Augmenting Coral Growth on Breakwaters: A Shelter-Based Approach

Abstract

With the increasing global population and migration toward coastal regions, and the rising demand for coastal urbanization, including the development of living spaces, ports, and tourism infrastructure, the need for coastal defense structures (CDSs) is also increasing. Traditional CDSs, such as breakwaters, typically composed of hard units designed to block and divert wave and current energy, often fail to support diverse and abundant marine communities because of their impact on current and sediment transport, the introduction of invasive species, and the loss of natural habitats. Marine ecoengineering aims at increasing CDS ecological services and the development of marine organisms on them. In this study, carried out in a coral reef environment, we examined the relationship between coral colony protection levels and three factors related to their development, namely, coral fragment survival rate, larval settlement, and water motion (flow rate), across three distinct niches: Exposed, Semi-sheltered, and Sheltered. Coral survivability was assessed through fragment planting, while recruitment was monitored using ceramic settlement tiles. Water motion was measured in all defined niches using plaster of Paris Clod-Cards. Additionally, concrete barrier structures were placed in Exposed niches to test whether artificially added protective elements could enhance coral fragment survival. No differences were found in coral settlement between the niches. Flow rate patterns remained similar in Exposed and Sheltered niches due to vortex formation in the Sheltered zones. Survival analysis revealed variability between niches, with the addition of artificial shelter barriers leading to the highest coral fragment survival on the breakwater. This study contributes to the development of ways to enhance coral development with the goal of transforming artificial barriers into functional artificial reefs.

1. Introduction

1.1. Coastal Defense Structures and Breakwaters

Coastal zones are highly populated by human settlements because of their wide range of economic and industrial opportunities [1]. These include tourism, high-value real estate, harbors allowing commercial and private shipping, and other coastal-related industries such as fisheries, aqua- and mariculture, and power and desalination facilities that are built on or near the shorelines [2]. The movement of large human populations to coastal areas has been documented in recent decades and is expected to only increase in the future [3]. This movement demands the urbanization and development of coastal regions in the form of housing, infrastructure, industries, ports, and marines. Accordingly, man-made coastal defense structures are widely built to protect the coasts and human investments from ocean waves, winds, and currents [4].

Coastal defense structures (CDSs) function by diverting, minimizing, quelling, and blocking waves, currents, and other weather-generated marine forces [5]. Breakwaters, which are a type of wave breaker, are one of the most widely used CDSs and contribute to the protection of coastal infrastructure, ports, and marinas, as well as beaches and shorelines.

They are one of the most prevalent, durable, and old types of CDS. Human communities built and used breakwaters for centuries, ever since there were vessels in the Mediterranean Sea and the Persian Gulf, i.e., as early as 7000 BC [6]. Traditional common breakwaters are constructed mostly from local or quarry-originated boulders of different sizes, piled from the seabed to the surface. Modern breakwaters use variously designed concrete units, such as antifers, accropodes, cubes, dolos, tetrapods, and more [7,8]. While breakwaters aim to reduce wave dynamics and shifts in current flow, these large and hard structures can also modify the substrate composition (for example, by creating a rocky environment over a sandy one) and cause alterations in the sedimentation regime (sediment transport and distribution), shore erosion, and local turbulences [9].

Coastal environments encompass numerous ecologically valuable habitats, including coral reefs, fish nursery grounds, oyster reefs, seagrass beds, sandy environments, and wetlands, all of which provide important ecological services [10]. Yet, currently, due to the consequences of global warming, such as the increase in flooding frequency, sea level rise, and shore erosion, more and more breakwaters are being established on vast sections of natural coastlines [11,12]. These structures are built first and foremost for the purpose of protecting human endeavors. They are not built with ecological needs in mind. Indeed, some studies reported that the establishment of breakwaters resulted in habitat loss affecting local marine life communities [13] and the disruption of species life cycles, resulting in the loss of native species assemblages (alga, invertebrates, and benthic species) and the attraction of invasive species [14]. On the other hand, due to various breakwaters’ structural complexity, durability, and direct interactions with the local coastal community, the introduction of new structures such as breakwaters may increase local biodiversity by creating novel habitats that provide additional substrate, shelters, and other amenities for the local marine organisms [15]. For example, studies by Burt (2014) [16], Wen et al. (2013) [17], and Tan et al. (2012) [18] demonstrated that the deployment of some breakwaters and seawalls has resulted in ecological enhancement in their surroundings and an increase in local biodiversity and species richness.

