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I’m swimming under a sidewalk. The cantilevered slab of concrete is a couple of meters over my head, part of Seattle’s Central Waterfront area that is famous for Pike Place Market and the tourist stroll of chowder houses and souvenir shops. It’s not your average sidewalk: it’s been embedded with translucent glass bricks that allow light to hit the seawater. Like many of the other enhancements to the recently rebuilt sea wall, it’s an act of eco-engineering intended to improve marine habitat in the waters of Elliott Bay, in Washington State.

Thanks to the glass bricks, I can see some of the other subsurface innovations. Most obvious is what’s right in front of me: the concrete face of the sea wall itself, which has a cobbled, river stone texture and angled shelves that encourage the growth of algae and invertebrates. Below me, the seafloor has been built up with mesh bags stuffed with rocks, known as marine mattresses; these reduce the water depth and make the sea wall area more hospitable to juvenile salmon, which are evolutionarily programmed to prefer shallow, nearshore waters. As for the light-delivering sidewalk, it’s intended to boost seaweed growth and create a more inviting passage to shade-avoidant salmon smolts.

It’s mid-September, still technically summer. Bobbing in a drysuit and snorkel in the polluted waters of Elliott Bay would not usually be my first choice for ocean recreation, but I’m tagging along with two University of Washington (UW) habitat biologists who are counting fish and other marine creatures near the sea wall to see these enhancements close-up. They’re all elements of the US $688-million Central Waterfront redesign and, together, they help create habitat that more closely mimics a natural shoreline. This includes niches, hidden surfaces, shadow, sunlight, and micro-currents that promote the growth of the tiny organisms young salmon feed on—and shallower waters that provide juvenile fish greater safety from predators.

While the improvements may sound humble, they’ve made Seattle a pioneer in a growing trend: cities that want to have their sea walls and their sea life too.

Sea walls have been around for as long as there have been cities built near the ocean. Some of the earliest-known sea walls were built in Byzantium (later Constantinople, now Istanbul) in the second century CE. There might not be a need for sea walls if humans built their homes 10 kilometers or so from the sea. But we don’t. Besides a love of waterfront views, one of the more obvious factors in our need to congregate on the coast is our use of ships for trade and transportation, and the need for infrastructure to receive them and their cargo.

Whatever the motive, humans continue to build close to the ocean, and sometimes—as with most of Seattle’s waterfront tourist attractions—right over top of it. Constructing barriers, like sea walls and breakwaters, is essential to protect architecture from extreme tides, waves, and storm surge (where winds drive seawater onto land at levels far above normal high tide). Left alone, coasts constantly change shape. Cities build barriers to control the shoreline—to keep erosion at bay and their infrastructure intact. Recent extreme weather events, like hurricanes Katrina and Sandy that hit New Orleans and New York City, have shown the devastation very large storm surges can wreak upon coastal towns and cities. In some areas, Hurricane Katrina’s surge was over eight meters high and reached inland over 19 kilometers.

The trouble is that coastal armoring almost always comes at a cost for marine species, juveniles and otherwise, that depend on natural shorelines.

Jason Toft, 47, is a senior researcher at UW’s School of Aquatic and Fishery Sciences (SAFS). He has conducted creature counts on Seattle’s waterfront for over 15 years. Sparked by the city’s recognition in the early 2000s that its downtown shoreline needed habitat improvements—resulting in a small beach near Olympic Sculpture Park created in 2006, and construction of the new sea wall that began in 2013—SAFS has embarked on a long-term effort to track the effects of the new sea wall on marine organisms. Gripping plastic clipboards and pencils in their neoprene-gloved hands, Toft and his colleague have spent the past hour snorkeling along the sea wall, scrawling observations—fish and crab numbers, notes about feeding behavior—on waterproof paper.

Among other species, today’s tally turns up 468 shiner perch, 57 striped sea perch, 16 red rock crabs, four kelp crabs, and two juvenile chinook. The low number of salmon seems alarming, but Toft says it’s typical for this time of year: their numbers spike in June and then decline as the majority of smolts migrate to the ocean. The number of shiners is also typical. “Down here, perch are like wallpaper,” he says.

