Scientists studying plastic pollution in a New Zealand harbour discovered that microscopic plastic materials could change how coastal sediments responded to a storm. Researchers buried small pieces of polyester netting in the seabed and measured changes in oxygen consumption and nutrient fluxes before and after severe weather. Scientific Reports published results in 2026 showing that sediments contaminated with plastics behaved differently to uncontaminated areas when the harbor was disturbed by strong winds and heavy rainfall. The results suggest that plastic pollution can influence important processes that help coastal ecosystems function.
Scientists tested the effects of plastic when a storm hit the seabed
The research was conducted by Ines Bartl, Samantha Ladewig and Simon Thrush from the University of Auckland’s Institute of Marine Science. Their study examined how plastic pollution interacted with sudden environmental changes in Mahurangi Harbour, New Zealand.According to the study, the team established 48 experimental plots across an area measuring 14 by 22 metres on an intertidal sandflat. These coastal areas are covered by seawater during high tide and exposed as the water recedes.Half of the plots served as controls, while the other half received polyester netting inserted vertically into the sediment. Each plastic-treated plot contained 81 pieces of netting measuring 6 by 2 centimetres and just 10 micrometres thick. The researchers positioned the material so that its upper edge sat level with the sediment surface.Although the pieces were thin, they provided a way to examine how plastic fibres embedded in coastal sediment might affect the surrounding environment. The researchers also disturbed the control plots using a spatula to ensure that the physical process of inserting the plastic did not create an unfair comparison.Over two months in 2022, the team collected measurements at 6 time points. 3 sampling sessions took place before the storm, followed by measurements three days and approximately two weeks after the event.
What happened when severe weather struck Mahurangi Harbour
The storm arrived on 20 and 21 March 2022, bringing strong northeasterly winds and heavy rainfall. The study recorded average wind speeds of up to 60 kilometres per hour, with gusts reaching 80 kilometres per hour for around 24 hours. Rainfall ranged from 3.8 to 52 millimetres over a 4-hour period.Such weather can disturb coastal seabeds by generating stronger waves and increasing the amount of material carried into the water from land. The movement of sediment can redistribute organic matter and expose buried material to oxygen, changing the conditions in which microorganisms process nutrients.The researchers observed signs of these changes at their study site. Three days after the storm, the water had lower light availability and higher nutrient concentrations than at several earlier sampling points. The team linked these conditions to increased river discharge and sediment stirred up by wind and waves.The research demonstrated that storms can affect not only the physical structure of the seabed but also the activity of microorganisms and marine animals living in it. These organisms help break down organic matter and regulate the movement of nutrients between sediments and surrounding water.The researchers wanted to determine whether the presence of plastic altered these processes when the seabed experienced this sudden disturbance.
Why did plastic-contaminated sediments respond differently
The results showed that the plastic pollution alone did not result in a clearly detectable overall ecosystem response during the experiment. However, statistical analysis showed a significant interaction between the plastic treatment and the time of sampling, i.e. the response depended on the time of measurement.One important difference involved sediment oxygen consumption, a measure of how much oxygen is used by processes occurring in the seabed.The study found that oxygen consumption in the control sediments was reduced by about 24 percent after the storm compared to pre-storm measurements. In contrast, oxygen consumption in the plastic-treated sediments was 43% higher than the storm response recorded in the control sediments.The researchers also saw changes in ammonium, a form of nitrogen that can move from the sediment into the surrounding water. In the control plots, ammonium concentrations in sediment pore water fell by around 53% after the storm, while the amount released from the sediment increased substantially.The increase in ammonium release was even more pronounced in the plastic-treated plots. The study reported a 53% higher release than in the control sediments three days after the storm.These differences suggest plastic-contaminated sediment may process and release nutrients differently during extreme weather. The study also found that pore-water concentrations of nitrate and nitrite, collectively measured as NOx, were 63% lower in the plastic plots than in the controls three days after the storm.
Significance of these changes for coastal ecosystems
These findings are significant because coastal sediments are significant in regulating the nutrients available to marine organisms. Nitrogen helps biological production, but changes in the way nitrogen moves through an ecosystem can affect the balance between sediment, water and the organisms that live there.The researchers suggested plastic pollution might change the balance between the processes that produce ammonium and those that break it down. This could help explain why plastic-treated sediments released more ammonium while retaining lower concentrations of nitrate and nitrite after the storm.The team considered a few possibilities. This may alter the physical properties of the sediment e.g. how easily it erodes, or how stable it is when waves disturb the seabed. Plastic surfaces can also provide places for microorganisms to grow.Chemical effects may have played a role as well. The researchers referred to earlier work from the same experiment that detected a rapid reduction in the concentration of DEHP, a plasticiser, in recovered polyester netting. They suggested that substances released from the material could have affected nearby microbial communities.However, the study did not establish which mechanism was responsible for the observed differences.
Why researchers are examining the combined effects of plastic and storms
According to the researchers, repeated storm-related releases of ammonium could change nutrient availability in coastal waters and potentially affect the ability of sediments to retain or process nutrients.The team estimated that, if the entire 15-square-kilometre intertidal area of Mahurangi Harbour behaved like their plastic-treated plots, ammonium release following a storm could reach approximately 0.761 tonnes per day. That would be about 1.6 times the estimated release from the control sediments under storm conditions.