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Spring 1999

Science Bits

Oysters and corals suffer from warmer climate

Two recent studies have highlighted the adverse impacts of a warmer, more polluted ocean on the seas' denizens. Both studies were reported at the January meeting of the American Association for the Advancement of Science.

One group of researchers studied the spread of two parasitic oyster diseases in the Chesapeake and Delaware Bays and northward as far as Maine. Eileen Hofmann, John Klinck (both of Old Dominion University), Susan Ford, and Erick Powell (both of Rutgers University) were funded by the National Sea Grant College Program's Oyster Disease Research Program. The scientists looked at the historical record of monitoring for the oyster diseases MSX and Dermo, warm-water parasites that infect a variety of oysters around the world. They found direct evidence that increased winter water temperatures have been important in the recent outbreak of MSX along the Mid-Atlantic and Northeastern Atlantic coastline.

The team developed two models that simulate the host-parasite-environmental interactions of Eastern oysters and the pathogens that cause MSX and Dermo. The models are physiologically based, structured around the proliferation and death rates of both parasites under different environmental conditions. Data were from long-term field observations and from field and laboratory experiments. Simulations using environmental conditions characteristic of Delaware and Chesapeake Bays reproduced the observed seasonal disease cycles and consequent oyster mortality.

Hofmann's report has implications for resource management of the shellfish. Oysters are valuable commercially and important environmentally because of their tremendous filtering ability, which can help stabilize coastal estuaries' environmental systems. Hofmann said, "We have to manage the disease populations with a long-term climate perspective, which means that you have to be aware of such occurrences as an El Niño or other climatic effects. You cannot set management strategies based simply on what you've done in the past."

Cornell University ecologists have discovered that hundred-year-old corals are succumbing to diseases they previously survived. In this case, worsening pollution adds to the stress of increasing ocean temperatures.

C. Drew Harvell of Cornell organized a session on ocean diseases at the AAAS meeting. Speaking in the session was Kiho Kim, a postdoctoral research associate with Harvell, who reported on an unusual disease in Florida Keys corals.

Kim said that monitoring of sea fan corals in the keys, where up to 40% of sea fans are infected by a fungal disease and many have already died, suggests that lower water quality and higher ocean temperatures stress corals and increase their susceptibility to disease. He said the Florida findings support a growing consensus among scientists worldwide that as ocean ecosystems become degraded they will offer more favorable places for disease outbreaks and the emergence of new pathogens.

"We didn't begin our study of sea fans to monitor death and destruction," Harvell said. "Originally, we were interested in the natural disease-resistance properties of corals, such as the antibacterial and antifungal chemicals they produce, because some of those compounds may be useful in human medicine. That disease resistance normally keeps a coral alive for hundreds of years, despite living in an ocean full of potential pathogens."

Harvell said that Garrett Smith (University of South Carolina at Aiken) was responsible for tracing the sea fan disease to a common, soil-dwelling type of Aspergillus fungus, washed out to sea by land erosion. "Somehow, a soil pathogen that was best known for infecting aged and immune-compromised humans has crossed the land-sea barrier," she said. "Now, one of our jobs is to discover what has compromised the resistance of the corals at some sites." Although a significant number of sea fans have died at a few sites, at many locales they recover from infections, pointing to the success of their natural resistance.

Lately in the Florida Keys, coral death has been occurring so suddenly and rapidly that Harvell and Kim must monitor their research sites three times a year. Harvell and Kim conduct their studies from the Keys Marine Laboratory in Long Key, with the assistance of Reef Relief in Key West. Their research is supported by NSF, NOAA, and the New England Bio Labs Foundation.

Old Dominion University, Rutgers University, Cornell University


La Niña puts a new twist on tornado season

This year's La Niña event may stir up twice as many tornadoes as occurred last year in Indiana, Arkansas, Mississippi, and the western parts of Kentucky and Tennessee, according to Purdue University graduate student Suzanne Zurn-Birkhimer.

In a study reported at the National Severe Storms Conference last fall, Zurn-Birkhimer compared the variation in tornadoes for El Niño years versus La Niña years on a state-by-state basis for an 81-year period. Her findings show more tornadoes in the central and southern plains and the Gulf Coast during strong El Niño years, with a shift to more tornadoes in the lower Midwest, the Ohio and Tennessee valleys, and the mid-Atlantic region during La Niña years.

Zurn-Birkhimer calculated the positions and strengths of the polar and subtropical jet streams during El Niño and La Niña events from 1916 to 1996 to study the effect on the distribution and strength of tornadoes in the Tornado Alley region of the United States. "During an El Niño event, the polar jet stream--which carries cold, dry air from the north--shifts south, bringing cooler air to the Midwest and Southeast," she says. "This cooling effect might also serve to suppress tornado activity in those areas." By contrast, during a La Niña event, the subtropical jet--the jet stream that brings warm, moist air from the south--shifts to the far north, bringing an influx of warmth and moisture to these regions and increasing the odds for tornadoes.

Despite the widespread blame El Niño received for all of last year's weather woes, Zurn-Birkhimer says her study shows there is little evidence that the phenomenon is associated with changes in national tornado activity.

"La Niña events, however, seem to favor an above-average annual number of tornadoes in select geographical regions," she says.

