Enhancing Sydney’s Marine Life with Colorful Concrete: The Impact of Red, Green, and Yellow

When marine scientists gave Sydney Harbor seawalls a colorful glow up, they discovered it boosted marine biodiversity along our increasingly urbanized, concrete-dominated shorelines.

A new study published in the Journal of Applied Ecology by researchers from Macquarie University and the Sydney Institute of Marine Science has found the color of concrete can significantly affect which marine organisms make their homes in urban seawalls.

Their findings suggest a simple, low-cost design tweak—adding color—could help revive marine life along concrete-dominated coastlines.

As cities expand into the sea, natural shorelines are increasingly replaced by concrete seawalls, pilings and pontoons. These built structures now dominate, replacing not only habitat structure, but also the rich palette of colors found in nature.

Most marine infrastructure is made from plain gray concrete which is strong, cheap, and durable, but visually uniform and biologically unfamiliar. Natural shorelines, by contrast, feature a rich palette of colors. These colors don’t just look good, they can influence how marine species interact with their environment, the scientists discovered.

“Many marine animals respond to light and color when choosing a place to settle,” says senior author Dr. Laura Ryan, from Macquarie University’s School of Natural Sciences.

“So we asked: if we make concrete more colorful, can we improve marine biodiversity?”

Testing the rainbow theory

To test this, the team created colored concrete panels—red, yellow, green, and standard gray—and attached them to seawalls around Sydney Harbor. Over 12 months, they tracked which organisms settled on each panel and whether fish eating them influenced the outcome.

They found marine invertebrates and seaweeds colonized panels differently depending on the panel color. Red panels in particular supported communities distinct from other colored panels, attracting higher numbers of green algae and barnacles.

These differences held even when fish grazed freely on the panels, suggesting that color effects were not driven by helping creatures blend in, but were influencing where larvae settle.

“We were surprised that even after the panels were fully covered in marine growth, the original color continued to influence which species were present,” says Holly Cunningham, first author on the study.

“It shows that surface color continues to matter long after the surface is no longer visible.”

The color effects also varied according to the panel’s location on the seawall, with color showing much stronger effects in the lower parts of seawalls that were underwater for longer periods.

Using the light

The researchers also discovered that the effects weren’t just about what organisms could see, they may relate to how different species use light.

“Red panels may represent high-quality habitat for green algae, which capture light for photosynthesis in the blue and red spectrum,” the study notes.

Meanwhile, brown algae, which absorb light differently, showed greater associations with gray and green surfaces than with red ones.

Perhaps most surprisingly, the study found these color preferences persisted throughout the entire 12-month experiment, with the original color continuing to influence which species thrived even when the concrete was no longer visible underneath layers of growth.

Until now, projects such as Living Seawalls aimed at restoring habitat to marine infrastructure have focused on adding texture, such as grooves and crevices, to mimic natural habitats.

This study suggests that adding color using long-lasting pigments like iron oxides, may also make a meaningful difference to seawalls.

As coastal cities grow, it’s becoming even more important to understand how changes in the colorscape affect marine life, says coastal ecologist Professor Melanie Bishop from Macquarie University, supervising author and co-leader of the Living Seawalls project.

“We are trialing more of these colored panels in the water now,” says Professor Bishop.

“Incorporating color into marine design is practical, affordable, and easy to scale, potentially bringing back a forgotten sensory cue that many species rely on, so if we keep finding that color matters, the eventual outcome would be to incorporate color into living seawalls in a way that mimics native environments.”

Professor Bishop says traditional gray infrastructure could inadvertently create environments hostile to many native species, by eliminating the visual cues these organisms have evolved to recognize and respond to over millions of years.

Co-designing infrastructure with nature in mind not only supports marine biodiversity, but also sustains vital marine ecosystems impacting clean water, fisheries and carbon storage.

“This study suggests engineers and planners should consider matching the brightness and colors of artificial structures to the dominant colors found in local natural habitats, so they can give native marine life the visual cues they’re naturally programmed to seek out.”

This content was originally published on The Macquarie University Lighthouse.