Among the local marine species, sessile marine species are mostly affected by breakwater installation and deployment. The process of ecological succession in benthic marine habitats (to newly immersed substrate) starts when microorganisms settle on the substrate, creating microbial and microalgal biofilms or mats [19]. Further phases are the settlement of more developed sessile species, such as, calcifying algae and mollusks, called “habitat-forming species” because they create an additional biogenic layer to the original substrate. The ongoing recruitment of organisms will then attract grazing and predatory species, consequently establishing a new community in the habitat [20]. When the local conditions are less favorable for those crucial habitat-forming species, they might not recruit to the structure units of the breakwaters, leading to low biodiversity and species abundance [21,22]. However, invasive species may take advantage of the new type of habitat and niches created by breakwaters, as well as the low competition by local species found on them [23].

Breakwater units usually contain quarry-originated boulders or strengthened and smooth concrete, which are characterized by smooth surfaces [12,24,25,26] and, as such, are less suited for the settlement of sessile organisms. For example, bivalves and corals prefer to settle onto porous and coarse surfaces [26]. Breakwater units that lack surface roughness and rugosity minimize species’ ability to penetrate and attach (i.e., settle) onto the structure [27]. An additional disadvantage of quarry and concrete-molded units that are being used in breakwaters is the lack of complexity in the spaces between them.

To mitigate such negative effects on marine organisms and to enhance biodiversity in breakwaters, scientists, civil engineers, and commercial maritime companies aim to modify and increase the ecological functionality of breakwaters by implementing “Ecological Services Enhancement” (ESE) solutions [28]. Several solutions have been so far tested and implemented [28]: soft engineering [29], complexity-enhancing tiles [21], ecological concrete antifers [30], water retaining [31], etc.

1.2. Diversity in Sheltered Niches in a Breakwater

A new theory proposes that, alongside substrate conditions, localized environmental factors within a breakwater also influence marine fauna settlement and development [12]. Studies have suggested that settlement on breakwaters is reduced at the outer-exposed parts of a structure, while the inner-sheltered niches may harbor diverse and abundant fauna [12,32]. Studies on cryptic or sheltered niches in breakwaters found that fish [32], invertebrate, and algal [12] communities are more diverse in these niches. This is due to the added protection from both the abiotic stress of water motion (wave, currents, flow rate, and transport of sediment and debris) and biotic stress (predators) [32].

All of these factors can also affect coral communities, as shallow coral community structures are often shaped by wave exposure, flow rate, and mass transfer [33,34]. Overall, it is postulated that there is a positive link between high flow rates and coral health, as increased flow creates thinner boundary layers, which increase the mass transfer of gases and metabolites, increasing the rates of physiological processes [35].

Therefore, we hypothesized that (1) coral communities on breakwaters have higher survival and/or recruitment rates at Sheltered niches compared with Exposed niches and that (2) a medium (Semi-sheltered) niche on a breakwater would have the highest levels of coral survival rate. To test these hypotheses, we divided a breakwater area into three shelter niches defined by the level of protection a coral receives in that niche from incoming abiotic stress. Protection is defined as a physical barrier that covers the full volume of the coral between the coral and incoming waves, currents, or sediment drift. The three niches tested were as follows (Figure 1):

(a)

Exposed—no protection for the coral.

(b)

Semi-sheltered—one to two sides of the coral are protected.

(c)

Sheltered—three to four sides of the coral are protected.

Next, we tested the effects of artificial protection structures on coral survival and recruitment.

2. Materials and Methods

2.1. Study Site

This study was conducted in Eilat, at the northern tip of the Gulf of Eilat/Aqaba, Israel, and took place in two breakwaters (Figure 2):

• Tur-Yam breakwater at Eilat’s south marina, located at Almog’s beach (29.515122, 34.926736);
• Herod’s breakwater on Eilat’s north beach, located in front of Herod’s hotel (29.546614, 34.966668).

