The Development of a Floating Mono-Particle “Sun Shield” to Protect Corals from High Irradiance during Bleaching Conditions

Abstract

Coral bleaching is occurring more frequently as the climate changes, with multiple mass mortality events recently recorded on the Great Barrier Reef. Thermal stress coupled with high irradiance have previously been shown to be primary causes for coral bleaching. Therefore, a reduction in either of these pressures could reduce coral stress and eventual bleaching. Herein, we report the early development of a novel technology capable of reducing the amount of light entering a water body by ~20% in open ocean conditions. This mono-particle “sun shield” consists of an ultra-thin monolayer material and reflective calcium carbonate particles. The monolayer enables spreading of the particles into a thin film across the water surface, with only small amounts of material needed: 7.1 g/m². A numerical modelling case study of residence times and the build-up of reactive oxygen stress in corals showed that the successful application of a stable film over the Lizard Island reef flat could reduce the reactive oxygen stress to below bleaching levels across approximately 1.5 km² of reef area. With further development, mono-particle films such as this have the potential to be deployed over at-risk coral reefs at relatively small scales during predicted heatwave conditions, potentially reducing the severity of bleaching on coral reefs.

1. Introduction

1.1. Influence of Thermal Stress and Solar Radiation on Coral Bleaching

Coral bleaching is a result of environmental stress disrupting the balance between coral and their intracellular algal symbionts, or zooxanthellae (Symbiodiniaceae). Under normal environmental conditions, zooxanthellae photosynthesis generates sugars which are translocated to the coral host cells, providing them with the majority of energy required for growth and reproduction [1]. Corals respond to some environmental stressors by expelling their symbionts, and when this occurs the corals turn pale or white [2]. When the stress occurs across large geographic ranges, this “bleaching” response can lead to mass mortality events as recorded on the Great Barrier Reef (GBR) in 1998, 2001, 2016, 2017, 2020, 2022, and 2024 [3,4,5]. The primary causes of mass coral bleaching include thermal stress during heatwave conditions, coupled with high irradiance (including photosynthetically active radiation (PAR 400–700 nm) and ultraviolet radiation (UVR, 290–400 nm) [6,7,8,9]. Mass coral bleaching occurs predictably globally when seawater temperatures reach ~1.5 °C above their long-term summer averages for durations of several weeks (often expressed as degree heating weeks) [10], especially during calm periods when light penetration is at its maximum [11]. The mechanisms leading to thermal coral bleaching involve the generation of reactive oxygen species in both the host and symbiont [2,8,12,13,14], ultimately resulting in the expulsion of symbionts by the host (bleaching) and apoptosis and tissue mortality under severe conditions [14].

1.2. Shading Interventions to Reduce Bleaching

Natural shading by turbid water [15,16,17,18,19,20], cloud cover [21,22] reef topography [23,24], algal canopies [25], and mangrove cover [26] have been shown to reduce irradiance stress, and that in turn reduces the severity of coral bleaching in some circumstances. A recent review [27] suggested that any level of shading across the UV and visible spectrum during severe bleaching conditions has the potential to reduce the negative effects of thermal stress, but this protection varies between coral species. Small-scale aquarium and reef pool trials have demonstrated that artificial shading ≥ 30% can significantly reduce coral bleaching in species from several families of coral during simulated bleaching conditions (by over 50% after 8 degree heating weeks) [28,29,30,31]. There are several proposed engineering approaches that could be applied to reduce solar radiation over coral reefs, from local to regional scales (reviewed in detail by Harrison (2024) [32]). The generation of microbubbles to “whiten” the first few meters of surface water and reflect solar radiation has been proposed [33], while “misting” with vaporised bio-oils [34] or “fogging” [32,35] with seawater droplets just above the ocean surface may scatter and absorb sunlight over areas over 10s of km². The regional protection of multiple reefs from solar radiation would require large-scale interventions such as “marine cloud brightening,” where the atmosphere is seeded with nano-sized crystals of salt and has the potential to reduce both light penetration and surface water temperatures [36,37,38,39].

Solid particles suspended in seawater attenuate light through scattering and absorption, and conditions of high turbidity have been shown in some cases to protect corals from bleaching (see above). However, chronic high turbidity can also have negative effects on corals, including reduced autotrophic energy acquisition and smothering by direct sediment deposition [40]. If particles can be concentrated at the water surface to form a mono-particle layer, then a significantly lower number of particles could be used to achieve temporary light attenuation. Myers et al. (1970) noted the application of a hydrophobic coating to particles improved formation and retention of an even and persistent film at the surface [41]. A potential solution to further improve the formation of dispersed mono-particle layer surface films is the co-application of monolayers. Langmuir monolayers are thin films which sit at the air–water interface and are self-assembled from molecules containing a hydrophilic head and a hydrophobic tail. Monolayers have a range of applications including the mitigation of water evaporation in reservoirs [42,43,44]. When applied to the surface of water, monolayers lower the surface tension, creating a surface pressure ($\Pi$), causing them to spread until they reach their equilibrium spreading pressure (${\Pi }^{eq}$) [43]. The interaction of monolayers and particles has previously been observed as a monolayer indicator when studying the spreading rate of a monolayer film [45]. During these experiments, small talc particles were incorporated into the monolayer film and were carried with the monolayer as it spread [45,46].