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About two kilometers south of the Central Waterfront lies the mouth of the Duwamish River. For millennia, the salmonid species that spawn here—including chinook, chum, and coho—used the Elliott Bay shallows as rearing habitat. Much of this was destroyed by the shoreline armoring that began when settlers arrived in the mid-1800s. Since then, development has transformed roughly 68 percent of Seattle’s foreshore. The increasingly artificial shoreline was part of a long series of blows—along with overfishing, pollution, and spawning ground degradation—for Seattle’s salmon population, and for the Duwamish people who depended on them.

Puget Sound chinook migrate into Canadian waters and range as far north as Alaska, and like other populations, are the primary food source for southern resident killer whales. The impacts noted above have reduced the historical populations of Puget Sound chinook by at least 90 percent, landing them on the endangered species list in the United States. Seattle’s willingness to invest in translucent sidewalks, cobbled surfaces, and marine mattresses has largely been a response to the desperate situation facing Puget Sound chinook.

Still, it took an earthquake to get things moving on the waterfront redesign. Seattle engineers built the Elliott Bay sea wall between 1911 and 1936 to protect the city’s urbanizing waterfront; inspections after a 2001 earthquake led government officials to realize it was overdue for replacement. Along with significant cracks in a column supporting the heavily trafficked Alaskan Way Viaduct that parallels the shoreline, inspectors discovered that the roughly 20,000 old-growth timber pilings comprising the sea wall had been eroded by salt water and tiny, wood-boring crustaceans called gribbles. This was allowing the tide to wash out the underlying soil through its holes and gaps—creating, in the city’s words, “dangerous voids” under the viaduct.

In 2010, the city began soliciting input on redesigning the Central Waterfront, including upgrading 2,160 meters of its sea wall. Government officials, local residents, and Indigenous groups all expressed a concern for the area’s native salmon populations. Undoing 150 years of urbanization is impossible, but Seattle officials hope the new sea wall will improve the situation for salmon and other marine species. To evaluate the effects, SAFS researchers like Toft will be collecting data for years to come.

The ongoing revitalization will also enhance human habitat, transforming 26 blocks of the city’s public harbor with increased pedestrian and cyclist access, better storm water management, and a rebuilt pier park with direct water access. The city began tearing out the aging Alaskan Way Viaduct altogether in February—rerouting the daily rush of 90,000 cars through a newly bored tunnel (an ambitious project that has had its share of costly complications). Phase one of the sea wall upgrade is mostly complete, but phase two, the north section that runs 1,050 meters between Olympic Sculpture Park and Pike Place Market, has not yet been scheduled or funded.

If the enthusiasm locals have shown to Toft’s team is any indication, the upgrades will continue. At the small beach near Olympic Sculpture Park where Toft and his colleague entered the water, curious visitors questioned them before and after the snorkel trip about their research—questioned enough, in fact, that the pair had to tactfully withdraw to get on with their work.

“People do get a kick out of it,” Toft says. “When we’re out here in our drysuits we get asked questions all the time. People want to know what we’re doing, and why, which is the kind of public engagement we want to see at the waterfront. Socially and culturally, it’s a really cool place to work.”

Despite the looming uncertainties around the project’s next phase, Toft calls the waterfront redesign “amazing,” giving a nod to the officials, citizens, and organizations that have made it possible, as well as the global attention it’s received.

“It’s one of the biggest efforts of its kind, in terms of a shoreline structure in a major city that’s been engineered to improve habitat,” he says. “We’ve talked about it at meetings all over the world, and everyone wants to know more about it.”

Eco-engineering urban waterfront areas is becoming a hot topic, as municipal governments worldwide attempt to be better neighbors to fish and other marine life. Major cities like Hong Kong, Baltimore, and Singapore are exploring ways to upgrade the habitat values of their existing infrastructure, while others are incorporating eco-design into new projects.

The United Kingdom encourages the trend through their 2011 Marine Act, which states that coastal development proposals “should aim to avoid harm to marine ecology, biodiversity, and geological conservation,” and offer opportunities for “building-in beneficial features” for fish and other ocean species.

