In The Lab

• Nov 01, 2005

Georges Bank Sea Squirt Colony Spreading Thin
Researchers have just completed a field survey of the invasive sea squirt colony on the Georges Bank, Mass. A wider area was searched for the sea squirt this year, and it was mapped over about twice the area observed in 2004; the colony was first discovered in 2003.

Results show that the species is present in two adjacent areas totaling 88 square miles in U.S. waters near the U.S.-Canada boundary. The very large, mat-like colonies observed in 2004 have been replaced by fewer smaller ones. The Georges Bank occurrence is the largest known infestation of colonial sea squirts in a major offshore fishing ground.

Scientists from the U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), and the University of Rhode Island (URI) conducted the survey aboard the NOAA Ship Delaware II during the last two weeks of August 2005. In the 2004 survey, the sea squirt colony was mapped over 40 square miles before weather truncated the cruise. This year's wider survey showed it to be present over at least 67 square miles of the same general area. Scientists also discovered an additional infestation of 21 square miles, possibly in an early stage, that lies 10 miles to the east of the original observations in an area that is now closed to fishing.

The research team also surveyed three sites in Canadian waters where seabed conditions are similar to those on the U.S. part of the bank and found no evidence of the sea squirt.

The mats were observed on gravel substrate that is highly productive for fish and sea scallops on the northern edge of Georges Bank. Video and photo transects made using the USGS seabed observation and sampling system (SeaBOSS) documented the distribution of the colonies in water depths of 45 meters to 65 meters (145 feet to 213 feet).

Scientists will analyze data collected on the cruise to determine if the tunicate invasion has the potential to alter seabed communities that sustain commercial fish species. There is concern that the sea squirt mats could form a barrier between fish and their prey that live in the seabed. Moreover, as no organisms have been observed to grow on the mats, it is possible that the tunicates would be an unfavorable surface for the settlement of scallop larvae and larvae of other species. Samples of the tunicate will be evaluated to determine its nutritional value to predators and to confirm identification of the species through DNA analysis.

Sea squirts are tunicates, a type of sea life with a primitive spinal cord in the larval stage and a firm, flexible outer covering, called a "tunic," in the adult stage. The Georges Bank occurrence is of the genus Didemnum. They form dense mats made of thousands of minute individuals by attaching to firm substrates, such as gravel, sea scallops, mussels, docks, seaweed, and other structures.

Tunicates can overgrow sea scallops and mussels, and they may affect other species of clams and worms that live in the seabed below the tunicate colony. This tunicate can reproduce and spread either sexually or asexually (by budding.) The free-swimming tadpoles produced by sexual reproduction live only a few hours before attaching to the seabed and forming new colonies. By contrast, fragments of colonies are long-lived and can be spread by tidal and storm currents. Controlled experiments in Cape Cod waters by USGS and Woods Hole Oceanographic Institution scientists have shown that small pieces removed from sea squirt colonies increase dramatically in a matter of weeks by budding. Thus, fragmentation of tunicate mats could promote the spread of the species.

The species is known to thrive in marine environments that lie within its preferred temperature range (28 degrees to 75 degrees Fahrenheit) and that have firm substrates and plentiful food -- conditions that are widespread off the coasts of New England and Atlantic Canada. The presence of tunicates could change gravel habitats that lie along the northern edge of Georges Bank. Didemnum cannot survive on habitats of moving sand, and therefore much of the shallow Bank crest is not threatened. The species is not yet known to occur on mud habitats that are typical of the deep basins of the Gulf of Maine.

This species of squirt also occurs along the coasts of the Netherlands and France. In North America, it has been documented in waters from Long Island, NY, to Eastport, Maine on the U.S.-Canadian border; California coastal waters; in Puget Sound, Wash.; and along the coast of British Columbia. The same species (or a close relative) is present at several localities in New Zealand. For more information, visit woodshole.er.usgs.gov/.

Dammed Dolphin Endangered
The Ganges River Dolphin has a unique look that would never be mistaken for the dolphins seen catapulting into the air at Sea World. Its long beak, stock body, and low, triangular hump in place of a "true" dorsal fin brings to mind the mythical sea monsters found in old sailors' stories.

In reality, the dolphin, Plantanista gangetica, is nowhere near the size of those mythical sea creatures. Its length measures between 4 feet and 8 feet, and it weighs between 180 pounds and 200 pounds. It has two subspecies, P. g. gangetica and P. g. minor, which were previously thought to be separate species.

Found in Bangladesh, India, and Nepal, the dolphins swim in extremely murky, silt-laden freshwater rivers. Their nearly blind eyes are believed to function only as a direction-finder. When feeding, the dolphin swims on one side, probing the river bottom with its snout and flipper, while using a process called echolocation, which emits sound waves that reflect patterns, to seek out specific fish it favors, such as gobies, catfish, and carp.