Here, we investigated the potential for monolayer and hydrophobic particles to form stable mono-particle layer films at the air–water interface and how they attenuate light by scattering and reflection (Figure 1). Calcium carbonate particles were selected for testing as they are naturally present in the marine environment (including the main component of coral skeleton and reef sand), and they have a good reflectivity. These particles were coated with a natural fatty acid to form a hydrophobic surface to assist in maintaining the particles at the air–water interface. The naturally-occurring monolayer components investigated were stearic acid, as it provides compatibility between the film and the particle, and cetyl alcohol which is known for its fast-spreading properties [47]. The films were optimised for particle size, composition, and application method before scaling up to outdoor tank trials and preliminary open sea trials. The attenuation achieved in the sea trials was applied in a process-driven model that predicts oxidative stress in coral symbionts [48] to demonstrate potential protection from bleaching over a heating event at Lizard Island.

2. Materials and Methods

2.1. Materials

Stearic acid-coated calcium carbonate particles with varying sizes were obtained from several suppliers and used as received, summarised in Table S1. Analytical grade ethanol and stearic acid (>98.5%, Fluka) were purchased from Chem-Supply (Port Adelaide, Australia). Cetyl alcohol (hexadecanol, C₁₆OH, > 95% purity, milled and sieved to less than 250 µm) was purchased from P&G Chemicals.

2.2. Preparation of Mono-Particle Films

Suspensions containing stearic acid and calcium carbonate particles were prepared by first dissolving the stearic acid (1 mg/mL) in ethanol. Calcium carbonate (already coated in stearic acid, see Table 1) was then dispersed in this mixture to give the required concentration, e.g., 61.6 mg/mL for the 1.8 µm particles. Ethanol was used at this early development stage as a solvent carrier to ensure that a homogeneous film was formed. Suspensions were mixed immediately prior to addition to ensure particles were well-dispersed, and the calculated volume was applied to the water surface using a micropipette to achieve the required surface coverage.

Solvent-free formulations were prepared by combining the solids directly. For example, the 1T calcium carbonate particles (d₅₀ = 1.8 μm)/cetyl alcohol (200:1 mass ratio) mixture was prepared by first weighing out cetyl alcohol (1 mg) into a glass vial followed by the addition of calcium carbonate particles (200 mg). For small-scale preparations, the solids were mixed by vortexing and stirring with a spatula before being applied to the water surface. For larger preparations, the solids were shaken with seawater in sealed containers to obtain a suspension.

The mass of particles needed for each composition was calculated using Equation (1) which is derived considering a geometric analysis with the approximation of all spheres having a radius of d₅₀, and located at the air–water interface, to calculate the actual area covered by the particles.

(1)${\displaystyle \frac{m}{S_A}}=\left({\displaystyle \frac{C}{100}}\right)\left({\displaystyle \frac{2\times \rho \times d_{50}}{3}}\right)$

where $m$ is the mass of particles (g); $S_A$ is the surface area of water to be covered (cm²); $C$ is the percent surface coverage; $\rho$ is the density of individual particles (2.7 g/cm³); and $d_{50}$ is the mean particle diameter (cm). The particle surface coverage ($C$) was held constant at 30% for most experiments, unless otherwise stated.

2.3. Light Microscope Imaging

Light microscope images of films with and without ethanol were taken using a Leica MZ16 stereomicroscope under 29× magnification (Leica, Wetzlar, Germany).

2.4. Small-Scale Light Attenuation Measurements of Mono-Particle Films

Small-scale films were produced in 1 L of water in a 2 L glass beaker (diameter 12.7 cm) modified with a quartz disc in the centre of the base (described in detail in Figure S1A). The light intensity below the beaker was measured with a Flame S UV–VIS-ES spectrometer (Ocean Optics, Inc., Orlando, FL, USA) over a range from 280–700 nm before and after addition to calculate attenuation (n = 3 independent replicate preparations) with an Ocean Optics DH2000 light source (Ocean Optics, Inc., Orlando, FL, USA) that produced UV and visible light.

For solid-powder films, light attenuation was measured using a Lux/Fc (foot candle) light meter with a Hydra LED aquarium light (AquaIllumination, Bethlehem, PA, USA) as the light source (described in detail in Figure S1B).

2.5. Outdoor Small-Tank Trials

Small-tank trials were conducted outdoors in full sunlight between the hours of 10:00–15:00 local time. Up to four black polypropylene tanks (55 cm × 34.5 cm) were used, each with a surface area of 1897.5 cm². The black tanks ensured only direct, and not scattered light from artificial surfaces, was measured. Seawater was kept at around a 28 cm depth and at temperatures of 27–30 °C. Each tank contained a submersible pump (Tunze Nanostream 6045, Penzberg, Germany) to generate moderate surface movement. Films were applied by either dry application of pre-mixed solid components or by suspending the components in seawater at a ratio of ~30:1 seawater:calcium carbonate prior to addition. The light intensity was measured typically within 2 h after the formation of stable films using identical calibrated light detectors (Licor LI-250A light meter with LI-192 flat cosine underwater sensors, Lincoln, NE, USA) that were placed in the tanks under the film (to measure shading) and simultaneously in an adjacent tank without film (to measure full sunlight intensity). To measure light attenuation across the spectrum of full sunlight, a Jaz spectrometer (Ocean Optics, Inc., Orlando, FL, USA) was used.

