Impact of microplastic pollution on breaking waves Open Access

Plastic is a ubiquitous feature of modern life. The durability, malleability, versatility, and lightweight of plastic have made it indispensable in many fields and industries. Due to the rise in plastic production during the latter half of the 20th century, there has been a corresponding increase in plastic waste accumulating in the environment.¹ Reports of plastic pollution in aqueous systems date back to the 1970s. More recently, this issue has also been examined from both physicochemical and biological perspectives concerning the sea surface microlayer (SML).^2–4 Every year, several million tons of plastic waste enter the ocean from land, primarily through rivers.^5,6 The plastic concentration in the ocean is predicted to increase dramatically^7,8 and seriously exceed the planetary boundaries.⁹ Without any drastic changes in waste management, no global “waste peak” is expected in this century.¹⁰ 

Extensive research has been conducted on various aspects of microplastics, including (a) past, present, and future plastic release into the environment;^1,7,8 (b) transportation pathways including hydrodynamic modeling;^5,6,11,12 (c) accumulation sites;^13–16 (d) impact on flora and fauna;^17–21 and (e) implication on human health.^22,23 However, a significant gap remains in our understanding of how microplastics interfere with geophysical and geochemical processes. Recent work revealed that airborne microplastics have direct radiative effects in the atmosphere,²⁴ demonstrating that microplastic pollution does affect global geophysical phenomena. In the ocean, the most abundant type of plastic waste consists of microplastic debris smaller than 1–5 mm, with an estimated microplastic concentration between 0.05 and 1.0 g/m².^14,25,26 The lower cutoff for microplastics is typically around 10–25 μm, below which smaller particles are classified as nanoplastic.^27–30 Since most plastics have a lower density than seawater, they are typically found at or near the surface. Larger particles (>1 mm) are usually no more than a few meters beneath the surface, but smaller particles can be mixed deeper into the water column (100–300 m according to Egger et al.²⁹) and may eventually sink as biogenic aggregates.^29,31,32 Wave breakage and subsequent sea foam, known as white caps, are strongly affected by the sea surface chemistry and physics.^33–35 The stability of sea foam affects environmental processes such as air–sea interaction and albedo, i.e., the reflection of solar radiation.^34–36 Given the frequency and number of wave breaking events, minor changes in sea foam stability can affect air–sea interaction and albedo on a global level.^33,37,38 Therefore, microplastics' alterations to the ocean surface should be considered.

Microplastics are hydrophobic, arbitrary shaped particles, which, when present at the air–water interface, affect the surface tension³⁹ and surface rheology^40–42 and lead to surface deformation.⁴³ Depending on the size, shape, and surface properties of the particles, they can either stabilize an interface and thereby help to form stable foams, known as Pickering stabilization, or destabilize foam interfaces.^39,44 The surface concentrations of microplastics, as currently measured, are insufficient (by several orders of magnitude) to either decrease the surface tension^{2,3,14,25,26,40} or impact surface rheology.^45,46 For particle-stabilized systems, the main process affecting foam stability, i.e., bubble coalescence can already be affected by a few particles trapped in the water film between two air bubbles (foam lamellae) or in the joining area of several films (Plateau border). Particle shape, which determines the contact angle of the particle–water–air contact line, surface hydrophobicity or hydrophilicity, and capillary forces govern the overall particle-stabilized foam. Microplastics present in the film can perforate the liquid film and impede foam stability, while particles can also act as steric barriers between air bubbles or reduce water drainage, thereby delaying coalescence. For example, a few particles with pointed edges and a contact angle of more than 90° (i.e., hydrophobic surface properties) will penetrate foam films immediately while roundish hydrophilic particles (contact angle below 90°) will stabilize the foam.^39,44,47 Hence, it is difficult to predict the interaction of sea foam with heterogeneous and arbitrary shaped microplastics.

