Posted on: Tuesday, September 4, 2001
Scientists probing mysteries of seaweed propagation
By Jan TenBruggencate
Advertiser Science Writer
A pile of rotting limu stinks up a beach at Kane'ohe on O'ahu, or perhaps at Kahului on Maui.
It's because of pollution, right? Or it's seasonal, or maybe a function of winds and currents.
It could be any of those things and any of several others, say Hawai'i's seaweed experts.
"One place is Kanaha Beach Park on Maui. There's always something on the beach there, sometimes rare algae," said retired University of Hawai'i botany professor Isabella Abbott, the dean of phycologists — seaweed experts — in the state.
Another Maui location, off Lipoa Street in Kihei, has an interesting mix of algae that aren't dead. They are drift algae that are always wet and wash in and out with the tides.
"That's a good place for collecting gracilaria," the edible seaweed known in Hawaiian as manauea, she said.
The winds and currents are probably the primary reasons these locations have piles of seaweeds. But there are many others.
Botany professor Celia Smith called the business of washed-up seaweed "a pretty complicated scenario."
It certainly is not entirely the fault of human activity, she said.
"Water motion will crop biomass from the reef even in a pristine environment," she said.
For some species, breaking off pieces may be a technique for spreading themselves. It may be that they have evolved to break off in the surf, so the free-floating chunks can "replant" themselves on the reef. Some may be driven ashore, but others may find places on the reef, or be washed off the back of the reef into deeper water.
"Some work out of my lab suggests that self-pruning — we call it fragmentation — is a competitive advantage for weedy species. Even if they tumble off the reef and still are in light, they can make it," she said.
Most seaweeds may be doing this at some level, and some of them have developed unique ways to get their genes spread. The native Halimeda discoidea is often bitten by fish, then spit out apparently because it doesn't taste good, and the bite-size pieces can settle and regrow, Smith said.
A few species may indeed react to human effects and the presence of nutrients, such as sewage and agricultural runoff, by growing so aggressively that the next set of big waves breaks them free.
Smith said nutrients may be associated with blooms of the alien red alga Hypnea musciformis and the native sea lettuce alga Ulva fasciata, whose Hawaiian name is limu palahalaha. But maybe not.
"Are they more responsive to nutrients? That's an active area of research," she said.
Much about seaweeds remains a mystery.
"We don't exactly know what makes seaweeds bloom," said Karla McDermid, a botanist in the marine sciences department at the University of Hawai'i at Hilo.
A pile of seaweed on the shore means that somewhere, seaweed is growing fast, presumably in response to some environmental condition, she said.
"It's not just nutrients. It might be warm water, long sunny days, or calm waters. Each species has a different suite of features that allows it to grow quickly and reproduce," McDermid said.
Overfishing can also play a role, because some limu may naturally be kept in check by the fish and crustaceans that eat them.
Maui residents have argued that the development of the Kahului Harbor breakwater prevented natural flushing of the bay there, allowing piles of smelly limu palahalaha to collect in the corner of the bay.
Ultimately, scientists may not yet have the answers in particular situations. Marine scientists are still trying to figure out exactly why the wispy, green marine alga known as Cladophora sericea causes massive blooms every few years off Maui. One such bloom has been under way this summer.
Early theories that nutrients from shore or sewage injection wells are causing the occasional cladophora blooms have been discounted. Maybe it's a particular weather pattern, current, or other feature.
Smith said the seaweed blooms may be messages about changing conditions in the environment.
All three researchers said there is still much to be learned about seaweed and the marine environment. In the past year, experts have located 28 species never before recorded in the Hawaiian Islands, and seven that are entirely new to scientists.