2.6. Outdoor Large-Tank Attenuation

Large-tank trials were conducted outdoors in full sunlight between the hours of 10:00–1500 local time. Two opaque 1000 L fibreglass tanks were used, each 60 cm deep with a surface area of 1.33 m² and containing a submersible pump (11,500 rpm, Hydro Wizard ECM 63, Brelingen, Germany) to generate moderate surface movement. Films were applied to one tank (same methods as for the small tanks), while the other tank was used as a film-free reference as per the small-tank trials above. The light intensity and shading were measured typically within 2 h after the formation of stable films (as described above).

2.7. Large-Scale Sea Trials

Films were applied within a 5 m × 5 m floating barrier constructed from polyethylene pipes (110 mm diameter, 6.6 mm wall thickness) joined and sealed with compression fitting elbows. The square barrier was tethered to a pontoon or boat with ropes. The film was prepared by adding the pre-mixed dry powder to a sealed 5 L plastic container along with 2 L seawater and thoroughly mixed by shaking. The container was sealed with a cap that included an inlet and outlet fitting to allow water to flow through the vessel under pressure. The inlet was connected to a bilge pump with a seawater supply to deliver water under pressure and the outlet to 4 m of 14 mm hose with a garden hose spray fitting to disperse the calcium carbonate suspension. Two sea trials were conducted under the Great Barrier Reef Marine Park Permit G18/41254.1. The first was conducted at the Australian Institute of Marine Science (AIMS) Jetty (19° 16.64′ S; 147° 3.53′ E) on 11 December 2018 between 14:00 and 16:00. The second trial was conducted near Red Rock Bay (19° 11.21′ S; 147° 0.84′ E) on 13 December 2018 between 12:00 and 15:00. A video of application in this sea trial can be found in the Supplementary Materials (Video S2). Attenuation of full sunlight was measured with a Licor LI250A light meter (with LI-192 underwater sensor, Lincoln, NE, USA) once the film had fully formed on the surface.

2.8. Modelling the Response of Coral Reefs to Shading

A coupled hydrodynamic–biogeochemical model of the GBR has recently been developed which predicts coral bleaching (assuming temperature-mediated light-driven oxidative stress leads to bleaching) [48]. This model, originating from the eReefs Project [49], has been downscaled to a number of individual reefs [50], and is used in a case study here to quantify the effectiveness of solar radiation reduction on reactive oxygen stress on Lizard Island corals (14.665° S, 145.465° E). More information on the bleaching component of the model appears in Text S1.

3. Results and Discussion

The development of a floating mono-particle “sun shield” to protect coral from high light intensity and thereby reduce the likelihood of bleaching during adverse conditions was conducted over several phases. In Part 1, initial proof-of-concept experiments were conducted in a laboratory using solvent (ethanol) to disperse the films. Following successful demonstration and optimisation of the concept, Part 2 moved onto tailoring the technology for application in the marine environment. This stage was also conducted in the laboratory but switched to a solid-based formulation and the use of an experimental setup which more closely mimicked outdoor conditions. In Part 3, testing was continued outdoors at an increasing scale, including on the open ocean. Part 4 describes a process-driven model applied as a case study to predict oxidative stress in coral symbionts to demonstrate the potential protection from bleaching by the sun shield over a heating event at Lizard Island. Finally, Part 5 discusses the benefits, limitations, and future directions for the research.

• Part 1: Proof of concept and initial development

• Mono-particle layer formation

Initial experiments to prove the concept and explore the impact of variables such as particle size, mass applied, and ratio to monolayer component were undertaken using stearic acid as the monolayer material and ethanol to create a suspension of particles. Light microscopy in Figure 2 shows 1.8 µm particles applied to the water surface with a stearic acid monolayer and solvent during (Figure 2A) and after film formation (Figure 2B). The application of particles to the water surface without a monolayer resulted in the particles remaining clumped together at the point of application and not spreading out across the water surface (Figure 2C). However, the addition of cetyl alcohol to the formulation provided a dispersive force which broke the clumps apart and formed a more evenly dispersed layer of particles across the water surface, even in the absence of the solvent (Figure 2D). The ability of this dispersed layer to attenuate light was then investigated.

• Influence of particle size on light attenuation properties

A range of particles of varying sizes from 0.15 to 3.5 µm (Table S1) were tested to determine the impact of particle size on light attenuation (coverage held at 30%, Table 1). Light attenuation at 500 nm is shown in Figure S2 (panel A) and over the light spectrum of 280–700 nm in Figure S2 (panel B). To test whether the observed light attenuation differences were due to the change in particle size and not only the mass of particles added, a second set of experiments held the mass loading of particles in suspension constant at 0.97 g/m². The resultant light attenuation at 500 nm is shown in Figure 3A, and the light attenuation results across 280–700 nm are shown in Figure 3B.

For a surface coverage of 30%, the attenuation measured at 500 nm increased with particle size from 12% ± 4.0 (0.15 µm) to 82% ± 5.5 (3.5 µm) (Table 1). However, a much larger mass of particles was required to achieve this result; for example, increasing the particle size from 1.8 µm to 3.5 µm increased the amount of calcium carbonate required from 0.97 g/m² to 1.9 g/m² (Table 1).