In addition to particles, surfactants influence foam formation and stability. Surfactants are amphiphilic surface-active molecules that adsorb to air–water interfaces and reduce the surface tension. In a marine environment, natural surfactants are substances released directly into ocean water by microbes, such as bacteria, protists, and single-cell plants,^48,49 or they may be embedded within marine microbial biofilms.⁵⁰ Major classes of natural surfactants include glycolipids, lipopeptides, lipoproteins, and phospholipids. The concentrations, molecular mass, and chemical composition of surfactants in the SML or biofilms are generally unknown. However, it is assumed that these factors depend on the biological species, salinity, water quality (e.g., oxygen level), and nutrient availability. The surfactant enrichment in the SML appears to be greater under oligotrophic conditions (nutrient-depleted waters of the Northwestern, Northern and Southern Pacific, Northern and Southern Atlantic, and Southern Indian Ocean) than in eutrophic waters.² These differences may originate from higher microbial activities in eutrophic water while direct surfactant production by bacteria and phytoplankton is dominant in oligotrophic conditions. In addition, the local fluid dynamics (water current and wind speed) corrupt the spatial and transient homogeneity of the surfactant adsorption layer.

In summary, the before-mentioned constraints make it nearly impossible to define particle size and particle distribution as well as particle and biosurfactant concentration in the SML and we, therefore, propose a model system composed of microplastics collected from the ocean surface and the nonionic surfactant Triton X-100 representing amphiphilic surface-active molecules. The selection of original oceanic microplastic and a well-defined surfactant enables us to study the fundamental physicochemical adsorption to the air–water interface and its consequence on wave foam formation and foam stability. The experimental design has limitations. To put our work into perspective, the use of original microplastic debris closely simulates the real world, but future work should address the influence of biofilm formation on particle surfaces as well as the photochemical degradation. Further, the use of a surfactant model system closer to the biosurfactant composition in the SML as well as wave channel experiments with seawater would be advantageous.

The artificial seawater (ASW) was prepared according to Kester et al.⁵¹ The used salts are listed in Table I. To mimic the surface-active microlayer on the sea surface, 0.2 mg/l of Triton X-100 (Sigma Aldrich, Switzerland) was added to the ASW. This represents a model of the SML surfactant concentration with medium productive oceanic conditions.² The used surfactant Triton X-100 is a nonionic surfactant, i.e., does not interfere with charges potentially present in the aqueous phase or on the microplastic particle surfaces.

Microplastics were kindly donated by The Ocean Cleanup, which were collected at the surface of the North Pacific Ocean from July 27 to September 19, 2015, in locations between 25–41°N and 129–156°W.¹⁴ The microplastics were collected with a 0.5-mm square mesh Manta trawl, thus in one dimension larger than 0.5 mm. In order to amplify any effect of microplastics on the air–water interface, we reduced the particle size. The particles were ground for 20 s (Durabase Coffee Mill). Due to the grinding, new surfaces were formed, which have not been in contact with seawater before.⁵² Therefore, they were suspended in ASW for 24 h to imitate the exposure to the ocean. Afterward, surface tension of the water was measured (Krüss Bubble Pressure Tensiometer BP50, Germany) to confirm no further leakage of surface active compounds (see Fig. S1). The particles were separated into three size classes 125–250, 250–320, and 320–1180 μm using a sieving machine (Retsch AS 200, Germany). The exact size distribution of each size class was analyzed using laser diffraction analysis (Beckman Coulter LS 13 320, USA) and is shown in Fig. S2. Microscopic images of the milled and sorted particles are shown in Fig. S3 (Leica Microscope DM6B, Germany). Various microplastic surface concentrations c_MP from 0.1 to 32 g/m² were used with concentrations of 0, 0.5, 1, 2, and 5 g/m² to study the influence of particle size classes.

To examine the foam height, a foam column setup as described by Joshi et al.⁵³ was used to generate foams and subsequently monitor the foam decay as a function of time. In brief, the foam is generated by injecting dry air (1.5 l/min) through a sintered glass filter (pore size: 16–40 μm, diameter 40 mm) into a water-filled glass column with diameter of 40 mm and height of 200 mm. A Fujifilm XT-4 with a Fujinon Aspherical XF 16–55 mm 1:2.8 zoom lens monitored the foam height from the side. The experimental setup is shown in Fig. 1. In a similar setup, it was confirmed that the resulting bubble size distribution resembles the distribution found in nature.⁵⁴ 

The measuring procedure is as follows: After Milli-Q or ASW was loaded into the column, airflow (1.5 l/min) was switched on and kept constant for 3 s to generate the foam. The foam height was measured first for the water alone (Milli-Q and ASW). Next, microplastics were added to the water in the glass column, and the experiment was repeated with the same setting as for the pure water. The foam height experiments were filmed and analyzed using the ImageJ software with Fiji manual tracking plugin. Each experiment produced a curve of foam height over a time period of 3 s. A plateau of the foam height was visible, as illustrated in Fig. 1(b). For the calculations, the plateau height was used. The foam height was measured five times for each microplastic particle concentration and each particle size distribution, and each measurement series was repeated five times. All experiments, unless further noted, were carried out at 22 ± 1 °C.

