Underwater photos help map artificial reefs for growing seaweed beds

Greg Howard
4th March, 2026

Underwater photos help map artificial reefs for growing seaweed beds

The study site at Himeshima Island, Japan (a, b) is an artificial reef where boulders were placed across a varied seascape of sandy bottoms and natural cobble/boulder ridges (c, d), allowing for a direct comparison of which bottom type is more suitable for restoring macroalgal beds.

Image adapted from: Kanki et al. / CC BY (Source)

Key Findings

  • This study, off Himeshima Island, Japan, mapped seafloor vegetation to find ideal locations for restoring macroalgal beds
  • Macroalgal growth thrived on artificial boulders placed on sandy bottoms, supporting diverse species like Sargassum and Undaria
  • Boulders on natural rocky reefs showed limited vegetation, often only short-lived seaweed, suggesting sandy bottoms are more favorable for restoration
Macroalgal beds – underwater ecosystems formed by large algae – are vital for healthy coastlines, providing oxygen, food, and shelter for marine life. However, these beds are declining globally due to various human impacts, prompting restoration efforts. A key challenge in these restoration projects is identifying locations where new macroalgal beds are likely to thrive.[1] Kyushu University and Okayama University researchers recently addressed this issue by investigating suitable conditions for establishing macroalgal beds, focusing on the use of artificial reefs. The study took place off Himeshima Island, Fukuoka, Japan, where the local government created an artificial reef in 2016 using man-made boulders to encourage macroalgal growth. The research team used a technique called underwater photogrammetry – essentially creating a detailed 3D model of the seafloor – with a very high resolution of 0.07 meters. This allowed them to map the distribution of seaweed vegetation on the artificial reef with great precision. The results revealed a striking difference in vegetation growth depending on the location of the boulders. Boulders placed on sandy bottoms supported rich macroalgal communities, including species like Sargassum and Undaria pinnatifida. However, boulders positioned on existing natural cobble or boulder reefs showed very limited vegetation, primarily a short-lived seaweed called Colpomenia sinuosa, and most had no vegetation at all. Furthermore, boulders near natural reefs, or those clustered together, also exhibited poor vegetation growth. This finding suggests that the type of seafloor substrate is crucial for successful macroalgal bed creation. Sandy bottoms appear to be more favorable than natural rocky reefs. The reasons for this aren’t fully explained in the study, but it may be related to competition with existing organisms on natural reefs, or differences in water flow and nutrient availability. These findings build upon earlier research highlighting the negative impacts of coastal urbanization on seaweed communities[2]. That study showed a loss of seaweed biodiversity in urban areas, including a decline in certain species and a shift in algal composition. While that research focused on the effects of urban development, it underscores the broader issue of habitat alteration affecting seaweed ecosystems. The current study moves beyond simply documenting decline, and actively seeks solutions for restoration. The importance of considering multiple factors affecting macroalgal recruitment is also supported by work on Cystoseira forests[3]. That study demonstrated that the combined effects of ocean warming and acidification negatively impact the recruitment of these forest-forming macroalgae, and that the presence of associated crustose coralline algae is also important. This highlights that successful restoration isn't just about finding a suitable physical location, but also about the environmental conditions and the presence of other species. Interestingly, the impact of grazing pressure on macroalgal beds, as shown in research on Cystoseira amentacea[4], wasn’t directly addressed in this study. However, it’s a factor that could potentially explain the limited vegetation observed on boulders near natural reefs. Higher fish populations around natural reefs could lead to increased grazing pressure, hindering macroalgal establishment on the artificial structures. The researchers concluded that installing boulders at a low density on sandy bottoms, and avoiding areas close to natural rocky reefs, would be the most effective strategy for creating new macroalgal beds. This provides valuable guidance for future restoration projects, emphasizing the importance of careful site selection and habitat consideration.

EnvironmentEcologyMarine Biology

References

Main Study

1) Seafloor vegetation map of man-made boulders reef by underwater photogrammetry: Suggestions for site selections in macroalgal bed creations

Published 2nd March, 2026

https://doi.org/10.1371/journal.pone.0341865

Related Studies

2) Coastal urbanization leads to remarkable seaweed species loss and community shifts along the SW Atlantic.

https://doi.org/10.1016/j.marpolbul.2013.09.019

3) Climate change and species facilitation affect the recruitment of macroalgal marine forests.

https://doi.org/10.1038/s41598-022-22845-2

4) Reduction of herbivorous fish pressure can facilitate focal algal species forestation on artificial structures.

https://doi.org/10.1016/j.marenvres.2018.04.007


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