The light attenuation increased as the particle size was increased from 0.15 µm to 1 µm and began to drop as the particle size was further increased past 2 µm (Figure 3A). This correlates well with models of the theoretical scattering coefficient for calcium carbonate particles calculated using anomalous diffraction theory by Balch et al. (1996) who demonstrated a maximum scattering coefficient between 1 and 2 µm [51]. This drop in light attenuation performance with larger particles was also observed by Baker et al. (1996) who determined that suspensions of particles with a mean size of 8.5 µm attenuated light 15 times more than larger particles with a diameter of 48 µm [52]. These results also agree with theoretical particle light scattering behaviour where smaller particles (i.e., 0.15 µm) display Rayleigh scattering and show dependence on light wavelength [53]. As particles increase in size (>1 µm), they display Mie scattering which is independent of light wavelength and the scattering properties are particle size-dependent [53]. Attenuation across the damaging UV and blue spectrum (280–500 nm) was greatest for the 1.0–3.5 µm diameter (Figure 3B). Particles of 1.8 µm in diameter were chosen as the optimal size for further testing as they offered high and virtually flat attenuation across the entire UV–visible spectrum. The reported attenuation by 1.8 µm particles can be applied to multiple intensity metrics, including watts m⁻², µmol m⁻² s⁻¹, and lux m⁻² regardless of wavelength.

• Impact of particle surface coverage on light attenuation

The effect of adding varying amounts of particles to the water surface was investigated using the optimal 1.8 µm-diameter particle. The masses of particles used to obtain the required surface coverages are summarised in Table 1. The light attenuation performance of these mono-particle films at 500 nm is shown in Figure S3, with the results over the wavelength range from 280 to 700 nm shown in Figure S4.

As the particle surface coverage of the film was increased from 10% (0.32 g/m²) to 30% (0.97 g/m²), there was an increase in the amount of light attenuated at 500 nm (26% ± 0.73 to 65% ± 11) (Table 1). However, increasing past 30% did not result in a further increase in light attenuation. Additionally, as the surface coverage was increased above 30%, not all the particles were able to spread over the surface, resulting in some particles settling to the bottom of the container (visual observations). This is likely due to the inherent surface pressure of the stearic acid monolayer, which limits the number of particles on the surface. Based on these results, the optimum surface coverage was determined to be 30% (0.97 g/m²), which was the coverage used for subsequent experiments.

• Influence of particle/stearic acid ratio

As previously shown, the addition of a spreading agent such as stearic acid is required to successfully form a particle layer on the water surface as it aids the spreading of the particles into an even film. To determine the influence of the particle to monolayer ratio on the light attenuation performance, the particle size and surface coverage were held constant (1.8 µm and 30%, respectively), while the ratio of particle to monolayer in the composition was decreased from 123:1 to 15.4:1 (see details in Table 1 and Figure S5). Mono-particle layers without stearic acid were not able to form homogeneous particle films as the particles did not disperse the films across the surface efficiently, instead forming islands of particles, as previously shown by light microscopy (Figure 2C). The optimum ratio required to form a homogeneous film with high light attenuation at 30% surface coverage was a particle to monolayer ratio of 61.6:1. Ratios below this showed a slightly reduced performance (Table 1, Figure S5), while higher ratios were not able to form homogeneous films (i.e., large islands of particles formed with gaps in between).

• Part 2: Development of solvent-free formulations

Solvent-free formulations were developed using the optimal 1.8 µm calcium carbonate particle size. To more closely mimic in situ conditions, a different light attenuation setup was used, including a dispersed light source rather than a point source, and a detector capable of measuring light from a wider range of angles, including light scattered by the film (Figure S1, panel B).

• Choice of monolayer material

As expected, initial experiments using a solid mixture of fine stearic acid powder and calcium carbonate particles did not adequately spread across the water surface to form a homogeneous film [54,55]. Cetyl alcohol was chosen as the replacement monolayer material as it has a much quicker spreading rate, is readily biodegradable and is naturally found in the marine environment [55,56,57]. While these are positive points for the suitability of cetyl alcohol in this application, further studies are needed to test its potential for environmental harm (see Discussion). The solid mono-particle formulation was prepared by mixing fine cetyl alcohol powder (<450 µm) with the calcium carbonate particles, before application to the water surface, forming a well-dispersed film (Figure 2D).

• Film optimisation

A loading of 7.1 g/m² was found to be the saturation point for the amount of calcium carbonate particles on the surface using cetyl alcohol as the monolayer, Table S2. The addition of 2.0 and 3.9 g/m² to the water surface was able to form a homogeneous film; however, clear patches were observed on the surface due to insufficient particles for 100% coverage. Adding more than 7.1 g/m² resulted in excess particles sitting on the surface and no increase in light attenuation.

The ratio of particles to cetyl alcohol was investigated to determine the optimal performance (Figure 4, Table 2). A mass ratio of between 50:1 to 200:1 was found to yield the best performance of ~40% light attenuation. Increasing the particle ratio to 300:1 resulted in a decrease in performance to 27% ± 3.2 as the low amount of monolayer material was not able to effectively spread the particles. Increasing the amount of cetyl alcohol (ratio of 25:1) resulted in a lower light reduction (30%). This is likely due to the monolayer spreading the particles out further, forming a less dense film. Therefore, a ratio of 200:1 was identified as the optimal mass ratio as it obtained a high light attenuation with the lowest amount of monolayer material.

