 m and 100 m depths, covering approximately 2 700 km2 of the WIO. I found high MP concentrations (457 n/m3) in the water samples from the WIO compared with other studies from the Indian Ocean. The samples had significantly higher fibre concentrations compared to fragment concentrations with the highest fibre concentrations at 20 m depth. Atmospheric deposition might explain the higher fibre concentrations in surface water. The largest difference between the MPs in the two currents were with fibre concentrations with fibre concentrations in the EACC almost one third higher than in the MC. The data indicated that the local coastal input was not a major source of MP contamination but were rather a result from the background marine input. Another explanation for the small amounts of MPs near the coast might be due to the dilution effect of river water in shallower water, but this should be further investigated. I also quantified the MPs in coral of remote Mascarene coral reef islands of the Republic of Mauritius. Coral samples were collected from Rodrigues, St Brandon’s Atoll, and Agalega. I sampled six different genera, Acropora, Stylophora, Pocillopora, Fungia, Sinularia, and Sarcophyton, from both hard- and soft coral types. I found lower MP concentrations (0.78 MPs n/g) in coral compared with other studies. The MP concentrations did not differ between the three islands, despite large differences in human populations. The data reflected that the MP concentrations found in the corals were a result from a homogenous marine input. I found higher fibre- than fragment concentrations in coral. However, the concentration differences were not statistically significant. The soft corals had higher MP concentrations than the hard corals, as predicted due to the differences in feeding methods. I did, however, conclude that the MP concentrations were genus specific, as the genera with the highest MPs were Sinularia, a soft coral, followed by Acropora, a hard coral. I also propose that Acropora be considered as a global monitoring species since it occurs in most tropical coral reefs. Comparisons with other studies proved to be difficult due to the differences in sampling method, lab analyses, units of measure, and other procedural differences, calling for method and reporting standardization. Comparisons between the MP properties from these two studies were possible as the same sieves and measurement protocols were applied. Concentrations cannot be compared due to the nature of the different matrixes.

When I compared the data from the two studies, I found that fibres were the dominant MP type, and the dominant size of MPs (fibres and fragments) were in the smaller size categories. The comparisons between the proportions of the MPs properties (size and colour, between fibres and fragments) in seawater and corals, shows that MPs in corals do not reflect the properties of MPs in seawater. Therefore, corals do not bio-indicate seawater MPs, but do indicate coral reef pollution.