Several factors have contributed to the species' decline: water pollution, poaching, and accidental capture in fishing nets.

The most critical factor has been the continued construction of numerous dams and barrages along the Ganges-Brahmaputra river system, which has severely fragmented the dolphins' habitat. Exact population numbers are not known -- approximately a few thousand exist worldwide -- but conservationists are convinced that the current population is large enough survive if adequate conservation measures are taken soon.

Numerous efforts have been made by the World Wildlife Fund, which has assisted in a number of educational initiatives, as well as the rescue of dolphins trapped in canals. Also, the recent discovery that fish oils could be used in place of dolphin oil as bait has offered some good news regarding reductions in poaching.

Some local governments, such as the Sind government, have given the dolphins full legal protection and have established reserves, a move that has shown some success at increasing the populations. For more information, visit www.panda.org.

Scientists Say Sun's Role In Global Warming Larger Than Previously Thought
According to reports by two Duke University physicists, at least 10 percent to 30 percent of global warming measured during the past two decades may be due to increased solar output rather than factors such as increased heat-absorbing carbon dioxide (CO₂) gas released by various human activities. The physicists said their findings indicate that climate models of global warming need to be corrected for the effects of changes in solar activity. However, they emphasized that their findings do not argue against the basic theory that significant global warming is occurring because of CO₂ and other "greenhouse" gases.

Nicola Scafetta, an associate research scientist working at Duke's physics department, and Bruce West, a Duke adjunct physics professor, published their findings online Sept. 28 in the research journal Geophysical Research Letters. West is also chief scientist in the mathematical and information sciences directorate of the Army Research Office in Research Triangle Park. Scafetta's and West's study follows a Columbia University researcher's report of previous errors in the interpretation of data on solar brightness collected by sun-observing satellites.

The Duke physicists also introduced new statistical methods, which, they assert, more accurately describe the atmosphere's delayed response to solar heating. In addition, these new methods filter out temperature-changing effects not tied to global warming.

According to Scafetta, records of sunspot activity suggest that solar output has been rising slightly for about 100 years. However, only measurements of what is known as total solar irradiance gathered by satellites orbiting since 1978 are considered scientifically reliable, he said.

But observations over those years were flawed by the space shuttle Challenger disaster, which prevented the launching of a new solar output detecting satellite called ACRIM 2 to replace a previous one called ACRIM 1.

That resulted in a two-year data gap that scientists had to try and bridge by relying on data from other satellites.

"But those data were not as precise as those from ACRIM 1 and ACRIM 2," Scafetta said in an interview.

Nevertheless, several research groups used the combined satellite data to conclude that that there was no increased heating from the Sun to contribute to the global surface warming observed between 1980 and 2002.

Lacking a standardized, uninterrupted data stream measuring any rising solar influence, those groups surmised that all global temperature increases measured during those years had to be caused by solar heat-trapping "greenhouse" gases, such as CO₂, introduced into the Earth's atmosphere by human activities. But a 2003 study by a group headed by Columbia's Richard Willson, principal investigator of the ACRIM experiments, challenged the previous satellite interpretations of solar output. Willson and his colleagues concluded rather that their analysis revealed a significant upward trend in average solar luminosity during the period.

Using the Columbia findings as the starting point for their study, Scafetta and West then statistically analyzed how the Earth's atmosphere would respond to slightly stronger solar heating. Importantly, they used an analytical method that could detect the subtle, complex relationships between solar output and terrestrial temperature patterns.

The Duke analyses examined solar changes over a period twice as long -- 22 versus 11 years -- as was previously covered by another group employing a different statistical approach.

"The problem is that Earth's atmosphere is not in thermodynamic equilibrium with the sun," Scafetta said. "The longer the time period, the stronger the effect will be on the atmosphere because it takes time to adapt."

Using a longer 22-year interval also allowed the Duke physicists to filter out shorter-range effects that can influence surface temperatures but are not related to global warming. Examples include volcanic eruptions, which can temporarily cool the climate, and ocean current changes, such as el Nino, that affect global weather patterns.

Applying their analytical method to the solar output estimates by the Columbia group, Scafetta's and West's paper concludes that "the sun may have minimally contributed about 10 to 30 percent of the 1980-2002 global surface warming."

This study does not discount that human-linked greenhouse gases contribute to global warming, they stressed.

"Those gases would still give a contribution, but not so strong as was thought," Scafetta said. "We don't know what the Sun will do in the future. For now, if our analysis is correct, I think it is important to correct the climate models so that they include reliable sensitivity to solar activity. "Once that is done, then it will be possible to better understand what has happened during the past hundred years."

For more information, contact Monte Basgall (919) 681-8057, or by e-mail at monte.basgall@duke.edu.

This article originally appeared in the 11/01/2005 issue of Environmental Protection.