It should be noted that the new light setup used to measure attenuation in this section included a more dispersed source and captured more scattered light. Therefore, the absolute attenuation results cannot be directly compared to those in Part 1 (Table 1). However, for reference, one formulation from Part 1, containing the particles in an ethanol suspension with stearic acid, was tested in the alternate setup for comparison with the results also included in Table 2. At the conclusion of Part 2, the optimised formulation was identified as a solid mixture containing 1.8 µm calcium carbonate particles and cetyl alcohol at a mass ratio of 200:1 and applied to the water surface at a loading of 7.1 g/m². This formulation was used for outdoor trials at increasing scale.

• Part 3: Outdoor and larger scale trials

• Small-scale outdoor light attenuation trials

The optimised, solvent-free formulation was applied to the surface of small outdoor tanks (55 cm × 34.5 cm) containing natural seawater (Figure 5A,B). The method of application had a clear effect on film formation in small tanks. Coarsely applying the solid formulation resulted in very patchy film formation. This was improved by carefully sprinkling/spreading the solids more evenly (Figure 5B). An alternative method of suspending the solids in seawater immediately before application to the water surface resulted in the most homogenous film and therefore became the preferred method for application in most of the subsequent large tank and field applications. The light attenuation values for coarse, fine, and water suspension applications were 6%, 23%, and 22%, respectively (Table 3), which are approximately half of those obtained in the laboratory tests. The reasons for this include a possible increase in scattered light transmitting through to the base of the tanks and the tendency for films to aggregate somewhat against the pressure of tank walls, likely due to water movement from the recirculation pumps. Films formed more rapidly and evenly when the pump was on and the water was moving; however, there was little difference in light attenuation (range 18–23%) for the films formed with or without water movement.

• Large-scale outdoor light attenuation trials

The deployment and testing of the particle formulation were then scaled up to larger tanks to study the impact of surface movement on film stability and light attenuation. These studies were conducted on 1000 L outdoor tanks (92.2 cm × 144.5 cm, surface area 13,320 cm²) as shown in Figure 5C. The light attenuation by the films in the large outdoor tanks was similar to the results from the smaller tanks (16% full sunlight), confirming that up-scaling the films should result in similar light attenuation.

• Open-sea deployment of formulation

In order to contain the film on the sea surface, a 5 m × 5 m rigid floating barrier was constructed and was able to successfully contain the film, although wave action did allow some of the film to escape from both below and above the barrier. Two trials were undertaken in two different locations near Townsville, Queensland, Australia: AIMS Jetty and Red Rock Bay. The wind was approximately 10–15 km/h at both sites and this was observed to cause the film to move towards the down-wind side of the barrier. When the barrier was moved around or the wind reduced, a complete film would re-form (Video S2). The deployment trial at the AIMS jetty achieved 14% light attenuation, while the trial at Red Rock Bay (Figure 5D) achieved 22% light attenuation. These results were again consistent with shading in the small and large tanks using the same formulation (Table 3).

• Part 4. Modelling shading responses in corals

To assess the impact of reduced surface solar radiation on oxidative stress in corals, we ran simulations of a coupled hydrodynamic–biogeochemical model of Lizard Island, Northern GBR, with solar radiation reduced by 10, 20 and 30%. The simulations ran through the bleaching summer of 2016/17, and the results are shown for 1 April 2017 (Figure 6), when the thermal stress was the greatest. For simplicity, we present only the change in the reactive oxygen species (ROS) of the zooxanthellae cells which become toxic above a certain threshold [48]. Our simulations show excess ROS production is generally proportional to solar radiation; thus, the response to reduced solar radiation is a reduction in ROS content. Here, we normalise the ROS content by the content at which expulsion starts (see Text S1 for more details). The grey scale shows reefs with normalised ROS content below the expulsion threshold of 1, while blue to red indicates a normalised ROS content greater than 1 which are assumed to lead to greater, or more widespread, expulsion rates and bleaching of corals. The full-sun example considers the solar radiation experienced on Lizard Island that year (see SI). The greatest bleaching occurs in the shallowest waters. The 20% reduction in solar radiation (80% sun) that is obtainable using mono-particle films moves 1.45 km² of coral to below the bleaching threshold, while also almost completely reducing all areas to below the normalised ROS values of 1.4 (Figure 6, Table S3).

The effectiveness of shading to reduce oxidative stress will depend on the duration of intervention, and for floating reflective films this is linked to the residence time of water over a reef. In order to assess the residence time of water on a variety of reefs on the GBR, a set of 200 m-resolution model simulations were undertaken [50]. Age tracer experiments have shown that on some small reefs (such as Davies Reef off Townsville) [48], tidal-driven currents remove all the water over the reef within the duration of a rising or ebbing tide. For these reefs, the surface film would be quickly lost. On large reefs, water can reside on the reef up to a few days (such as Cockburn Reef in the northern GBR, Baird et al. [50], or Heron Island in the southern GBR) [58]. However, often this “time” is accumulated on reef sand flats with low coral cover such that there is little benefit of shading. Of more than 20 reefs assessed in Baird et al. [50], the ones with the highest residence time over shallow water and with significant coral cover are those surrounding Lizard Island [50]. This is a result of a relatively low tide range and the presence of the island which helps to steer flows. More work is needed to identify other reefs with long residence times over shallow corals that would benefit most from the application of reflective surface films during summer heatwave conditions.

