Tracking microplastics in the Mekong: a research overview

This week I would have been presenting my work in Vienna at the European Geoscience Union (EGU). However, EGU have been amazing, moving everything online and making the conference free and much more inclusive. Instead of presenting, online discussions are taking place with researchers uploading their presentation materials for everyone to see. It’s exciting because this allows more people to view your work as well as no longer restricting the people that can attend due to not getting funding or being able to travel. 18,000+ abstracts have been uploaded with a record 12,000 “attendees” on day 1!

Credit Nguyễn Anh Vũ via Pexels

I was supposed to be presenting the data analysis of my most recent field trip but due to lockdown that wasn’t possible. So instead I decided to present an overview of my PhD and what I plan to achieve as I feel like I finally have a solid set of goals and clear structure (although who knows this may change!). I thought it would be nice to share that on here too and explain each section a little. Each picture is from my presentation/display that I uploaded for the session “Plastic is freshwater environments” for EGU 2020. I could go into a lot more detail but have tried to keep it simple just because I could go on forever and don’t want to get too technical- please comment or email me if you have any questions!

The first slide below gives an overview of my PhD – I am researching the controls on microplastic flux mechanisms in a major river, the Mekong and out to sea. These controls or mechanisms are summarised in Fig.1. They range from the hydrology of the river, to interaction with sediment (aggregation/flocculation) and to being buried in the sediment. As a marine biologist, what I’m really interested in are the biological interactions with microplastics and how that effects microplastic transport. This includes ingestion and egestion but also biofouling where organisms grow on the surface of plastic and therefore change its density and cause it to sink.

The different sections of my project are then summarised:

  • I will conduct settling experiments with different types of microplastics and biofilmed plastic to understand how this affects settling velocity in the water column.
  • This will be used to model and produce a map of the Mekong which shows the different concentration of microplastics depending on the flow of the river and highlight areas of ecological risk.
  • These predictions will then be supported by field work including water column and fish sampling as well as investigating whether coral reefs act as sinks for microplastics.

The slide below goes into some more detail as to why I am researching microplastic in the Mekong and the settling experiments. The global input of plastic from rivers is highest in Asia, including the Mekong River which spans 6 countries. It is important to gain an understanding of what governs transport of microplastic in rivers as the majority of marine plastic debris originates from riverine sources. If we can predict where microplastics move we can implement more effective mitigation and prevention measures.

The settling experiments will take place in the lab using a water column. I will be looking at how polymer type, salinity, sediment and presence of biofilms alters settling velocity. This is vital to understand how microplastics move. This will then be combined with historical hydrological data to model a risk map of the Mekong.

Next I summarised the third stage of my research which is the field work and sampling section. For my PhD I am focussing on Cambodia and Vietnam. I have already had 2 field work campaigns and will hopefully do 2 more. We have been working with Paññāsāstra University of Cambodia, Can Tho University and the The Southern Institute of Water Resources Research, Vietnam. The diagram in the middle shows my set up. I have 5 nets, 4m apart tied together by rope with a weight on the end. This is thrown over the side of a boat and allows me to collect data on the vertical distribution of microplastics. I leave them in for a set amount of time and then rinse the contents into a glass bottle. At the same time there is an Acoustic Doppler Current Profiler (ADCP) in the water. An ADCP collects the water current velocity with depth using the Doppler effect of sound waves scattered back from particles in the water column. This is important to collect to allow us to calculate the concentrations and amount of plastic flowing in the river.

This section of the project is also part of the National Geographic River of Plastic project which is sampling plastic from Laos, through Cambodia and Vietnam. There is also a pubic engagement aspect of the project aiming to work with local Universities and organisations to understand the local perceptions of plastic pollution and support efforts to find solutions to plastic pollution. It’s really exciting to be a part of!

The bottom part is a summary of how I process the samples. I filter the water samples and then have used potassium hydroxide (KOH) to dissolve the organic material that has been collected. Otherwise when you put your sample under the microscope you can’t pick out the plastic! It’s very debatable what is the best way to do this though and I will experiment a bit with this.

Next I look at the samples under the microscope and identify any plastics. I count and weigh them and then use Fourier-transform infrared spectroscopy (FT-IR) to confirm they are definitely plastics. This works by shining a beam of light at a sample to get an infrared spectrum of absorption. This gives you a pattern of wavelengths absorbed or not which depends on the type of material the sample is. The pattern is matched to a library of patterns from known materials to determine what type of material the sample is. We can therefore confirm whether the sample is plastic or not and also find out what type of plastic it is too! This is the part that takes the longest as you can only identify one tiny plastic at a time. This data can then be combined with the flow data from the ADCP to calculate concentration, vertical distribution and annual flux of microplastic through the Mekong River.

The final part of my project is all about corals in the South China Sea. I was awarded a National Geographic Early Career Grant which is allowing me to travel to Con Dao, a group of islands off the coast of Vietnam is the discharge path of the Mekong River. It is a marine reserve and home to numerous coral reefs and vulnerable species such as dugong.

Corals are at risk from microplastic pollution because microplastics are in the size range of what corals normally ingest. Once ingested this can cause lots of problems and result in the coral being unable to function properly and may reduce survival and trigger bleaching. Microplastics may also get attached to the outside of corals on their skeletons. Through both these ways, coral reefs may act as a sink for microplastic pollution. It is important to understand the effects of this on coral ecosystems as they support a huge diversity of marine life and provide numerous ecosystem services through fishing, coastal protection and tourism. Corals are already at risk from climate change, careless tourism, fishing and ocean acidification. Microplastic pollution may add another risk to reefs and potentially accentuate the other threats by making them less able to tolerate other stressors. For my MSc I looked at the effects of microplastics on coral photosynthesis and respiration (which will hopefully be published this year), so I am very excited I get the opportunity to carry on this work!

To understand whether corals around Con Dao are being impacted by microplastic pollution from the Mekong, I will be constructing 3D images of the reefs (see below). This will indicate the species diversity and cover, which is an indication of reef health. I will also be collecting water and sediment samples and determine microplastic concentration on the reefs to see if there is a relationship between this and coral health. Obviously correlation does not equal causation and other factors will have to be taken into account which can cause coral reefs decline and this will be taken into consideration. It will be interesting to see if there is a difference in microplastic concentration from one side of the island to the other and in comparison to other aquatic areas such as the open ocean and the Mekong.

So that’s the plan so far! I have already collected some samples in Cambodia and Vietnam. I was just about to get into the lab to conduct my settling experiments and process my samples but then lockdown happened. I was also supposed to be doing more sampling in the Mekong in March and then going to Con Dao in June but all my field work has now been postponed until next year. It’s very frustrating and I will definitely have to get an extension on my PhD. I wish I had all my data and could just be writing up! Of course I am very lucky I can still be getting on with things and haven’t lost my job or anything so I am feeling very grateful. It has given me time to reflect and do more research and writing but I can’t wait to get back to the lab and field.

Hope you are safe and well, please let me know if you have any questions or would like to discuss anything!

One Comment Add yours

  1. Philip James says:

    Fabulous, thank you Freija for explaining this in a lesser scientific manner so that we who would never have been invited to the EGU2020 conference, can share in all the highly valuable work that you have been doing.

    Like

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