The impact of plastic in our oceans is a growing threat to the marine ecosystem and a new study conducted at LIRS is helping to understand the magnitude of the problem associated with microplastics – those plastics that are <5mm.
The aim of the project is to understand the potential effects of microplastics on tropical marine ecosystems, including through ingestion by coral reef fish, an area that is not well understood. A team of scientists from the Australian Institute of Marine Sciences conducted research in the waters off Lizard Island in November and December 2020 and focussed on three questions.
- Do microplastics and mesozooplankton accumulate in slicks on the water surface?
Sea slicks, or ocean films, are naturally occurring areas of aggregation for materials, including larval forms of fishes and other mesozooplankton, as well as microplastics. As a result, larval fishes in these sea slicks may experience a higher exposure to, and higher probability of ingestion of, microplastics. To examine this, Dr Kroon’s team collected water and mesozooplankton samples in and outside slicks using neuston tows (photo 2) and purse seines around Lizard Island in November and December 2020. Both water samples and fish samples are currently being analysed at AIMS for the presence of microplastics. The findings will show whether microplastic contamination is indeed higher in slicks, and if so, whether this leads to higher consumption of microplastics by larval fish present in those slicks.
- Has microplastic contamination of fish larvae changed over time? Sea slicks, and other physical processes also deliver late stage fish larvae to the nearshore area where fish settle into adult coral reef habitats. As a result, larval fish may be an important vector for the transfer of microplastics into these habitats. To examine this, Dr Kroon’s team collected water samples (using submergible pumps) and juvenile reef fishes (using light traps) in November and December 2020. Light trapping is a well-established method to collect late stage fish larvae and has been used at Lizard Island since the 1990s. In addition to analysing these water and fish samples for microplastic contamination, historical samples of the same fish species curated by the Australian Museum are being compared to document decadal patterns in microplastic contamination and ingestion by larval reef fish.
- Do fish larvae actively select or avoid ingestion of microplastics? Microplastic ingestion by fish larvae is expected to be random (i.e. non-selective), although some non-random ingestion has recently been reported for coral reef fish. To examine this for mesozooplankton, we will test the prediction that natural food and microplastics will be ingested by coral reef fish larvae in ratios that they are exposed to. Of particular interest is whether this prediction holds in future projected increases in plastic contamination. In December 2020, we conducted various pilot studies at Lizard Island to inform laboratory experiments to test this prediction in year 2 of the project. Specifically, we tested (i) behavioral similarities between fish food and different microplastics in seawater (i.e. floating or sinking); (ii) uptake of different microplastics by individual damselfish, and (iii) influence of social grouping on uptake of different microplastics by individual damselfish. The findings of these pilot studies, combined with those from field sampling, will be used to design the laboratory exposures to determine selectivity under current and projected future microplastic contamination scenarios.
Practical implications of this research
This project hopes to determine the susceptibility of coral reef mesozooplankton to microplastic contamination. Specifically, the results will help to elucidate the role of physical processes in aggregating mesozooplankton and microplastics into slicks and document the potential transfer of microplastics by juvenile fish onto coral reefs. Combined, this will improve understanding of the potential current and future effects of microplastic contamination on coral reef ecosystems.
This two year project is generously funded by LIRRF through a grant received from the Banyer Family and has enabled Dr Kroon to assess the marine ecosystems and organisms from the area to gain a closer look at the current state of contamination in the area.