Scientists intrigued by Scientists are watching Hawaiian waters closely this summer for a recurrence of a mysterious algal bloom that surrounded the islands last year.
blooms of plant life
in Pacific 'desert'
The phenomenon
affects fisheries and
atmospheric carbonBy Helen Altonn
haltonn@starbulletin.comThey are puzzled as to what caused rich plant life in an ocean that is generally barren in summer months.
"We'd like to know how nature did it, turned the desert into a rain forest, at least for a month," said Dave Karl, University of Hawaii oceanographer.
The bloom was an enormous mass of rapidly growing large-celled phytoplankton (diatoms), seldom seen around Hawaii.
It spanned more than 600 miles, from south of Hawaii to the north. Another bloom was seen later in the north. The two were believed to be separate features, scientists said."We're trying to pull together the pieces and find out what caused the bloom," Robert R. Bidigare of the UH Oceanography Department said. "We're not sure why it happened, but we know it's important for two main reasons: It was very large and it lasted a long time. It came and went, but it was around for six months (June to December)."
It blanketed island waters for about a month.
"We've actually made a movie of this," Bidigare said. "You can see chlorophyll explode around the Hawaiian Islands."
It appears to propagate to the northeast, he said, but the scientists are questioning what would cause that.
Hawaiian waters in the summer normally are nutrient-poor with very low chlorophyll concentrations, scientists said.
"Something caused a nutrient increase that may have stimulated these blooms," Bidigare said, speculating that possibly several events were responsible.
Such blooms are significant because phytoplankton concentrations allow more carbon up the food chain and provide more foodstuff for fish, he said. The northern area was an active swordfish fishery before it was closed to fishing, he noted.
The phenomenon raises a lot of questions, Karl said: "Why does the open ocean bloom? How does it bloom? What makes it tick?"
The blooms were tracked by NASA's SeaWiFS, a sensor on the Orbview-2 spacecraft, and monthly measurements in the Hawaii Ocean Time series at Station Aloha, about 60 miles north of Hawaii.
Sampling at the Hawaii Ocean Time site confirmed that what the scientists were seeing from the satellite images was a phytoplankton bloom, and the organisms were diatoms, scientists with the National Marine Fisheries Service said.
They were looking at satellite images, studying monk seals in the Northwest Hawaiian Islands, and the bloom "just popped up," said David G. Foley of the Fisheries Service. "Suddenly, there was a huge injection of biomass in an area where we normally don't have any.
"It looked like starfish crawling," he said, describing the huge blob of organisms.
"We've been monitoring phytoplankton around the islands since the fall of 1997, and this is a very striking feature -- we've never seen anything like this before around Hawaii," said Jeffrey J. Polovina of the Fisheries Service.
Phytoplankton density increased threefold at the surface, he said. High mahimahi catches occurred in the bloom area in July, but there are no data to determine the bloom was responsible, Polovina said.
Since a low-nutrient environment "favors small guys," Hawaii usually has small-celled plankton, Foley said. "The diatoms were 10 times the normal size of critters we are used to seeing."
He said measurements at Station Aloha showed diatoms in 1997 and 1998 but nothing in 1999, so scientists were very surprised to see them last year in such high density.
Karl said last year's bloom was larger and more visible on the surface than anything seen in the past, but he believes it is a recurrent phenomenon and that scientists will start seeing it again this summer.
Satellite images have been available only three years, and Hawaii Ocean Time provided the first series of ocean measurements, he pointed out.
"The satellite sees something anomalous because we're not expecting to see it. Station Aloha data say it's weird and unexplained but not without precedent."
Karl said other blooms have been seen at Station Aloha, where scientists have been studying the ocean and microbial life since 1988.
Bidigare said the UH Oceanography Department, National Marine Fisheries Service and Joint Institute for Marine and Atmospheric Research have been collaborating for five years to study blooms, eddies and other features in an ocean region where fisheries are extremely important.
NASA recently funded mathematical models to estimate the productivity of the blooms, and ships and satellites are being used to look at the physics and biology and try to understand how the marine food web responds to physical forces, he said.
Researchers are continuing measurements this month at the ocean station, Karl said, noting that the National Science Foundation has approved a two-year extension of the Hawaii Ocean Time program because of the accomplishments.
"We have an instrument sitting on the bottom of the ocean looking up," Karl said. "It catches everything that comes up in the upper water column."
Foley, Hawaii Coastwatch coordinator and Fisheries Service scientist with the Joint Institute for Marine and Atmospheric Research, said there could have been a bunch of diatom patches last year that looked like a big mat.
He said the phenomenon is important because "these patches comprise a rich, verdant, as yet unidentified potential habitat for a variety of pelagic organisms."
Diatoms tend to concentrate organic carbon in large particles that can transfer quickly up the food chain, he said.
High diatom concentrations also can stimulate release of toxins, he said.
Bidigare said UH oceanographer Mike Landry analyzed the phytoplankton and confirmed three species of diatoms in the bloom. "As far as we know, they were not toxic."
Karl said the open ocean needs to be fertilized with nutrients to get greening, just as in a garden, and all the nutrients are found in the deep water.
"It's easier to get those nutrients into the surface ocean in wintertime than summertime," he explained, "because in the summer you have a heated surface that makes it harder to mix, by the wind, let's say."
He said the blooms may result from added iron in the ocean. But most of it comes here from the Gobi Desert in the spring, so the summer diatom outburst remains a mystery.
Karl said diatoms are a potential pathway for removal of carbon dioxide from the atmosphere because they grow quickly and produce a large biomass. "If they sink out, they take with them a lot of carbon, which initially was carbon dioxide.
"We would very much like to understand how these blooms come and go because ultimately they're impacting the carbon balance of Earth, more than just the local environment," he said.