We investigated alterations of the foam evolution by microplastic pollution by studying the foam height in a foam column setup to determine the relevant physicochemical foaming conditions as well as to elucidate if differences in foam amount and lifetime are to expected from Milli-Q, tap, and artificial seawater. In a second step, we investigated the surface foam area after a breaking wave event with a wave generator in an 11 × 0.5 × 1 m³ tank (length × width × height) with a still water depth of 0.23 m to imitate the effect of microplastic on the foam in an actual breaking wave event. The microplastic samples are oceanic surface debris collected in the North Pacific by The Ocean Cleanup and conditioned as outline in Sec. II.

We measured the effect of microplastic concentration, plastic particle size, water salinity, and temperature on the foam height with the foam column setup. The foam height evolution, including the foam formation and decay phases, reveals only marginal differences between Milli-Q water (water resistivity of 18.2 MΩ cm, conductivity <0.055 μS/cm) and artificial seawater (ASW, 35.0 ‰ salinity, conductivity 44 000–58 000 μS/cm) as depicted in Fig. 3(a). The foam height evolution of Milli-Q water is, in particular toward the end of the plateau, about 3% lower than for ASW. The foam bubbles produced in Milli-Q water are visually larger than those produced in ASW (Fig. S4) due to charge separation inhibited coalescence.⁵⁸ The foam height evolution in Milli-Q and ASW water with or without added microplastic (c_MP = 5.0 g/m²) is depicted in Figs. 3(b) and 3(c), respectively. The foam height increases about 8% and 12% for Milli-Q and ASW when microplastic particles are present. This increase is seen in particular at the beginning of plateau. Figure 3(d) summarizes the foam height as a function of microplastic particle concentration (c_MP = 0.1–32.0 g/m²) in ASW and Milli-Q water in comparison to plastic-free waters (c_MP = 0.0 g/m²), indicated by the dashed baseline. The results show that the foam height primarily depends on the microplastic surface concentration, while the water salinity has only a minor effect. Statistical analysis of all experiments was performed with RStudio 1 and showed a statistical significance with p  $≤$ 0.05. The measurements in the foam column [Figs. 3(a)–3(c)] confirmed that the foam properties marginally depend on the water quality, and we, therefore, are able to use tap water for the experiments in the wave channel without jeopardizing the comparison to ASW. Finally, the increased foam height is independent of the water temperature between 5 °C and 22 °C, as shown in Fig. S5.

When comparing different particle sizes at low concentrations (c_MP = 0.05–1.5 g/m²) as shown in Fig. 4, smaller particles [diameter d = 125–250 μm, Fig. 4(a)] reveal an increase in foam height already at low surface concentration below 0.25 g/m², whereas larger particles [d = 250–320 μm, Fig. 4(b)] have no effect on the foam height up to c_MP = 0.25 g/m². The largest tested microplastics (d = 320–1180 μm) show a considerable foam height rise starting from c_MP = 0.5 g/m² [Fig. 4(c)]. Eventually, at high microplastic surface concentrations up to c_MP = 32 g/m², the different particle sizes affect the foam height indistinguishably [Fig. 4(d)]. In conclusion, the foam height evolution is independent of the water salinity, temperature, and particle size in the tested range. However, the minimum surface concentration required to affect the foam height relates to the number of particles.

There are two mechanisms responsible for increased foam heights observed in the foam column tests. First, interfacial particle adsorption alters the liquid film curvature, resulting in a change in capillary pressure inside the film. As a result, the flow rate inside the film is lowered and a delay in bubble coalescence is observed.⁵⁹ Second, the particles aggregate and act as a physical barrier at the air–water surface, in the film, or the Plateau border. The liquid inside the film is, therefore, unable to drain; as a result, the bubbles have a longer lifetime.³⁹ In the later case, wettability and the contact angle strongly influence the behavior of particles at air–water interfaces. If particles are not wetted (contact angle > 90°), the film will rupture. On the other hand, if particles are partially wetted (contact angle < 90°), they maintain the liquid of the film and delay drainage.³⁹ Based on our observations, we propose that the arbitrary shaped microplastics are partly wetted and lead to a hindrance to the draining liquid. This is supported by the microscopic images in Fig. S4, showing that the microplastics are in the water phase.