Two megatrends are also feeding interest and investment in eco-engineered marine habitat—urbanization and climate change. In the next decades, it’s estimated that about 75 percent of the human population will live in coastal zones located less than 100 kilometers from the ocean—despite this being only 10 to 15 percent of available land area on the planet. “Coastal land is therefore in high demand,” one 2017 eco-engineering study noted, “and development and seaward land reclamation is occurring at unprecedented scales.” As these urban areas expand, they are transforming formerly vibrant shoreline ecosystems into built-up waterfronts and coastal defenses—a phenomenon known to researchers as ocean sprawl.

The fact that people are swarming into urban coastal zones in search of better lives is understandable. But life in coastal cities may become less desirable, and less tenable, due to the effects of climate change. Accelerating sea-level rise, storm surges, and flood events are some of the hallmarks of global warming, and they’re sending officials in coastal cities scrambling to erect shoreline defenses—to protect all the people currently pouring into them.

For marine species, more artificial structures means more negative effects on food webs that are already suffering the manifold pressures of human civilization and climate change, from overfishing to ocean acidification. As a poster child for an ocean barrier that tries to better mimic natural habitat, Seattle’s new sea wall raises two big questions. Do these interventions actually work, and if they do, can we deploy enough of them to make a difference?

Singapore, the city-state at the tip of the Malaysian peninsula, is actually an island. A thriving port for seafaring traders on the “Silk Road of the sea” since the 14th century, it expanded after British colonization began in 1819. Now Singapore is a coastal city of over five million people, and its harbors host over 100,000 large vessels—freighters, tankers, cruise ships, and barges—per month. Urban and industrial development along its coastline has drastically reduced the area’s formerly rich habitats of mangrove forests, intertidal mudflats, and coral reefs. Sea walls now armor about 63 percent of Singapore’s coastline.





Lynette Loke, a 31-year-old postdoctoral research fellow at the National University of Singapore’s Experimental Marine Ecology Laboratory (EMEL), has taken great interest in the artificial shorelines that ring much of her city. She’s been involved in sea wall habitat enhancement since 2009 and is now one of the leading researchers in the field.

With the aid of a computer modeling program Loke developed called Complexity for Artificial Substrates (CASU), she has designed, formed, and tested several generations of concrete tiles intended to boost biodiversity on and around Singapore’s hardened shoreline. Loke and her EMEL colleagues have molded tiles from cement that feature pits, towers, grooves, and triangular shapes called darts—some in consistent patterns, and some with more complex, random arrangements.

Singapore’s sea walls are mostly constructed of granite boulders, and at high tide are not a terrible place for the humble species that make up the understory of the local marine food web: seaweeds, crabs, marine worms, and snails, along with anemones, and sea squirts. But when a retreating tide exposes the rock slopes to sunlight and tropical temperatures, conditions quickly become brutal for these organisms. Loke’s tile designs help dissipate the deathly heat and retain moisture critical for survival.

“The idea of taking away a natural shoreline and replacing it with artificial systems always seemed a bit funny,” says Loke. “But after a while, when I thought more about it, I realized that what we’re doing with our tiles is basically making the best we can out of the worst.”

In the past decade, habitat researchers have been applying some rigor to the common sense assertion that foreshores transformed by urban or industrial development are not equal, habitat-wise, to those in their more natural state. Most urban marine infrastructure—such as sea walls and pilings—is vertical and uniform, in contrast with the sloping and heterogeneous topography that fish and other coastal organisms have relied on, and evolved within, for millions of years. The relative sterility of artificial structures from a marine biodiversity point of view was quantified in a 2013 study in the United Kingdom that showed its artificial coastal defenses—sea walls and sediment-trapping structures called groynes—are 40 percent less biodiverse than natural areas.

“People are beginning to think more about what’s going on at the interface between land and sea,” says Loke. “We haven’t really given much thought to that space, despite people having manipulated coastlines for millennia.”

