Using hydrodynamic models to understand the impacts and risks of plastic pollution

Critchell, Kay Lilian (2018) Using hydrodynamic models to understand the impacts and risks of plastic pollution. PhD thesis, James Cook University.

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Anthropogenic marine debris, mainly of plastic origin, is accumulating in estuarine and coastal environments around the world, causing damage to multiple species of fauna and flora, as well as habitats. Plastics have the potential to accumulate in food webs, and cause economic losses to tourism and sea-going industries, like commercial fishing. The production and use of plastic products is growing, from 230 million tonnes produced globally in 2005 to 320 million tonnes in 2015, a 40% increase in production over 10 years. If we are to manage the increasing input and threat, we must understand where plastic pollution is accumulating in the environment and what the impacts to organisms in these areas are.

The goal of this thesis was to explore the dispersal and risks of plastic pollution in the coastal environment, at a scale that is useful to local management authorities. I used four research aims to achieve this goal. The aim of the first data chapter (Chapter 2) was to prioritise research that would improve modelling outputs in the future. In the second data chapter (Chapter 3), the aim was to locate the areas of highest exposure to plastic pollution for three vulnerable habitats. In the third data chapter (Chapter 4), I aimed to explore the dominant sources and processes of plastic accumulation. Lastly, in the final data chapter (Chapter 5), I aimed to understand the sublethal consequence of plastic exposure on a tropic reef fish.

The first data chapter of my thesis presents an advection-diffusion model that includes beaching, settling, resuspension/re-floating, degradation and topographic effects on the wind in nearshore waters to quantify the relative importance of these physical processes in governing plastic debris accumulation. I found that the source location has by far the largest effect on the accumulation location of the debris. The diffusivity, used to parameterise the sub-grid scale movements, and the relationship between debris resuspension/re-floating from beaches and the presence of a wind shadow created by high islands also has a dramatic impact on the modelled accumulation areas. The rate of degradation of macroplastics into microplastics also had a large influence in the prediction of debris dispersal and accumulation. These findings may help prioritise research on the physical processes that affect plastic accumulation, leading to more accurate modelling, and subsequently an improved empirical basis for management in the future.

In the second data chapter, I used the model described in Chapter 2 to predict the potential exposure of vulnerable habitats and species to plastic pollution using a spatial risk assessment approach. The effect of plastics on the marine environment is well documented, however the physical location of these interactions are largely unknown. I assessed the potential exposure of mangroves, coral reefs and marine turtles to plastics during the two main wind conditions of the region; the trade winds and monsoon wind seasons. By creating relative exposure categories based on the density of particles in modelling outputs of nil, low, medium and high exposure. I found that in the trade wind season (April to September, dominated by strong south-easterly winds) marine turtles, mangroves and reef habitats had lower exposure than during the monsoon wind season (October to March, dominated by lighter and more variable winds). A small proportion of coral reef habitat was in the high exposure categories, whereas the turtle home-range had a large area in high exposure categories (16% and 26% exposed to high microplastics during monsoon season, respectively). Unlike the other two case studies, the mangrove habitat had consistent hotspots of high exposure across both wind seasons. The outputs of this chapter can inform local scale management action, for example turtle management and recovery plans. The method presented here can also be transferred to other species and habitats and scaled up for larger jurisdictions.

In the third data chapter, I built on Chapter 2 (a sensitivity analysis of physical/modelled processes) by using field data for macro- and microplastics to interrogate the model. The aim was to find the likely sources of plastics to the Whitsunday region and understand the limitations for the model in a complex coastline and at a management-relevant scale. I found that, for microplastics, offshore sources are likely to be more important than onshore, and for macroplastics, local (onshore) sources are more important than they are for microplastics. Of the physical characteristics I examined, I found none that make a site more or less predictable in the modelling. Field data on sources at local scales is necessary, although, this is recognised as a difficult task.

In the last data chapter, I assessed the consequence of plastic exposure by quantifying the effect of microplastic exposure on juveniles of a widespread and abundant planktivorous fish (Acanthochromis polyacanthus). Under five different plastic concentrations, with plastics the same size as the natural food particles (mean 2 mm diameter), consumption of microplastic was low and there was no significant effect of plastic exposure on fish growth, body condition or behaviour. However, the number of plastics found in the gut of the fish vastly increased when plastic particle size was reduced to approximately one quarter the size of the normal food particles, with a maximum of 2102 small (< 300 μm diameter) particles present in the gut of individual fish after a 1-week plastic exposure period. Under conditions where food was replaced by plastic, there was a negative effect on the growth and body condition of the fish. These results suggest plastics could become more of a problem as they breakup into smaller size classes, and that environmental changes that lead to a decrease in plankton concentrations likely have a greater influence on fish populations than microplastic presence alone.

The risks of plastic pollution to environmental features remain largely unquantified. However, my thesis demonstrates significant gains in understanding of mechanisms that can be used to determine where plastics are likely to accumulate, and identifies priorities for future research to improve the statistical power of the models. For example, by understanding the resuspension of plastic in areas without wind driven waves. This thesis also highlights the need to understand different types of plastics separately, microplastics have different consequences to macroplastics, different areas of accumulation and different sources, therefore the risks and appropriate management actions are very different.

Item ID: 55959
Item Type: Thesis (PhD)
Keywords: marine debris, coastal, oceanography, wind shadow, modelling, sensitivity analysis, plastics, plastic pollution, planktivorous reef fish
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Copyright Information: Copyright © 2018 Kay Lilian Critchell
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Critchell, Kay, and Lambrechts, Jonathan (2016) Modelling accumulation of marine plastics in the coastal zone; what are the dominant physical processes? Estuarine, Coastal and Shelf Science , 171. pp. 111-122.

Date Deposited: 25 Oct 2018 23:55
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050204 Environmental Impact Assessment @ 50%
04 EARTH SCIENCES > 0405 Oceanography > 040503 Physical Oceanography @ 50%
SEO Codes: 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
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