• Part 5. Potential benefits, issues, and future directions

The deployment of sun shield films to reduce irradiance is one of several intervention options with the potential to reduce bleaching [32]. Other approaches include shade cloth [28], the generation of sub-surface micro-bubbles [33], surface misting with a biogenic oil vapour [34], fogging above the surface [32,35], and marine cloud brightening [32,36,37,39]. Each of these strategies have their own advantages, limitations, and risks, and all remain in early development (recently reviewed in Harrison (2024) [32] and Tagliafico et al. (2022) [27]). The main strength of the sun shield films approach is that they could be rapidly applied in specific areas identified as being at high risk of bleaching due to predicted heatwave conditions that coincide with periods of high irradiance. This intervention may also have substantial flexibility in scale: it could be applied to protect very small patches of an ecologically or economically (tourism) important reef or scaled up to square-kilometer areas of a whole reef. In contrast to some other forms of shading or water cooling, application of the film requires no permanent or large-scale infrastructure other than deployment equipment on boats. These potential benefits, as well as issues and future directions, are discussed further below.

Like marine cloud brightening, the application of the sun shield film reduces incident solar radiation at the ocean surface. The consistent attenuation by the films across the UV–visible spectrum (280–700 nm) means that the results from this study can be directly contrasted with studies that apply different intensity metrics. However, the attenuation of incident light achieved by other interventions (apart from small-scale shade cloths and umbrellas) is not well described so direct comparisons are difficult. Furthermore, the absolute attenuation achieved by each intervention might not be directly comparable as the intensities measured by the different sensors used in each study are likely to be differentially affected by the light source (artificial vs. full sunlight) and light scattering in the water [59]. The measured attenuation may be further affected by differential scattering and absorption between water bodies and the benthic habitat itself, potentially explaining the apparent differences in attenuation between the two field sites in this study. The effects of surface attenuation by sun shield films on underwater reef light fields should be studied further.

The complex interplay between the main drivers of coral bleaching: thermal- and light-driven stress [2,60], means that reducing one or both of these pressures has the potential to reduce the bleaching response [27,32,39]. In addition to reducing light pressure, shading also has the potential to reduce the heating of seawater over a reef [32,61]. Modelling indicates that the shading and cooling effects of cloud brightening, assuming a reduction in incident solar radiation by 6.7%, would substantially reduce bleaching stress across multiple reefs of the GBR [39]. In this context, the ~20% incident solar attenuation achieved by the sun shield films indicates a promising performance. Modelling (Part 4) the attenuation of solar radiation shows that even without cooling the water, this reduction in incident solar radiation is likely to be sufficient in some circumstances to reduce the severity of bleaching. It is possible that sun shield deployment on a large scale adjacent to reefs may also reduce the heat load of surface water before it moves across the reef (but that was not modelled here). Further experimental work is needed to confirm the modelling presented here to better understand under what circumstances (i.e., film longevity, duration of heat and light stress) attenuating sunlight by 20% might sufficiently reduce “light pressure” and oxidative stress to prevent bleaching across a diversity of species.

The potential scale of shading interventions varies from the colony and sub-reef scale (shade cloths), to the regional or even the entire GBR scale (cloud brightening) [32]. Challenges to the implementation of surface film intervention at scale include sufficient production of the required materials and efficient deployment at sea. At the optimum loading identified in the current study, approximately 70 kg calcium carbonate would be required per hectare (10,000 m²). The materials are already commercially available at a cost of AUD 40 (USD ~26) per 25 kg bag. This would correspond to USD 75 per hectare. Further cost reductions could be achieved for large-scale commercial supply. The potential deployment methods of surface films depend on the scale, site, and duration of intervention and could include application by aircraft (under-wing granule applicator or fertiliser spreader attachments), vessel (spraying equipment currently used to deploy oil spill dispersants), or automated from reservoirs on static buoys. For square-kilometer reef-scale interventions, deployment by ship is the only realistic option, where mechanical mixing with large volumes of seawater prior to application could be achieved. While the materials costs of this intervention could be considered low, the application costs have not been investigated and are likely to vary widely with each application method and with scale.

Surface film deployments would be informed by bleaching forecasts and guided by the modelled local circulation dynamics to ensure the reflective film is retained over the reef during periods of high irradiance [48]. Circulation modelling shows the residence time on the reef varies [50], but is rarely more than one day, so this intervention is suitable for only a subset of reefs (as described in Part 4). Increasing the residence time by physical containment of the films (e.g. by oil booms) is possible; however, our field trials indicated that films accumulate against a barrier even in light wind conditions. This accumulation can lead to film instability due to the stacking of heavy calcium carbonate particles which can start to sink from the surface. As a result of the breakdown of the film, and the generally short to moderate residence time of reefs, it is likely that films would need to be re-deployed daily. The films were stable on the water surface during the experimental trials, but more work is needed to determine whether they remain sufficiently stable to require only daily application without boom containment. The transient nature of the films means that they could be deployed only when needed (i.e., days with a high potential of bleaching) and when conditions are suitable; when no longer required, they are likely to rapidly dissipate.

