Module V: Monitoring

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In the former modules there have been a lot of explanations on what macroplastics are and the importance of rivers as pathways of plastic litter from the sources, through rivers, to seas and oceans and the potential. However, if you want to take informed action you need data on marine or riverine litter. Moreover, it is also important to have exact information if you want to evaluate measures that are taken and you need understanding of the context specific factors for success (Löhr et al., 2017). This module therefore provides you with current knowledge on monitoring plastic litter.

This module focuses on current knowledge on monitoring plastic litter and it is structured into three parts:

  1. Introduction – What is monitoring? 
  2. How do I find out what kind of monitoring I should do? 
  3. Examples riverine litter – The Meuse River – Citizen science

In this module, we have prepared some explanatory videos, figures, informative texts and assignments for you which will help you to understand monitoring.


1. Introduction text: WHAT IS MONITORING

Worldwide there is a growing concern about the risks and possible adverse effects of litter, as reflected in the most recent report by GESAMP WG40 “Proceedings of the GESAMP International Workshop on assessing the risks associated with plastics and microplastics in the marine environment”. Marine litter pollution is a complex worldwide environmental problem with diverse marine and land-based sources. The amount of plastics in the marine environment has gradually increased, mainly as a result of increasing use and poor waste management (Breukelman et al., 2019). Although marine litter pollution is recognised worldwide as a serious environmental problem it is unknown how much in total has entered the ocean. It is estimated that between 4.8 and 12.7 million tonnes of land-based plastic waste end up in the ocean every year (Jambeck et al., 2015). Lebreton et al. (2017) estimated that between 1.15-2.41 million tonnes of plastic waste enter the ocean every year from rivers. 

Plastic pollution is a serious problem in rivers around the world although the problem differs greatly between rivers and parts of the world. The main focus of the United Nations Environment Programme with regard to marine litter is nowadays on the reduction and prevention of litter entering the oceans (a good example are the litter traps that are being placed in rivers as well as clean-up actions at river banks). Monitoring the riverine environment for the presence of litter is therefore of utmost importance. 

Monitoring is needed to gather scientific knowledge on the amount, sources, transport and spread of litter and especially plastic litter in the environment. The knowledge is a necessary part of assessing the extent and possible impact of riverine litter. Moreover, it is important to come up with possible mitigation methods to reduce inputs, and it is also needed if you want to evaluate the effectiveness of such measures. In monitoring it is crucial to use consistent and reliable methods of sampling and sample characterisation (e.g. number, size, shape, mass and type of material) to come to accurate and reliable data.  

The following video will introduce you to the topic of monitoring riverine litter and its importance. It will also give insight in how it can help us to better understand the problem of riverine litter accumulation around the world. It gives an impression of the current role of monitoring litter in the Meuse River explained by Lea Crijns from Rijkswaterstaat.

“Watch the video: River litter monitoring


Watch the video and give three reasons why monitoring is crucial if you want to tackle the problem of riverine pollution.


  • to get accurate knowledge on the amounts of plastics in the river and on the river banks.
  • to get insights in the sources of the plastics.
  • to identify the so called “hotspots”.

Further reading

An interesting overview paper has been written by Tim van Emmerik and Anna Schwarz: “Plastic debris in rivers”. The paper discusses the current scientific state and monitoring techniques on plastic debris in rivers and evaluates existing knowledge gaps. 

Link to the paper


When setting up a sampling programme the design needs to take into account the management objectives (e.g. compliance, efficacy of reduction measures), the environmental setting and the most appropriate indicators to be targeted. Indicators are selected to describe the ‘state’ of the environment, such as the quantity of litter per unit of measurement (i.e. area, length, number of organisms if you look at microplastics in animals). It is common to compare the measured ‘state’ against a baseline or reference state. But, as plastic litter is everywhere in the ocean and rivers, it is unlikely that the baseline will be zero. There needs to be a degree of consistency in the techniques used and in the frequency and location of sampling, to allow reliable estimates of changes in space and time. The magnitude of the change to be detected, coupled with the inherent variability in the measured parameter, determines the sampling effort required to reliably detect spatial and temporal trends. 


An in-depth report is available for marine litter monitoring. Although this is an example for marine litter, the example is also applicable for riverine pollution by river tourism. 

GESAMP (2019). Guidelines for the monitoring and assessment of plastic litter in the ocean

Examples of key questions if you want to start monitoring


Question: We are worried about the impact of plastic on tourism. How could we get good data on this?

