Closing the Mediterranean Floating Plastic Mass Budget

It is currently not well known how much plastic is entering our oceans, and what happens with this plastic in the environment. Most plastic particles are lighter than water, and are therefore expected to float. However, over time a layer of algae increases the weight of these plastic particles, possibly leading to these particles sinking down. Another significant part of plastic particles might eventually end up on our coastlines.

Here, we developed a method to estimate the total input of plastic particles in the Mediterranean, and where these plastic particles are expected to end up. The Mediterranean is a good test case: it is confined, a lot of people are living in this area, which likely means a lot of plastic pollution as well, and there are many plastic concentration measurements available from the surface water.

A model of floating particles is made to represent floating plastic particles as accurately as possible, by comparing it with these plastic concentration measurements. Results from the model show that the total plastic input into the Mediterranean Sea likely ranges between 2,100-3,400 tonnes for the year 2015, that only a small part of the plastic remains afloat (about 170-420 tonnes), that about half of the plastic starts sinking down, and that about half of the plastic is expected to end up on the coastlines.

author: Mikael Kaandorp, date: May 2020

Estimates of plastic inputs into the ocean are orders of magnitude larger than what is found in the surface waters. This can be due to discrepancies in the sources of plastic released into the ocean, but can also be explained due to the fact that it is not well known what the most dominant sinks of marine plastics are, and on what time scales these operate. To get a better understanding on possible sources and sinks, an inverse modelling methodology is presented here for a Lagrangian ocean model, estimating floating plastic quantities in the Mediterranean Sea.

Field measurements of plastic concentrations in the Mediterranean are used to inform parametrizations defining various sources of marine plastics, and removal of plastic particles due to beaching and sinking. The parameters of the model are found using inverse modelling, by comparison of model results and measurements of floating plastic concentrations. Time scales for the sinks are found, and likely sources of plastics can be ranked in importance.

A new mass balance is made for floating plastics in the Mediterranean: for 2015 there is an estimated input of 2,100-3,400 tonnes, and of plastics released since 2006, about 170-420 tonnes remain afloat in the surface waters, 49-63% ended up on coastlines, and 37-51% have sunk down.

Introduction: where is the 'missing' plastic?

In the TOPIOS project, we try to answer the question why the estimated amount of plastic entering our ocean is orders of magnitude larger than the amount of plastic found at the surface of the ocean.

Solving this problem can be seen as putting together a complex puzzle: it is a good idea to start with some of the easier but most essential pieces before moving onwards.

In this work, the first pieces we put together are for the upper ocean: this is the area for which most measurements are available to validate our model. We have a certain input (or source) of plastic particles into the surface water, and a certain output (or sink). For the output we assume beaching and sinking are the most dominant for now, as compared to e.g. ingestion by animals or fragmentation of particles into smaller and smaller pieces.

Simulation of floating plastic particles

Floating plastic particles are simulated using a Lagrangian model: this means virtual particles, representing plastic particles, are released in a virtual ocean.

The particles are moved around using reanalysis data for the currents [1], and influence of the waves (Stokes drift) [2]. The simulation is run between 2006-2016.

Where does plastic pollution come from?

The virtual plastic particles are released at different locations, where we expect a lot of plastic input, see the figure on the left.

The different sources taken in account are: river mouths (green circles) [3], fishing activity (blue shading) [4], and densely populated coastal areas (red shading) [5][6].

It is currently not well known how much each source contributes to the total amount of plastic pollution. We therefore set the importance of the sources as a parameter in the model.

Where are the floating plastic particles going?

Environmental sinks of plastic are added to the model. We consider plastic particles sinking down (due to e.g. biofouling [7]), and plastic particles ending up on beaches as the most important sinks.

It is currently not well known what is the most important environmental sink. We therefore parametrize the sinks to be able to test different scenarios.

With the parametrization for the sinks, we can now calculate the probability that a plastic particle is still afloat. This is displayed in the animation on the left: particles light up in red when they disappear due to beaching. By combining the sinks with the source parametrization, we can calculate the amount of floating plastic particles at a given location at a given time.

Using measurements to find our model parameters

Now that we have a model with possible sources and sinks of floating plastic particles, it is time to start comparing our model with in situ measurements of plastic concentrations.

About a thousand measurements of plastic concentrations were gathered from literature. By using a neuston net, researchers have measured the plastic concentration at different locations and times in the Mediterranean: see the animation on the left.

We will try to find out what the parameters for the sources and sinks should be, such that our model matches these observations as well as possible. This is what we call inverse modelling. In this process we try out a lot of parameter combinations, to see what the most likely values are.

Model results

The importance of the different sources and sinks of floating plastic are found using the inverse modelling process described above.

The most important source of plastic in the Mediterranean is likely the densely populated coastal areas: these are estimated to contribute to 61% of the pollution. Rivers are estimated to contribute to 32% of the pollution, fishing activity to 6%.

There is a roughly equal amount of plastic particles ending up on beaches and plastic particles sinking down. A map of where the plastic particles are expected to end up is shown on the left.

Conclusions and outlook

By combining a model of floating particles with measurements of plastic concentrations, we tried to get a better overview of plastic pollution in the Mediterranean Sea. This kind of analysis can be used to answer questions like: what causes most plastic pollution and where is plastic pollution a problem.

Future work will focus on making the model more realistic, by taking for example in account how plastic particles fragment into smaller pieces over time. More measurements can be included from beaches and sea sediment as well, and the approach can be extended globally. What happens to plastic particles once they start sinking down can be taken in account by using the entire 3D flow field instead of only the surface water.

For more information, you can have a look at our scientific paper in Environmental Science & Technology, or send us an email.

References

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