We wish to emphasize that no significant sea storm occurred during the period of the study (December 2022 to July 2024).

2.2. Spatial Coral Distribution at Different Niches of a Breakwater

Coral surveys conducted during December 2022 examined coral coverage at three niches at the Tur-Yam breakwater: Exposed, Semi-sheltered, and Sheltered. In total, 30 random 20 × 20 cm quadrats (total surveyed area = 5.625 m²) were surveyed, with 10 quadrats at each niche type. The results were compared by one-way ANOVA and Tukey’s HSD post hoc with RStudio (Version 4.3.3) [36].

2.3. Coral Recruitment to Breakwaters

Coral recruitment was investigated by using 10 × 10 cm ceramic recruitment tiles. The tiles were placed in an ambient running seawater aquarium for one month for acclimation and conditioning at the Interuniversity Institute for Marine Sciences in Eilat. The tiles were then deployed at the Tur-Yam breakwater on a metal net using zip ties. During July 2023, two series of recruitment plates were placed in two niches of the breakwater, Exposed (30 tiles) and Sheltered (30 tiles), using bolts (Figure 3). Following May 2024, the main coral spawning season, underwater surveys of coral spats on the tiles were performed using a blue light torch on a weekly basis. The total number of colonies settled in each set was counted.

2.4. Coral Survivability at Breakwaters With and Without Artificial Protection Structures

Coral Survivability on a Breakwater

Coral fragment survivability was tested in two layouts: (a) on different niches of the breakwater boulders and (b) adjacent to artificial protection structures (APSs).

The APSs, made of concrete with embedded metal mesh, were designed to provide varying levels of protection. They were built as simple walls without floors, intended to block water motion to different levels. The fully sheltered structure was a rectangular enclosure (15 cm high and 20 × 15 cm in length). The semi-sheltered structure had a “three-pointed star” shape, consisting of three 15 cm high and 15 cm long walls set at 120° angles to each other (Figure 4). The structures were bolted to the breakwater boulders to prevent movement. The choice of these simplified geometries was deliberate: they were designed to produce clear and repeatable differences in shelter level without introducing complex hydrodynamic effects that could obscure the influence of structural protection alone. This approach allowed the isolation of shelter as a variable affecting coral survival, providing a controlled baseline for future hydrodynamically optimized designs.

Fragments of the branching corals Stylophora pistillata and Acropora variolosa were collected from colonies grown in a coral nursery in the Northern Gulf of Aqaba. The corals were fragmented into 5 cm long straight fragments.

Coral planting was performed on the boulders of the Tur-Yam breakwater, Eilat. The boulder’s surface was cleaned using hard brushes and iron wool. The coral fragments were glued to the boulder surface, 5 cm from each other and from the walls (artificial and natural), using epoxy glue. For testing different niches, corals were planted on boulders in the defined niches, while the APS was bolted (and corals planted) on boulders in the Exposed niche only (Figure 5).

After planting, surveys were performed monthly via scuba diving for one year (from July 2023 to July 2024). Pictures of the corals were taken and then analyzed.

During the one-year period, growth was highly affected by frequent fish biting [37]. At the conclusion of this study, all fragments were returned to their original location in the coral nursery.

Data were analyzed using a Kaplan–Meier survival curve (KMSC), a widely used statistical method in medical and biological studies. KMSC accounts for both deceased and missing individuals by classifying them as “censored,” ensuring that the analysis considers coral fragments that were lost or detached without known outcomes. This approach allows for a more accurate estimation of survival probabilities despite missing data. A Log-rank test [38] was performed in RStudio (Version 4.3.3) to assess statistical differences between survival distributions.

2.5. Water Flows at Different Niches in the Field (Breakwaters) and in Laboratory Experiments

2.5.1. Water Motion Measurements

To examine the difference in water flow at each niche, the water flow rate was measured using a technique described by Doty (1971) [39], which includes the conversion of the dissolving rate of plaster of Paris Clod-Cards into flow rate in a uniform flow field. In a complex flow field, such as that in the sea, this method integrates water motion, which is equivalent to the flow rate used in the calibration. The Clod-Cards, manufactured following Doty’s protocol (Appendix A), were weighed three times (for the purpose of an average) before deployment in the test niches. After a 4-h period, the Clod-Card was taken out of the test area, rinsed in freshwater, and left to dry for 7 days. Afterward, the Clod-Cards were weighed three times.

