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Algae, macroalgae, kelp – there are many different names for the plants of the ocean, but in Hawaii it’s limu.
Before contact with the west, limu was an important part of Hawaiian culture and way of life. It is commonly found in food, but has also been used as a medicine and in cultural practices such as making lei or dyeing clothes. One variety, limu kala, was often part of hoooponopono practices — ceremonies of reconciliation — as a way to ask for forgiveness while participants ate or held the plant.
As the basis of the marine food chain, limu also plays a crucial role in tidal ecosystems, providing food and shelter for smaller invertebrates and herbivores.
But in recent decades, native limu and their myriad species have faced many challenges in Hawaii’s waters. Land development and groundwater pollution, along with invasive algae species and climate change, have created a deadly combination for Limu.
Veronica Gibson, a graduate student at the University of Hawaii Manoa, has been studying limu for over 10 years, and even she says we’re just beginning to understand it. What has become clear, however, is the role people play in shaping the future.
“We as humans are the ecosystem engineers who decide what becomes invasive and how we control our impact on those ecosystems,” she said.
Gibson believes that when more people know what native ecosystems look like, they can report abnormal changes.
“We want to manage it for many generations in the future so they can enjoy these things and not lose the biodiversity, productivity and culture associated with these systems,” she said.
Solving the problem is complicated. But it starts with understanding what is invasive and why.
The Department of Land and Natural Resources has a comprehensive list of invasive algae species in Hawaii. Known for taking over native species, dungweed, harrier, and even a so-called “suffocating 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 outrun corals, and introduced limu can learn to adapt to their environment.
“I’m thinking of ‘private’ or ‘alien’ as a status,” said Ryan Okano, program manager for DLNR’s Aquatic Resources Division. “To me, invasive is a trait that can be expressed by introduced species under unnatural circumstances.”
Gracilaria salicornia, known as Gorilla ogo, was originally brought to Hawaii as food. The small and stocky species has been introduced from Hilo to Oahu, and although some use it for pork or poke, supply has outstripped demand.
Gibson has monitored the proliferation of gorilla ogo in Oahu waters for the past 12 years, and even participated in ogo cleanups in Waikiki, and believes that serves as cautionary tale.
“Be careful what you introduce because it’s really hard to predict what’s going to happen,” Gibson said.
Due to fragmentation, or asexual reproduction by a single fragment, gorilla Ogo quickly conquered the east and north coasts, 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 coordinator at Kua’āina Ulu ‘Auamo, has watched invasive limu overtake native species over the past 50 years.
The coasts of Ewa and Kahe were once prized for their many varieties of limu. Ito recalls that the beaches were covered with different shades of green that locals took home.
Limu populations took a hit when increased urbanization and inland agriculture affected the groundwater Limu need to thrive in coastal waters. Native limu didn’t stand a chance when invasive species were introduced to Hawaiian waters for aquaculture in the 1970s and 1980s.
Ito now spends time sharing his knowledge of Limu with communities looking to restore it to their shores. Known as Uncle Wally, he often takes students and other community members on “limu walks” where he studies the growth and species of limu on various Oahu beaches.
There are many shades of limu, both literal and figurative. It’s not just about good limu versus bad limu. Scientists need to think about how invasive algae negatively and positively affect entire ecosystems.
Limu is a food source for limpets, sea urchins and fish and acts as a protective home for small marine life. Some Limu even help create sand and build reefs.
“It’s not just about controlling ‘bad’ limu,” Okano said. “We also need to think about what we’ve done to these ecosystems.”
Limu needs nutrient-rich groundwater to live, but when human impacts pollute it, even native Limu can take on invasive traits. Sewage, cesspools, land development and conventional farming can have serious consequences.
“Native species will grow very quickly and try to absorb all of these nutrients,” Gibson said. “But when there’s too much algae, it starts to rot and reduces oxygen levels, forcing the fish to leave.”
Matters are further complicated by climate change, particularly the effects of rising sea levels and warmer waters.
Kanoe Morishige, a coordinator of Na Maka Onaona, has studied populations of limu, opihi and haukeuke (sea urchins). She predicts that prolonged periods of high temperatures and low wave action will cause Limu to die off. This, in turn, can alter the food and habitat for fish while providing space for invasive species to thrive.
“Changing the timing of these types of aspects of our environment can really offset the growth of these populations in general,” she said.
It’s difficult to maintain an ecosystem’s required balance, she said, and it’s only degraded by invasive species and off-season changes in the water bodies.
With over 500 species identified in Hawaii, Nicole Yamase knows we are just beginning to grow in understanding of Limu and its species. The Micronesian graduate student appreciates studying Limu in Hawaii for its importance to Hawaiian culture.
“I really want to bring that connection home and fill those knowledge gaps,” she said.
There has been so much cultural practice involving limu in Hawaii, in part because of its sheer abundance and accessibility. So what happens when there is less native limu?
“No Limu, no culture,” Yamase said.
With Hawaii proclaiming 2022 the “Year of Limu,” there’s still a lot to learn. But for Yamase, it shows that people care about bringing indigenous Limu knowledge and awareness to the community. She is currently studying Limu Kala, a style Ito hopes will become the state Limu.
Morishige said there was more to learning about limu as a diet; It is a way of bringing people together and promoting traditional knowledge.
“Limu traditions are linked to an intimate understanding of place and a kuleana that our fishermen and people have for their broader communities,” she said.
And Limu will be the proverbial canary in the coal mine when it comes to the health of our coastal ecosystems, she said, so paying attention is important. She knows that changing the limu will affect opihi and haukeuke populations, then fish populations and further up the food chain as all of these systems are interconnected and rely on each other to thrive.
“Limu will be our first indicator on the coast of what’s happening in the ocean and on land,” Morishige said.
Civil Beat’s climate change reporting is supported by the Hawaii Community Foundation’s Environmental Funders Group, the Hawaii Community Foundation’s Marisla Fund, and the Frost Family Foundation.
“Grown in Hawaiiis funded in part by grants from the Ulupono Fund of the Hawaii Community Foundation and the Frost Family Foundation.