As the floating film is visible to the naked eye due to the calcium carbonate particles (Videos S1 and S2), there is the potential for concern that harm could be done by the film itself [62]. Overcoming this issue will require adequate risk assessments for the natural ingredients of the films. Calcium carbonate particles are already ubiquitous on coral reefs as suspended sediments and are considered the least harmful particulates to adult corals [40] and their gametes [63]. Daily breakdown of the film at the application rate of 7.1 g m² into a temporary suspension in a 3 m water column would result in a maximum short-term concentration of 2.4 mg L⁻¹, and this in turn could result in the deposition of up to 0.7 mg cm⁻² day⁻¹; however, these levels are below the concentrations known to affect corals [40]. For comparison, carbonate sediment produced by parrotfish can reach up to 0.8 kg/m².year (or 8000 kg/ha.year) compared with 71 kg/ha per addition of the calcium carbonate film reported here [64]. Similarly, stearic acid and cetyl alcohol are both natural products found in the marine environment, and they have very low toxicities (stearic acid) or are not soluble at concentrations likely to harm aquatic species (cetyl alcohol) and are readily biodegradable [56,57,65,66,67,68]. Stearic acid and cetyl alcohol could be considered nutrients; however, proportionally they only contribute ~0.5% calcium carbonate particulate mass, within the normal range for organic carbon content within sediments of the Great Barrier Reef [69]. Furthermore, the monolayer is only one molecule thick and is not a barrier to animals at the surface or gas diffusion [70]. Nevertheless, this is an early proof-of-concept study and further experimental work will be required, before large-scale application, to confirm that these or other potential ingredients are not harmful (toxic, smothering), do not act as a substantial nutrition source, do not disorientate fauna, do not over-shade (leading to light limitation), and are rapidly biodegradable under coral reef conditions [71].

Interventions to protect reefs from coral bleaching need to satisfy strict regulatory frameworks administered by authorities designed to protect coral reefs of high conservation value such as the GBR [62,72]. The assessments undertaken as part of this process would not only evaluate potential ecological, economic, social, and cultural risks but could influence where, when, and how the intervention can be applied and should also consider the outcome of not intervening during heatwave conditions [73]. As a relatively small reef-scale intervention, the floating sun shield is not considered “large-scale” climate manipulation (geoengineering) and as such would be classified by regulators as a local rather than regional or global intervention [62]. While reducing carbon emissions remains the best strategy to protect coral reefs in the long term, the potential of this and other interventions to reduce the total number of stressful heating/irradiance days by temporarily reducing light intensity in the surface waters of reefs should be considered [27,32]. In particular, the application of floating sun shields could be further developed and applied to protect sections of reefs that are valuable for tourism or important source reefs of larvae for natural reef recovery.

Author Contributions

Conceptualization, E.L.P., A.P.N., and G.G.Q.; methodology, J.M.P.S., E.L.P., F.F., A.P.N., and G.G.Q.; software, M.E.B., E.B., and M.M.; validation, F.F.; formal analysis, J.M.P.S., F.F., A.S., M.M., E.B., and M.E.B.; investigation, J.M.P.S., F.F., and M.E.B.; resources, G.G.Q., A.P.N., and M.E.B.; data curation, J.M.P.S., F.F., A.P.N., A.S., M.M., E.B., and M.E.B.; writing—original draft preparation, J.M.P.S., F.F., M.E.B., and A.P.N.; writing—review and editing, J.M.P.S., E.L.P., F.F., M.E.B., A.P.N., and G.G.Q.; visualization, A.P.N.; supervision, G.G.Q., E.L.P., and A.P.N.; project administration, G.G.Q.; funding acquisition, G.G.Q., E.L.P., and A.P.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We also thank Paul Kurtenbach, Briony Stephenson, Joe Gioffre, and the staff of the National Sea Simulator for experimental assistance. The authors acknowledge the Bindal and Wulgurukaba people as the Traditional Owners of the land and sea where this work took place.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Floating mono-particle layer “sun shield” shading coral reef by reflecting and scattering light (A) conceptual diagram and (B) prototype monolayer floating over coral in the National Sea Simulator. A video of the sun shield application can be found in the Supplementary Materials (Video S1).

Figure 2. Light microscope images of stearic acid-coated calcium carbonate particles (1.8 µm) applied in the following conditions: (A) with a stearic acid monolayer using ethanol solvent during formation and (B) after film stabilisation, (C) alone without monolayer or solvent (D) as an aqueous suspension with a cetyl alcohol monolayer. The scale bar is relevant to all panels (A–D).

Figure 3. Light attenuation of mono-particle films with particles sizes from 0.15 µm to 3.5 µm, at a surface concentration of 0.97 g/m2 (A) at 500 nm, and (B) over a light spectrum from 280 to 700 nm. An Ocean Optics DH2000 light source was used with a Flame S UV–VIS-ES spectrometer to measure the light attenuation.

Figure 4. Light attenuation of calcium carbonate (d50 = 1.8 µm) blended with cetyl alcohol in a mixture of mass ratios. All measurements were taken with a particle loading of 7.1 g/m2. Light attenuation was measured using the second setup as shown in Figure S1 (panel B). 25:1 and 200:1 are based on one trial.

Figure 5. Outdoor trials of the solid formulation (200:1 particle: monolayer ratio, loading of 7.1 g/m2) in small-scale (55 cm × 34.5 cm) tanks: (A) experimental setup, (B) film applied as a fine powder; (C) film applied in large-scale (92.2 cm × 144.5 cm) tanks: left tank is the film-free control and the right tank has the film applied; and (D) Red Rock Bay deployment of the solid calcium carbonate and cetyl alcohol suspension in a 5 m × 5 m floating barrier (Video S2).