2.5.2. Calibration Curve (Flow Tank)

To determine the relationship between the Clod-Card dissolving rate and the flow rate, a preliminary calibration curve was conducted. The flow tank (L = 3 m W = 38 cm H = 42 cm; Figure 6) includes a flow stabilizer that turns the incoming turbulent flow into laminar flow. Sea salt (@Red Sea salt) was used to mimic the ocean salinity conditions of 35 ppt. Sets of 12 Clod-Cards were placed at different flow rates of 32.5, 40.8, 50, 53, 83.6, and 86 mm/s. The linear equation derived from this calibration is y = 0.0011x + 2.4331 (R² = 0.9987), where y represents weight loss, and x represents flow rate. The Clod-Card method does not measure flow but rather the dissolving of the material and mass transfer caused by the flow. Therefore, although we report results as flow speed, it is based on a calibration curve obtained in a laminar flow. While this is true for flume experiments, in the field, the flow was most likely of some turbulence. Hence, the reported values are those of the equivalent of a unidirectional laminar flow.

2.5.3. Water Flow in Defined Niches on a Breakwater (Field Experiment)

To estimate the effect of the protection niches within a breakwater on water motion, we used the Clod-Card dissolvement method. In Herod’s breakwater, 11 Clod-Cards were placed in each niche and the control (a total of 33 Clod-Cards) (Figure 7). The results were analyzed using an ANOVA type I test.

2.5.4. Water Flow in Laboratory Conditions (Laboratory Experiment)

Using rocks, Sheltered (Figure 6, highlighted in pink) and Semi-sheltered niches were created within the flow chamber. Sets of 5 Clod-Cards were placed in each niche as placed in the field. According to the calibration curve equation, the flow rate in this experiment was fitted to the flow rate in the field (Figure 7).

2.5.5. Water Motion Inside APSs

To estimate the effect of the protection structures on water motion, we used the Clod-Card dissolvement method. In the Tur-Yam breakwater, 5 Clod-Cards were placed in each structure and the control (a total of 15 Clod-Cards) (Figure 7).

2.5.6. Data Analysis

The results were compared with RStudio (Version 4.3.3, R Core Team, 2021). Due to differences in sample size, we used a one-way ANOVA and Tukey’s HSD post hoc for the field experiment (n = 33) and a Kruskal–Wallis and a Dunn post hoc test for the laboratory and APS experiments (n = 15 in each experiment).

2.6. Sedimentation at Different Niches of Breakwater

Sediment accumulation was tested in the Sheltered and Exposed niches of the Tur-Yam breakwater. In each niche, eleven 50 mL tubes were used as sediment traps and set in an upright position with an “open lid” for two weeks. After retrieving the tubes from the breakwater, the seawater was drained, and the sample was rinsed with distilled water. Then, the samples were put in a drying oven for five days. After drying, shells and invertebrates were taken out of the sample. Each of the samples was put in an aluminum crucible that was weighed three times without and with the samples. The final weights from each site were compared.

3. Results

3.1. Coral Distribution

The coral cover percentage was predominantly composed of branching coral of the genera Acropora, Stylophora, Pocillopora, and Seriatopora. Coral cover was significantly different between the Sheltered and Exposed niches (Tukey’s HSD, p < 0.018; Figure 8). However, no significant differences were revealed between the Exposed and Semi-sheltered niches (p = 0.47), nor between the Semi-sheltered and Sheltered niches (p = 0.497).

3.2. Coral Recruitment

Coral larva recruitment showed no preference for either the Exposed (three spats recruited in general) or Sheltered (four spats recruited) niches of the breakwater during the eleven-month study.

