Plastic pollution in Moreton Bay

While the sight of plastic bottles, straws and bags is distressing, these larger plastics degrade and break down to form microplastics, which are pieces of plastic less than 5 mm in size, and nanoplastics, which are smaller than 1 mm. Due to their small size, the presence and impact of these micro- and nanoplastics go largely unnoticed in the environment. However, these tiny plastic particles can be ingested by microscopic organisms and find their way into commonly consumed seafood. The plastics also leach harmful chemicals into the environment.

In Australia, knowledge is limited about the amount and type of plastic pollution in marine and coastal environments. To manage the problem of plastic pollution in the marine environment, first we must understand the scale of the problem. In our paper led by Dr Elvis Okoffo and published in Science of the Total Environment, we focused on the semi-enclosed coastal embayment of Moreton Bay.

Our method

Part of the reason for this lack of knowledge is that current methods for measuring microplastics are time- consuming and semi-quantitative. We used a new quantitative method, Pyr-GCMS, to measure the concentration of seven common plastic polymer types (i.e., polystyrene (PS), polycarbonate (PC), poly-(methyl methacrylate) (PMMA), polypropylene (PP), polyethylene terephthalate (PET), polyethylene (PE) and polyvinyl chloride (PVC)). This method involves dissolving the plastic in dichloromethane and then pyrolising (heating up) the sample to vaporise the plastic from the samples. The vapour is then analysed in a gas chromatograph mass spectrometer to determine the types of plastic. The advantage of this method is that we can quantitatively measure plastic concentrations of any size range.
 

Sampling in Moreton Bay

We measured the plastic stored in around 50 surface sediment samples from across Moreton Bay, from a range of different Bay ecosystems, including mangroves, seagrass and mud from its main tidal channels. Most plastic is denser than water, and the majority of plastic particles sink to the sea floor. Ecosystems like mangroves and seagrass are very effective at trapping sediment in coastal ecosystems and have also been found in other regions of the world to have high concentrations of microplastics.

We found that plastics are pervasive across the Bay and were present in all the sediment samples, although they displayed a wide range of concentrations from 3.3 to 2194.2 µg/g. There was no clear pattern in the plastic concentration, suggesting a wide range of sources of plastics to the Bay that transport down waterways such as the Brisbane River, and it is likely that the plastics are transported within the Bay by tidal currents. Using the average concentration of the plastics in different regions and ecosystems within the Bay, we estimate that the total mass of plastic stored within the surface sediment (the top 10 cm) of the 1500 km2 of Moreton Bay to be 7000 tonnes. This level of plastic contamination is equivalent to three Olympic swimming pools full of plastic, or 1.5 million single-use plastic bags.

Among the seven polymers analysed, PE and PVC were found at the highest concentrations. These were the most commonly used plastics in Australia in 2019–20, suggesting a direct link between the consumption of plastics in Australia and their prominence in the coastal environment. PE is one of the cheapest types of plastic and is widely used for single-use plastic items such as food wrappings, plastic bags and plastic bottles. PVC is used in wastewater treatment plant pipes, building materials, electronics and clothing. Knowledge of these main plastic types polluting our coastal environments is critical to inform the management strategies for reducing future plastic pollution in the coastal environment.
We are currently investigating how plastic has changed over time in Moreton Bay from sediment cores from the centre of the Bay, and we will be looking at the level of plastic contamination in organisms that live there.

The research paper was published in Science of the Total Environment. Read it here.

Read more of these stories in Issue 18 of the CBCS Newsletter and follow us on X (Twitter) and LinkedIn.