In one experiment, Loke and her team bolted five engineered tiles to granite rip-rap on two small Singaporean islands. (The islands, Pulau Hantu and Kusu Island, are each less than five kilometers from the mainland; they were chosen instead of the mainland harbors, Loke says, to limit any human interaction with the tiles.) The 40-centimeter-square tiles were about six centimeters thick. Two of them featured concentric grooves, and the other two were inset with square pits. Each tile type had, in turn, two versions: one with consistent patterning and the other with a more complex, random pattern. The researchers also fixed a control tile that mimicked the background granite.

A year later, Loke and her colleagues returned to the islands and plucked and scraped all the organisms off each of the tiles. The resulting tally showed 4,234 invertebrates from 56 taxonomic groups. The upshot—published in a 2017 paper—was that the grooved tile with the more complex, random pattern had the highest level of species richness, about 2.5 times that of the control tile, which had the least abundance of any of them.

In short, that’s proof that eco-engineering does work, at least in the way that Loke and EMEL have pursued it. Loke herself has been a little surprised to see positive results like this emerge from her years of experimentation. “Back when I first started this work I wasn’t sure it was going to go anywhere, in terms of how a few centimeters of difference in the tile features could do anything to increase diversity. I was unexpectedly and pleasantly surprised by the results.”

EMEL’s most recent tile prototype, the BioBoss v2, is more complex than first-generation designs. Hexagonal and inset with towers, grooves, and pits, it looks like a 3D board game from a Star Trek episode. Singapore is considering deploying about 7,000 of them on Pulau Tekong, a large island east of the city, and more on the shoreline surrounding nearby Changi Airport⁠—areas that encompass a distance of about four kilometers.

These are tiny interventions on the global scale. But like others in her field, Loke sees her work as part of a larger shift in our attitudes toward coastal infrastructure. That category includes sea walls, but also the other structures that are part of ocean sprawl like jetties, piers, and ferry terminals. Such installations should no longer be looked at, Loke says, as solely serving the interests of humans and their property.

Protecting coastlines with what are called “soft options”—restoring mangrove forests and other natural barriers like salt marshes—is part of the mix. Mangrove plantings are happening in tropical countries such as Indonesia and Kiribati, and some subtropical ones like Bangladesh, and research on salt marshes as coastal flood defense has been done in the United Kingdom and the United States. But for coastal cities the predominant trend in coping with sea-level rise and other climate change impacts is building artificial, hard engineering interventions. And to date, all of these appear to diminish marine habitats rather than improve upon them.

Loke’s pioneering work at EMEL is paralleled by similar work around the world. Most of these efforts are unique engineering approaches—developed in situ to respond to the specific realities of a local ecosystem.

In the United Kingdom—that enlightened land, you’ll recall, where a plug for eco-engineering found its way into maritime legislation—Louise Firth of the University of Plymouth is working with colleagues to design one-meter high Bioblocks for shoreline protection projects. The concrete cubes, which look like giant dice, are inset with grooves and depressions to offer habitat niches.

Teams in the United Kingdom are also testing another technique to make breakwaters hospitable habitat: drilling shallow tide pools and pits into the rocks and concrete slabs of existing breakwaters. One of these experimenters is Alice Hall of Bournemouth University. The cracks, crevices, and holes she creates promote invertebrate growth and offer hiding places for crabs, juvenile fish, and other creatures. “We’ve found that these really, really simple techniques can double the diversity in some areas. Those tiny areas of refuge can make a huge difference.”

Hall is also collaborating with citizens on the Isle of Wight in monitoring the impacts of Vertipools, artificial pools shaped like vases with pointed bottoms. Underwater at high tide and exposed at low tide, they retain up to 10 liters of seawater and provide a refuge for fish, sea squirts, crabs, and the like. A local group called Artecology developed the structures, and they incorporated decorative design suggestions from local school children. Vertipools have already been tested on concrete sea walls along the coastline and fixed to the vertical walls of a local ferry terminal.

“The Vertipools have been absolutely incredible,” says Hall. “Some of them have been in place for over five years, and the biotic communities have really developed.”