3.3. Coral Survival

Survival Rate

Coral fragment survival did not present differences among the three natural niches (Exposed, Semi-sheltered, and Sheltered) after a 407-day study (Log-rank test, p = 0.74; Figure 9a). However, survival patterns evolved over time, with a noticeable divergence at ~180 days, where the coral fragments at the Exposed niche began to exhibit higher survival rates, compared with the fragments at the Semi-sheltered and Sheltered niches (repeated-measures ANOVA, p < 0.05). By approximately 300 days, the survival rates of the fragments at the Exposed niche had declined sharply, leading to a convergence of the survival curves across all locations (Figure 9a). No difference in survival was shown between Stylophora pistillata and Acropora variolosa (Figure 9b). Among the two genera tested, Acropora fragments showed the most noticeable growth among fragments, yet this did not correspond to a higher survival rate.

In contrast, coral fragments that were planted near artificial protection structures (APSs) presented significant differences in survival rates after 317 days (Log-rank test, p < 0.05; Figure 10). The survival curves diverged distinctly, with the Sheltered APS demonstrating significantly higher survival rates than both the Semi-sheltered and Exposed niches (pairwise, p = 0.01; Figure 10).

In both experiments, surviving coral fragments began to extend tissue onto the hardened epoxy glue used for attachment. In some cases, fragments that were broken, partly overgrown by algae, or partially bleached displayed new coral tissue and skeleton growth.

3.4. Water Flow

3.4.1. Water Flow in Protection Niches (Field and Laboratory)

Clod-Card experiments conducted both at Herod’s breakwater and in the laboratory revealed consistent trends in flow rate (water motion) reduction across niches. At Herod’s breakwater, flow rates presented significant differences among the Exposed, Semi-sheltered, and Sheltered niches (ANOVA, p < 0.05; Figure 11a). The Exposed and Sheltered niches experienced a mean flow rate of 58.95 ± 3.32 mm/s and 59.06 ± 2.71 mm/s, respectively, while the Semi-sheltered niche showed a lower flow rate of 56.64 mm/s. Tukey’s HSD post hoc found significant differences between all niches.

Similarly, the results from the laboratory study on the different rock formations exhibit a significant difference in flow rate among niches (Kruskal–Wallis, p < 0.05; Figure 11b). Post hoc pairwise comparisons using Dunn’s test with Holm–Bonferroni correction revealed that Semi-sheltered niches had a higher reduction in flow rate compared with Exposed niches (p = 0.0017), with an average flow rate of 52.7 mm/s ± 0.34. There were no significant differences between Semi-sheltered and Sheltered niches (p = 0.1275) or between Sheltered and Exposed niches (p = 0.2195), with average flow rates of 55.37 ± 2.09 and 54.6 ± 0.59, mm/s, respectively.

Dye spatial distribution demonstrated clear suction and vortex motion at the Sheltered rock formation (Figure 11(c.1–c.3)).

3.4.2. Water Flows near the APSs

Water flow at the Tur-Yam breakwater near APSs exhibited distinct flow reduction patterns (Kruskal–Wallis, p < 0.05; Figure 12). Post hoc pairwise comparisons using Dunn’s test with Holm–Bonferroni correction revealed that Semi-sheltered niches had a higher reduction in flow rate compared with Exposed (p < 0.007) and Sheltered (p < 0.02) niches, but this was not the case between Sheltered and Exposed niches (p = 1).

3.5. Sedimentation

Sediment accumulation levels were ~0.350 g sediment per two-week period, which remained consistent across all niches throughout the three-month study. While minor variations were observed among individual replicates, these fluctuations were inconsistent and did not exhibit a clear trend or significant pattern.

4. Discussion

In this study, we aimed to assess the influence of water motion on coral survival and recruitment rate at breakwaters. It was hypothesized that coral communities in Sheltered niches would exhibit higher survival and recruitment rates compared with those in Exposed niches, with optimal survival expected in Semi-sheltered niches. To examine these hypotheses, flow rates were measured across three niche types, Exposed, Semi-sheltered, and Sheltered, while one-year-long monitoring of coral fragments’ survival and larval recruitment was conducted in both Exposed and Sheltered niches.

Our findings provide insights into these hypotheses through the analysis of recruitment and survival data. After one year, no significant difference in coral recruitment rates was observed between Sheltered and Exposed niches, partially rejecting our first hypothesis. Recruitment rates in both niches were low, averaging only 13.33 spats/m² per niche, which contrasts with the higher spat recruitment of 60 spats/m² recorded by the Israeli National Monitoring Program (NMP) on nearby natural reefs during similar time periods [40].