  • You would like to find out what would be a good method to monitor the impact of plastics on tourism. Have a look at Table 10.2 from the report (GESAMP (2019). Guidelines for the monitoring and assessment of plastic litter in the ocean) where you can find the different policy concerns, in this case, the impact on tourism. If you look at line 1, which would be the most feasible way forward (Feasibility 1) you can see that the recommended sampling approach would be: 
  • Shoreline sampling at a beach looking at macroplastics:
    • Compartment SL = shoreline
    • Sub-compartment BE – beach
    • Plastic size MA: Macroplastic
  • The resources you would need are people and basic field equipment:
    • People R
    • Basic field equipment: R
  • The table indicates that Chapter 4 “Monitoring methods for shorelines” of the Guidelines can give you the in-depth information you would need to carry out this shoreline sampling.



  • The next step would be to look in Chapter 4 of the Guidelines “Monitoring methods for shorelines”. The chapter describes the tools to survey and monitor marine litter in the intertidal and associated zones (e.g. backshore, saltmarsh, dunes).

This would lead, for example to the following beach survey method:


Currently there is quite some concern on plastic pollution in rivers being exacerbated by the Covid-19 pandemic. See here for example the research carried out in the Thames.

Could you think of a good method on how to study the possible effects in the Meuse River of the Covid-19 pandemic? 


Click here and write your idea in the comments or write an email to  [email protected]


In this study carried out by Cordova et al. in Jakarta Bay they used this method “a 75 m-long, 1.5 m-deep net with a 5 cm mesh size were placed across each river during low tides for 15 min for four replicates”. The results were remarkable: “Unique to the pandemic, we observed an unprecedented presence of PPE (medical masks, gloves, hazard suits, face shields, raincoats) that accounted for 15–16% of the collected river debris of 780 ± 138 items (abundance) or 0.13 ± 0.02 tons (weight) daily.” 

You can also join the Schone rivieren program (Clean rivers program) as a volunteer (see the next section) and add masks to the items on the data sheets. 



Introduction text

Monitoring of riverine litter with Citizen science in the Meuse River

Citizen science (CS) is defined as “scientific work undertaken by members of the general public, often in collaboration with or under the direction of professional scientists and scientific instructions, which is often linked with outreach activities, science education or various forms of public engagement with science” (European Commission, 2016). Most CS projects concerned with plastic litter monitoring not only monitor plastic litter, but all kind of debris found in the monitoring area. However, the majority of items found are plastic items: 81% of the items found in monitoring river bank litter along the Meuse and Waal from 2017-2019 were plastic (Schone Rivieren, 2019). 

Typically, only the macro-sized fraction of plastic debris (macroplastic, defined as items > 5 mm in diameter, which can be identified with the naked eye) are counted in plastic monitoring through CS, although in some projects the larger microplastics (1-5 mm) are also monitored, with specific attention to nurdles, small plastic pellets which are used as raw material in the manufacture of plastic products. The smaller microplastics (0,1 µm-1 mm) and nanoplastics (< 0,1 µm) are outside the scope of the definition. 

Setup of CS projects is defined as a precise description of the CS project such as the goal of the project, the geographical area and the timeframe in which the project is carried out, the protocol that is used to collect the data, the type of data that is collected, number of measurement locations and frequency of measurements, training of volunteers, data quality and reference measurements, data processing and communication to stakeholders (including volunteers) about the project. 

The issues surrounding the pollution of the Dutch rivers have brought together three non-profit organisations (IVN, PSF & SDN) that have collectively founded Schone Rivieren (i.e. Clean Rivers). One of the actions that is carried out is a citizen science monitoring project in which the OSPAR Guidelines are used. 


  • Question: How could we include citizen science for our monitoring program?

Watch the following video on the role of citizen science for the Meuse River and within the Clean rivers project. Winnie de Winter from the North Sea Foundation will answer some important questions on monitoring in rivers. 

Take a look at this video!

More in-depth information:

Van Emmerik, T., Roebroek, C., de Winter, W., Vriend, P., Boonstra, M., & Hougee, M. (2020). Riverbank macrolitter in the Dutch Rhine-Meuse delta. Environmental research letters, 15(10), 104087.

Link to the Open Access paper


EXAMPLE TOP 10 items

How could you get good insight in the top 10 items found on the river banks?

  • Ordering you results in top items can support possible reduction and management measures. First you need to set up a good method to collect your data, as was shown in the different video’s in this chapter. You can for example use the protocols and datasheets of Schone rivieren (in Dutch) (Schone rivieren), or one of the other examples presented in this chapter.
  • Once you collected all the data it is important to make an ordering of the results and a nice way to present them as for example the results were presented in the factsheet of Clean rivers (in Dutch)

Apr 19, 2021 | 0 comments


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The author of module 5:

Ansje Löhr

Ansje Löhr

Associate professor at the Department of Environmental Sciences, Open Universiteit

Ansje Löhr is involved in varying (international) projects related to marine and riverine plastic pollution and waste management. She works in close cooperation with the United Nations Environment Programme (UNEP) and the Global Partnership on Marine Litter (GPML).

More information on research and education of the Department of Environmental Sciences of the Open Universiteit is available here! 

Click her for her Scholar Google account with current publications!