Figure 6. ROS content in zooxanthellae (normalised to the concentration at which expulsion is initiated) at Lizard Island at midday on 1 April 2017 with no, 10%, 20%, and 30% reductions in solar radiation over the bleaching season. Pink is the land and white is the ocean without corals. The thin black lines show 5 m and 10 m depth contours.

Table 1

Summary of film compositions tested in the laboratory and their corresponding light attenuation at 500 nm. All films contained stearic acid as the monolayer material and were formed from an ethanol suspension.

Experiment	Particle Diameter(µm)	Ratio Particle: Monolayer	Particle Loading (g/m2)(% Surface Coverage)	Attenuation(% at ʎ = 500 nm)
1: Particle diameter	0.15	5.1:1	0.081 (30%)	12 ± 4.0
	1.0	34:1	0.54 (30%)	50 ± 6.5
	1.8	62:1	0.97 (30%)	65 ± 11
	3.5	121:1	1.89 (30%)	82 ± 5.5
2: Particle loading	1.8	20.5:1	0.32 (10%)	26 ± 0.7
	1.8	41:1	0.65 (20%)	46 ± 3.4
	1.8	61.6:1	0.97 (30%)	65 ± 11
	1.8	82.1:1	1.30 (40%)	69 ± 2.0
3: Ratios	1.8	123:1	0.97 (30%)	69 ± 3.0
	1.8	61.6:1	0.97 (30%)	65 ± 10
	1.8	30.8:1	0.97 (30%)	57 ± 3.9
	1.8	15.4:1	0.97 (30%)	55 ± 11

Table 2

Summary of solvent-free film compositions developed in Part 2 and their corresponding light attenuation at 500 nm. * based on one trial.

Experiment	ParticleDiameter(µm)	Monolayer Material	Ratio Particle: Monolayer	Solvent	Particle Loading(g/m2) (% Coverage)	Attenuation(% at ʎ = 500 nm)
4: Solvent-free	1.8	Cetyl alcohol	25:1	None	7.1	30 *
	1.8	Cetyl alcohol	50:1	None	7.1	41 ± 3.5
	1.8	Cetyl alcohol	100:1	None	7.1	40 ± 1.7
	1.8	Cetyl alcohol	200:1	None	7.1	41 *
	1.8	Cetyl alcohol	300:1	None	7.1	30 ± 3.2
5: Alternate setup	1.8	Stearic acid	61.6:1	Ethanol	0.97 (30%)	18.6 ± 2.3

Table 3

Summary of details and results from the film tested in a range of outdoor trials. All used 1.8 µm calcium carbonate particles with cetyl alcohol at a ratio of 200:1 and applied at 7.1 g/m². Mean % attenuation ± SD (PAR, 400–700 nm). * range of two tests.

Experiment	Deployment Method	No. Tests	Attenuation (%)
Small-scale tank trials	Coarse powder	2	4.2–7.3 *
	Fine sprinkling	6	23.4 ± 8.7
	Suspension in seawater	3	20.7 ± 3.8
Large-scale tank trials	Suspension in seawater	4	17.2 ± 3.1
Open sea trial: AIMS jetty	Suspension in seawater	3	18.8 ± 3.4
Open sea trial: Red Rock Bay	Suspension in seawater	3	23.1 ± 1.7

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/jmse12101809/s1, Table S1. Summary of the stearic acid-coated calcium carbonate particles used in this project. d₅₀ is the median particle diameter, and d₉₈ is the diameter below which 98% of the particles reside; Figure S1: Light attenuation setups used to measure the particle film performance of (A) films added in solution, and (B) films added as a powder; Figure S2: Light attenuation of mono-particle films with particles sizes from 0.15 µm to 3.5 µm (A) at 500 nm for films with 30% coverage (0.97 g/m²), (B) at 30 % coverage over a light spectrum from 280 to 700 nm; Figure S3. Light attenuation at 500 nm for mono-particle films (1.8 µm-diameter particle) with a range of particle surface coverages (10, 20, 30, and 40%); Figure S4: Light attenuation testing results of Omyacarb^® 1T (d₅₀ = 1.8 µm) containing monolayers with a range of particle surface coverages (0, 10, 20, 30, and 40%) over a light spectrum from 280 to 700 nm; Figure S5: Light attenuation of particles with d₅₀ = 1.8 µm and a surface coverage of 30% with increasing particle to monolayer ratios (A) at 500 nm and (B) over a light spectrum from 280 to 700 nm; Table S2: Light attenuation testing results of Omyacarb^® 1T (d₅₀ = 1.8 µm) containing cetyl alcohol (50:1) solid-powder formulations. Light attenuation was measured using the second setup as shown in Figure S1. Figure S6: Schematic of the bleaching model. Note the temperature dependence of bleaching occurs because the process of RuBisCO-mediated carbon fixation is inhibited at temperatures above the summer maximum; Text S1: Modelling shading responses in corals (Part 4); Table S3. Area of corals with zooxanthellae above the expulsion threshold in the 100, 90, 80, and 70% of full-sun scenarios on reefs surrounding Lizard Island on 1 April 2017. Based on analysis of results presented in Figure 6; Video S1: A video of the sun shield application; Video S2: A video of the Red Rock Bay deployment of the solid calcium carbonate and cetyl alcohol suspension in a 5 m × 5 m floating barrier. References [74,75,76,77] are cited in the Supplementary Materials.

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