This discrepancy may be due to the breakwater granite substrate compared with natural reefs. Nevertheless, the presence of large grown colonies demonstrates that the granite boulder can sustain and recruit coral colonies. It is also likely that the breakwater’s isolation from sense coral reefs, which can limit the availability of local larvae [41], led to low recruitment. Prior research has shown that proximity to established reefs can significantly enhance larval supply to nearby structures [42,43]. The Tur-Yam breakwater is situated nearshore at a depth of 5 m, with the closest coral sources being the “Paradise” reef, 61 m east at ca. 25–45 m depth, and the “Katza” oil jetty reef, located 1.3 km north at a depth of ca. 10 m and deeper. Sea currents in the Gulf of Aqaba move predominantly along the coastline [44], with nearly no upwelling near the west coast, possibly limiting the larval supply. Further, Guerrini et al. (2020) [45] reported a significant decline in coral recruitment in the Gulf of Aqaba/Eilat over a four-year study (2013–2017). While previous studies documented recruitment rates ranging from 18 to 96 recruits per m² per year, their findings showed a reduction to 12.01 ± 5.09 recruits per m² per year at the Interuniversity Institute (IUI) reef and 1.82 ± 0.81 recruits per m² per year at Kisoski Beach (KIS). The authors attributed this recruitment failure to a combination of biotic and anthropogenic stressors, including reproductive failures, reduced larval settlement, high post-settlement mortality, and environmental degradation associated with urban development and tourism.

Survival rates of coral fragments within the Sheltered APSs were significantly higher compared with other niches (Figure 10). In contrast, coral fragments in the breakwater Sheltered niches did not show significantly improved survival rates, indicating that these niches did not offer sufficient additional protection. These findings underscore the potential of survival-oriented design in artificial structures. The high survival rate in the Sheltered APSs highlights the benefits of incorporating elements specifically designed to enhance coral protection from physical stressors.

Throughout this study, two primary causes of coral fragment mortality were identified: natural predation by corallivorous fish, particularly parrotfish, and anthropogenic disturbances caused by scuba divers, such as fin contact and reef trampling [46,47]. These factors, however, likely varied across niche types and may have acted as confounding variables—impacting coral survival independently of shelter level. The small, enclosed design of the Sheltered APSs effectively reduced access for both large corallivores and unintentional diver contact, illustrating how niche scale and shape can significantly influence the efficacy of a shelter in protecting corals. While the direct quantification of predation intensity and human disturbance was beyond the scope of this study, future research should aim to monitor these variables explicitly in order to disentangle their effects from those of structural design. This study nonetheless demonstrates that purpose-built sheltered structures, such as APSs, can enhance coral survival on artificial structures in high-stress environments by mitigating multiple stressors simultaneously.

To date, most coastal defense structures incorporate shelter incidentally as a byproduct of construction limitations rather than intentional design. Our findings suggest that future research should focus on developing optimized APS designs to modify existing breakwaters and other manufactured structures. The APSs in this study evaluated the efficacy of enhancing the survival of small coral fragments from pioneer species, focusing primarily on early life stages. A limitation of the current APS design is the finite space available, which may restrict coral growth beyond a certain size. While results show that limited space within the APSs increased coral survival, possibly by reducing exposure to physical stressors or grazers, this configuration may not support long-term growth due to potential crowding and competition for space and resources. To enhance long-term outcomes, future deployments should reduce the number of corals per APS and potentially adopt a “one coral per APS” strategy. Moreover, future APS designs should incorporate structural features that allow coral colonies to expand beyond the initial protective zone, minimizing intraspecific competition. From a scalability perspective, the broader application of APSs in restoration must go beyond individual coral protection and align with the surrounding habitat to ensure ecological compatibility. APS designs should be adapted to local environmental conditions and attract native species, thereby enhancing biodiversity and ecological function. Effective scaling also requires the consideration of resource demands, installation feasibility, and the cumulative ecological footprint of large-scale deployment. Our design and installation required intensive manual labor to install each device on the breakwaters. New designs should take into account the ease of installation as well as impact size. It may be needed to spread out areas in which one improves coral survival to allow for spillover effects and fish movement connectivity. Ultimately, scalable APS solutions should contribute to the ecological enhancement of breakwaters and promote positive, long-term impacts on surrounding benthic communities. A limitation of the current APS design is the finite space available, potentially restricting the growth of corals beyond a certain size. Limited space within the APSs could lead to crowding and competition for resources, potentially hindering coral growth and survival.