In Australia, a team led by marine biologist Beth Strain of the University of Melbourne’s National Centre for Coasts and Climate is experimenting with tile designs to support native bivalve populations in Sydney and in other global ports via the World Harbour Project (WHP). Started by the Sydney Institute of Marine Science, the WHP’s goal is to promote eco-engineering and other approaches that will help build ecological resiliency into urban ports and harbors. The organization has partners in 27 cities, from Abu Dhabi in the United Arab Emirates to Xiamen in China. This kind of global collaboration for sharing best practices and experimental results is more good news for urban marine ecosystems.

But are all these varied efforts yielding real results for marine biodiversity? On that question there is additional good news, thanks to research conducted by Loke’s EMEL colleague, Eliza Heery, and by 10 other eco-engineering luminaries from universities in Australia, Europe, and the United States. After analyzing 109 studies on enhanced marine structures around the world, the group found that eco-engineering efforts boosted species abundance in all but one case.

The fact that these types of interventions work is something to celebrate. The field of eco-engineering ocean habitat is young—past its infancy, yet not beyond early adolescence. But to even partially restore and preserve coastal marine ecosystems in the face of the climate change impacts barreling toward us, it needs to grow up fast.

“Sea-level rise is going to happen,” says Hall. “We can’t do anything about that. And people want to defend the land, so there are going to be coastal defenses constructed.” Planners, engineers, and government officials need to do their best to incorporate ecological enhancements into these structures, she says: there’s no need for a sea wall to be a smooth wall of straight concrete.

Artificial sea walls and other barriers will never equal natural shorelines for habitat quality, but Hall, Loke, and others have proven ways to build them much, much better. One question remains to be answered in the long-term: could eco-engineered structures benefit the burgeoning human populations of coastal cities as much as they benefit marine snails, juvenile fish, and other sea life?

When I swam with Jason Toft along Seattle’s rebuilt sea wall in September 2018, his team hadn’t yet had a chance to analyze the data from their creature counts. Since then, they’ve reviewed their findings against baseline data collected before the sea wall upgrade, and—as they begin their second year of snorkeling surveys this spring—the results already look encouraging.

“We did see a really good first-year result as far as juvenile salmon goes,” says Toft. “It is very early to tell, but it seems that they’re responding to the new eco-engineering designs.”

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Any optimism needs to be taken with a grain of sea salt, however, says Toft, since the upgrades are so new. It takes a few years after construction of eco-engineered habitat—somewhere between three and 10—to assess how the situation is stabilizing for marine organisms. But even after the end of this field season in October, Toft says, “we’ll have a lot more to say.”

That’s a cautiously upbeat note about restoring biodiversity in urban waters. One additional thought about eco-engineered urban habitats might be worth considering: the direct impacts they have, or could have, on humans. After all, since we seem to love living close to the water, there are selfish reasons to improve marine habitat. Perhaps one day cities will clean up their harbors and restore salmon populations and other species such that we could safely fish in them—plating up our very local, sustainably harvested catches at home or in upscale restaurants. That’s a predator-prey relationship that most humans would celebrate.

In this context, a moment from my swimming tour of the sea wall comes to mind—when the UW researchers and I attracted the attention of some families leaning on the sea wall railing.

“Look, Mom!” yelled one boy. “Scuba divers!” We were snorkeling, but the finger pointing was the same given to any mammal swimming in the harbor, regardless of gear. I felt like a harbor seal.

The boy and his family—along with many other people walking the sea wall—were shocked to see people swimming the city shoreline, amid the frequent debris of styrofoam pellets and Starbucks cup lids. Mostly, they were delighted. The same was true for the Seattle beachgoers who were keen to ask questions of the neoprene-suited frogmen they watched emerge from Elliott Bay.

While I couldn’t put my finger on it at the time, what now strikes me about those interactions is that the sight of humans swimming in city waters perhaps doesn’t need to be quite such a rarity. In coastal cities densely populated by hundreds of thousands of people, humans ought to be at least as common a sight in our harbors as, say, seals.

Perhaps our more ambitious goal for urban shorelines should be designing, restoring, and cleaning them to the point that they’re inviting habitat for sea squirts and salmon smolts, and for humans—where anyone who wants to will feel free to put on a snorkel and some neoprene, dive into the chuck and see what’s below the surface.