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Seaweed, macroalgae, kelp — there are many different names for the plants of the ocean but in Hawaii, it’s limu.

Before Western contact, limu was an important part of Hawaiian culture and lifestyle. Commonly found in food dishes, it was also used as medicine and in cultural practices like making lei or dyeing clothes. One variety, , was often part of hooponopono practices — reconciliation ceremonies — as a way to seek forgiveness as participants ate or held the plant.

As the basis of the marine food chain, limu also plays a critical role in intertidal ecosystems as it provides food and shelter for smaller invertebrates and herbivores.

But over the last couple of decades, native limu and its innumerable varieties have encountered many challenges in Hawaii’s waters. Land development and groundwater contamination, along with invasive algal species and climate change, have created a deadly combination for limu.

Veronica Gibson, a doctoral student at the University of Hawaii Manoa, has been studying limu for over 10 years and even she says that we are just at the beginning of understanding it. What has become clear though is the role people are playing in shaping its future.

“We as humans are the ecosystem engineers who decide what becomes invasive and how we control our impacts on these ecosystems,” she said.

Gibson believes that if more people are aware of what native ecosystems look like, they will be able to report abnormal changes.

“We want to manage it for many generations into the future, so they can enjoy these things and not lose the biodiversity, productivity and culture associated with these systems,” she said.

Tackling the problem is complicated. But it starts with understanding what’s invasive and why.

The Department of Land and Natural Resources has a comprehensive in Hawaii. Known for taking over native species, muck weed, hook weed and even one called “smothering seaweed” make the cut.

Typically, invasive macroalgae are defined as alien species that dominate reefs and inhibit the growth of other plant, invertebrate and fish populations. But even native limu can overtake coral and introduced limu can learn to adapt to their environments.

“I think of ‘introduced’ or ‘alien’ as a status,” Ryan Okano, program manager for the DLNR Division of Aquatic Resources, said. “Invasive, to me, is a characteristic that can be expressed by introduced species under unnatural circumstances.”

, known as gorilla ogo, was originally brought to Hawaii as food. The small and stocky species was introduced from Hilo to Oahu and although some use it for pork or poke, supply outgrew the demand.

Gibson has watched the spread of gorilla ogo in Oahu waters over the last 12 years, even participating in ogo cleanups in Waikiki, and believes it can stand as a cautionary tale.

“Be careful of what you introduce because it’s really hard to predict what will happen,” Gibson said.

Due to fragmentation, or the asexual reproduction through a single fragment, gorilla ogo quickly took over east and north shores where its native counterpart, limu manauea, thrived.

“It changes the ecosystem with its abundance, but it’s not desirable,” Gibson said.

Local limu expert Wally Ito, who recently retired as a coordinator with , has observed over the past 50 years how invasive limu has overtaken native species.

The were once prized for their abundant limu varieties. Ito remembers the beaches being covered in various hues of green, picked up by locals to take home.

Limu populations took a hit when increased urbanization and agriculture inland affected the groundwater that limu needs to thrive in coastal waters. Native limu didn’t stand a chance when waters in the 1970s and ’80s for aquaculture.

Ito now spends time sharing his knowledge of limu with communities looking to restore it on their shorelines. Known as Uncle Wally, he often takes students and other community members out on “limu walks,” where he’ll survey the growth and types of limu at various Oahu beaches.

There are many shades of limu, both literally and figuratively. It’s not just good limu versus bad limu. Scientists have to think about how invasive algae are impacting entire ecosystems, negatively and positively.

Limu is a source of food for limpets, urchins and fish, and it functions as a protective home for small marine life. Some limu even help to create sand and build up reefs.

“It’s not just about controlling ‘bad’ limu,” Okano said. “We also have to think about what we did to these ecosystems.”

Limu needs nutrient-rich groundwater to live, but when human impacts pollute it, even native limu can take on invasive traits. Wastewater, cesspools, land development and conventional agriculture can all have grave effects.

“Native species will grow really fast, trying to take up all those nutrients,” Gibson said. “But if there is too much algae, it will start to rot and decrease the oxygen, forcing the fish to leave.”

Further complicating the matter is climate change, especially the effects of rising sea levels and warmer waters.

Kanoe Morishige, a coordinator for , has studied limu, opihi and haukeuke (urchin) populations. She predicts that extended periods of high temperatures and little wave movement will cause limu to die back. That, in turn, can change the food and habitat for fish while creating room for invasive species to thrive.

“If the timing of these kinds of aspects of our environment change, that can really offset the growth of these populations in general,” she said.

Keeping the needed balance of an ecosystem is hard, she said, and it’s only worsened by invasive species and out-of-season changes happening in the waters.

With over 500 identified species in Hawaii, Nicole Yamase knows we are just at the beginning of understanding limu and how it grows. The Micronesian doctoral candidate appreciates studying limu in Hawaii because of its importance to Hawaiian culture.

“I really want to bring home this connection and bridge these knowledge gaps,” she said.

There were so many cultural practices with limu in Hawaii in part because of its sheer abundance and accessibility. So what happens if there’s less native limu?

“No limu, no culture,” Yamase said.

As Hawaii deemed 2022 “,” there is still much to be learned. But to Yamase, it shows that people care about bringing native limu knowledge and awareness to the community. She is currently studying limu kala, a species that Ito hopes will become the state limu.

Morishige said it’s more than just learning about limu as food; it’s a way to bring people together and champion traditional knowledge.

“Limu traditions are tied to an intimate understanding of place and a kuleana that our fishermen and people have to their wider communities,” she said.

And limu will be the proverbial canary in the coal mine when it comes to the health of our nearshore ecosystems, she said, so it’s important to pay attention to it. She knows that if limu changes, it will affect the opihi and haukeuke populations, then the fish populations and on up the food chain because all of these systems are interconnected and rely on each other to thrive.

“Limu will be our first indicator on the shoreline as far as what’s happening in the ocean and on land,” Morishige said.

Civil Beat’s coverage of climate change is supported by the Environmental Funders Group of the Hawaii Community Foundation, Marisla Fund of the Hawaii Community Foundation and the Frost Family Foundation.

“Hawaii Grown” is funded in part by grants from the Ulupono Fund at the Hawaii Community Foundation and the Frost Family Foundation.