The effect of microplastic on the foam formation created by a breaking wave and its subsequent decay was monitored by measuring the foam area evolution in a wave channel. Figure 5 shows photographic images of a breaking wave as a function of time filmed from the top view. We conducted wave breaking experiments with solitary waves at different wave heights with five different microplastic concentrations (0, 0.5, 1, 2, and 5 g/m²). Each experiment was performed ten times, and Fig. 6 depicts the foam area evolution of waves with relative wave height of (a) ε = 0.45, (b) ε = 0.60, and (c) ε = 0.75 after wave breaking at t = 0. For all waves, the foam area increases steeply after the breaking of the wave (yellow area), followed by a flattening of the foam area curve into a plateau phase (orange area), and finally, a rapid collapse of the foam (red area). Thus, the evolution of the foam area resembles the behavior reported by Callaghan et al.³⁵ Microplastic does not affect the foam formation and decay, but the peak foam area is increased. This effect is most apparent for ε = 0.45 [Fig. 6(a)] and decreases gradually with increasing wave height for ε = 0.6 and 0.75 (Fig. 6). In Fig. 7, the average foam area in the plateau phase (orange phase indicated in Fig. 6) is depicted. The foam area is stabilized more strongly at low wave heights. These results indicate that microplastic pollution increases whitecap stability and peak whitecap area up to 30%.

With the experiments in the wave tank being conducted with tap water, it is assumed that the foam formation in seawater is slightly more pronounced considering the results from the foam column setup in Fig. 3(c). Furthermore, surfactants and biological matter, e.g., plankton and proteins, in the sea surface microlayer (SML) are known to affect breaking waves, stabilize sea foam, and increase bubble persistence.^{4,35,37,60,61} Marangoni flow originating from a surfactant concentration gradient opposes drainage of the liquid foam cell walls between individual air bubbles.³⁷ Finally, biological material affects the fluid mechanics of the water surface, resulting, for example, in long-lasting surface foams.⁶² In summary, as surfactants are expected to have more impact on foam stability compared to microplastic particles, it is critical to confirm the observed effects of microplastic on foam stability also in the presence of surfactants. Therefore, we measured the effect of surfactants alone (c_surf = 0.2 mg/l, c_MP = 0.0 g/m²) and surfactants combined with microplastic (c_surf = 0.2 mg/l, c_MP = 5 g/m²) on the surface foam area in Fig. 8. The surfactant concentration of c_surf = 0.2 mg/l of the nonionic surfactant Triton X-100 is used as a model system to sea surface water with a medium surfactant concentration.² For both wave heights, ε = 0.60 in Fig. 8(a) and ε = 0.75 in Fig. 8(b), surfactants not only increase the maximum foam area similar to microplastic particles but also delay the foam decay. As expected, the stabilizing effect of surfactants is higher compared to microplastic particles. Nevertheless, the foam stabilization of microplastic particles is still apparent in the presence of surfactants. This indicates that microplastic particles impart an additional stabilization of sea foam even when other surface-active species are present.