Purdue University


Filamentary structure of atmospheric sprites confirmed

Stanford researchers have put red sprites under a telescope and found that they consist of thousands of fiery streamers, each only a few meters wide. The streamers are caused by a buildup of electrostatic charge in the atmosphere created by strokes of lightning.

For several years, members of Stanford's Very Low Frequency Research Group (VLF) have been studying this dazzlingly bright but subliminally brief form of lightning that occurs high in the atmosphere above large thunderstorms. Last year the researchers proposed that sprites may have such a filamentary structure.

To test this hypothesis, the team acquired a small telescope capable of viewing the features of red sprites in unprecedented detail and installed it in a prime sprite-viewing location, the Langmuir Laboratory of the New Mexico Institute of Mining and Technology. Before this study, observations of sprites had been made from a considerable distance with low-light-level video cameras. These images could not distinguish features less than about 150 meters in size, so details the size of the predicted streamers were not visible.

The main problem with getting detailed pictures of sprites is their brevity. With a lifetime of milliseconds, individual sprites do not last long enough for a telescope operator to zero in on them. "Fortunately, our knowledge of sprite behavior came to the rescue," said Umran Inan, head of the VLF research group. "We knew from past observations that, when a storm kicks up sprites, they tend to appear repeatedly at about the same place for 10, 20, even 30 minutes." That knowledge allowed the telescope operator, Elizabeth Gerken, to catch sprites in the act by positioning the telescope at the location of a previous sprite and waiting for its successors.

The pictures show the streamers that the researchers had predicted, but their shape and complex arrangement came as a complete surprise. "What we are seeing is not explained by any theory. We are all somewhat at a loss," said Stanford graduate student Christopher Barrington-Leigh, a member of the research team.

In the images, most of the streamers are vertical, but many are tilted at a variety of angles. Many streamers are present in one frame and absent in the next, meaning that they have lifetimes of less than 17 milliseconds. But some individual branches seem to persist through several frames. Most leave solid streaks. Others turn on and off, creating what appear as dashed lines or beaded forms.

Stanford University


Environmental engineers unravel evaporation paradox

Temperature, precipitation, stream flow, and cloud cover records all indicate that warmer, rainier weather is now more common in many regions of the world. But one set of figures has not conformed: the readings from the simple metal pans used to measure evaporation at weather stations. These data indicate that less moisture has been rising back into the air from these pans. How, some scholars have asked, could there be less evaporation when more rain and snow are falling?

Marc Parlange (Johns Hopkins University) and Wilfried Brutsaert (Cornell University) say that evaporation-pan records do indeed support predictions made by computer simulations of the impact of increased greenhouse gases. The paradox arose, they say, because many researchers have misinterpreted the evaporation measurements.

Parlange and Brutsaert found that decreasing pan evaporation does not necessarily mean that less evaporation is occurring in the surrounding landscape. A key reason is that the interpretation of the pan measurements has not taken into account the role of humidity in the air or the moisture that already exists in the surrounding landscape. For example, a pan of water placed outside in a hot, dry desert would evaporate very quickly. But put the same pan in a cool rain forest, and the water would evaporate much more slowly. Thus, evaporation figures may drop in some areas merely because more rain and snow have saturated the terrain.

The moisture level of the region around the weather station must be factored in when climate researchers interpret raw measurements from evaporation pans, the authors say. When it is, the paradox disappears, and evaporation rates fall in line with other signs of climate change. In their Nature article, Parlange and Brutsaert assert that "in many situations, decreasing pan evaporation actually provides a strong indication of increasing terrestrial evaporation."

Johns Hopkins University, Cornell University


GOES data show impending volcanic eruption

By using a satellite as a long-distance thermometer, scientists from the University of Hawaii (UH) were able to see signs of an impending volcanic eruption from space for the first time.

"We've never had a way to remotely monitor volcanoes for impending eruptions," said Luke Flynn, a UH volcanologist. The research, supported by the state of Hawaii and NASA, was presented at the December meeting of the American Geophysical Union.

On 20 May, a Guatemalan volcano named Pacaya erupted an ash cloud that blanketed Guatemala City and the local airport 30 kilometers (19 miles) away. University of Hawaii scientists saw it coming. Seven days before the eruption, the volcano started to heat up. A NOAA Geostationary Operational Environmental Satellite (GOES) saw the heat in the form of radiation. And an automated computer system at UH acted as an eruption alarm by picking the hot spot out of the satellite data.

University of Hawaii researchers Andrew Harris, Eric Pilger, and Harold Garbeil, along with Flynn, developed the new processing technique that allows data from two GOES satellites to go straight to the World Wide Web via the UH computer within about 11 minutes. The system has been used in Hawaii for the past two years to monitor fires and lava flows.

Using the UH system, observers have noticed eruptions starting on Mexico's Popocatepetl and in the Galapagos Islands. The system has also been used to monitor fires in Florida, California, and Hawaii, and it continually monitors a section of the Amazon rainforest where there is ongoing burning due to deforestation.

The GOES data need careful interpretation because sometimes GOES detects features that are not related to volcanoes or fires, such as cloud effects or areas of solar-heated ground. "We are simply providing a widely accessible tool to view eruptions and fires in real time," Flynn said. "We leave the final decision of whether or not an eruption is in progress to the experienced scientists at the respective observatories."

University of Hawaii


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Last revised: Tue Apr 4 15:10:30 MDT 2000