Despite the differences shown in flow reduction in Semi-sheltered niches, this did not correspond to the higher survival rates as hypothesized. In addition, although statistically different, the level of flow reduction was low in all locations, being about 5% of the water motion levels of the exposed areas (Figure 11). This unexpected result could be attributed to relatively moderate water conditions occurring in Eilat during the study period [40], resulting in small fluctuations in flow rate, which did not affect survival rate. The absence of storm events during this period also reduced the differences between field and laboratory conditions, allowing the steady flow chamber to serve as a reasonable approximation for structural flow effects. While the flow chamber operates with a predominantly unidirectional current, its design permits some back-and-forth movement, and under the calm conditions observed, it provides a simplified yet valid model to assess the influence of structural configuration on relative flow exposure. It should be noted that sedimentation was consistently low across all niches, indicating minimal water turbulence and the limited resuspension of sediments throughout the study period. Low sediment accumulation suggests that fine particulate matter was either transported away by ambient currents or settled without substantial redistribution. Without high hydrodynamic events, minor differences in water motion appeared to have limited influence on survival compared with other stressors, such as damage from corallivores and divers. Coral distribution data showed the highest presence in Sheltered niches, followed by Semi-sheltered ones (Figure 8). This distribution pattern may suggest that partial protection from both biotic and anthropogenic disturbance may be a significant factor for coral survival in moderate water motion conditions, while increased flow and wave protection will become relevant only under high-stress hydrodynamic events, such as storms.

Despite low recruitment rates, the presence of large coral colonies on the breakwaters suggests that recruitment does occur and that in suitable environments, breakwaters can support coral communities. The overall coral fragment survival rate of 55% in this study aligns with prior survival rates reported in the Gulf of Aqaba [37], underscoring the feasibility of coral planting on breakwater structures.

The success of Sheltered APSs in enhancing coral fragment survival underscores their potential value for both existing and newly constructed CDSs. Current “Ecological Services Enhancement” (ESE) for CDSs often focuses on exposed sections, neglecting the sheltered inner sides or breakwaters, which typically support diverse marine communities [29]. Developing tools such as APSs offers a promising approach to accelerating otherwise slow natural recovery processes and mitigating the impacts of both biotic and anthropogenic disturbance. For broader adoption, integrating sheltered-based eco-engineering principles into coastal construction project management, including collaboration between regional councils, local governments, businesses, and communities, is crucial [29,48,49]. The simplicity and low-cost production of APSs make them a practical and scalable means to be used for existing coastal infrastructure. As the demand for CDSs and their ecological enhancement grows, the APS method contributes to the broader global effort to develop environmentally sound coastal defense strategies.

5. Conclusions

This study advances our understanding of coral survival and recruitment dynamics on breakwaters, specifically the role of shelter levels within these structures. While recruitment rates remained low and consistent across all niches, coral survival rates were significantly higher in artificial Sheltered niches. These findings provide partial support for our initial hypothesis that Sheltered conditions promote greater coral survival, highlighting the effectiveness of APSs as protective habitats within high-stress environments. Although Semi-sheltered niches exhibited the greatest reduction in water flow rate, they did not show the expected increase in survival rates, leading us to reject the hypothesis that Semi-sheltered niches would offer optimal survival conditions. This discrepancy suggests that moderate reductions in flow rate alone may be insufficient for enhancing survival in the absence of more substantial hydrodynamic events.

Overall, this study emphasizes the potential of APSs to enhance coral resilience on artificial structures, demonstrating their practical value for eco-engineering in coastal defense design. By implementing strategically designed APSs and other purpose-built shelters in both existing and future coastal infrastructure, breakwaters can serve a dual function as protective habitats, transforming traditional defense structures into biologically supportive environments. This integration offers a promising pathway toward sustainable coastal urbanization that aligns human development with marine conservation goals.