The geophysical implications of increased sea foam stability by microplastics suggest increased air–sea interaction and ocean albedo, i.e., reflection of solar radiation. The constant exchange of climate-relevant gases between the ocean and the atmosphere is currently of special interest as part of the carbon cycle and driver of oceanic acidification. The stability of sea foam is acknowledged to contribute to the air–sea interaction, although its contribution is difficult to assess on a global scale.^38,63,64 Regarding wave albedo, it was shown by Gordon and Jacobs⁶⁵ using Monte Carlo simulations that a relatively small amount of additional sea foam can induce an increase in ocean albedo and, therefore, affect the global energy balance. As shown in Fig. 6, the water surface area covered by foam can increase up to 30%. With a more conservative estimate of 10%, the overall global coverage with albedo could, therefore, increase from 2%–4% to about 2.2%–4.4% of the ocean surfaces. As we see the increase by microplastic particles only in the foam area but not in the foam lifetime, we recommend to use these numbers with extreme caution. Another important factor that affects the solar reflection of waves is the formation of aerosols, i.e., microscopic water droplets formed during wave breaking that also reflect solar radiation.⁶⁶ It is not clear, to date, whether microplastic affects aerosol formation during wave breaking. However, microplastics may themselves be aerosolized during wave breaking and thereby enhance wave solar reflection.²⁴ Finally, from an ecological perspective, sea foam is an important habitat for marine microorganisms, which show an increased presence in sea foam compared to the pelagic zone.^67,68 Based on our results, we speculate that microplastic-stimulated biofilm formation as well as extended foam lifetime could act as a starting point of planktonic-based food supply, in particular, oligotrophic areas of the ocean.²² 

The increasing anthropogenic microplastic pollution of global water surfaces endangers surface habitats and the overall ecosystem. However, the influence of plastic debris on geophysical processes is widely unexplored. Here we investigated the effect of microplastic debris on the foam stability of breaking waves. We show that microplastic particles increase foam stability in fresh and seawater in a foam column setup. In wave breaking experiments, the formation of foams from smaller breaking waves is more affected, resulting in a foam area increase in up to 30% confirming results from the foam column experiments. In water that contains both surfactants and microplastic pollution, the foam stability is even more enhanced. We propose that microplastic particles at the sea surface are able to hinder drainage in the liquid film and possibly alter the curvature in the liquid film, which subsequently leads to lower flow rates inside the film. This was supported by microscopic pictures of microplastic at the air–water interface. The found increase in foam height and stability could impact essential geophysical and geochemical processes at the sea surface. In future work, more sophisticated on-site sampling of the microplastic debris would minimize artifacts due to predetermined particle size distributions and particle shape diversity set by the mesh size of the trawls. Further, the influence of biofilm formation on the particle surfaces as well as the photochemical degradation should be addressed in future work. Finally, the use of a surfactant model system closer to the biosurfactant composition in the SML as well as wave channel experiments with seawater would be advantageous.

See the supplementary material for Fig. S1, Surface tension for water with and without added microplastic and surfactant; Fig. S2, Microplastic particle size distribution; Fig. S3, Images of plastic particles after grinding, sieving, and ultrasonication; Fig. S4, Comparison of foam bubbles produced in Milli-Q water and ASW; and Fig. S5, Differences in foam height of different microplastic surface concentrations in ASW at 22 ± 1 °C and 5 ± 1.5 °C.

We acknowledge Stefan Gstöhl, Lucas Grob, Lukas Böcker, Bernd Nowack, Charlotte Laufkötter, and Robert M. Boes for helpful discussions. Caroline Giacomin is acknowledged for critically reading the final manuscript. Further, we thank Thomas Mani for the support with microplastic samples. Finally, J.B. and P.F. gratefully acknowledge support from the Swiss National Science Foundation (Grant No. 200021-175994).

The authors have no conflicts to disclose.

Jotam Bergfreund: Conceptualization (equal); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (equal); Resources (equal); Supervision (lead); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Ciatta Wobill: Conceptualization (equal); Data curation (equal); Formal analysis (lead); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Frederic M. Evers: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Project administration (supporting); Visualization (supporting); Writing – original draft (supporting). Benjamin Hohermuth: Conceptualization (equal); Investigation (equal); Methodology (supporting); Project administration (supporting); Resources (equal); Visualization (equal); Writing – original draft (supporting). Pascal Bertsch: Conceptualization (equal); Investigation (equal); Methodology (equal); Resources (equal); Supervision (equal); Visualization (equal); Writing – original draft (equal). Laurent Lebreton: Conceptualization (equal); Investigation (supporting); Methodology (equal); Resources (equal); Supervision (equal); Validation (supporting); Visualization (equal); Writing – original draft (equal). Erich J. Windhab: Conceptualization (supporting); Funding acquisition (supporting); Investigation (supporting); Methodology (equal); Resources (equal); Validation (equal); Writing – original draft (supporting). Peter Fischer: Conceptualization (equal); Funding acquisition (lead); Methodology (equal); Supervision (lead); Validation (equal); Writing – original draft (equal); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.