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Humboldt Field Research Institute

Posted August 23, 1999

Satellite data shed light on Penobscot Bay

Every day, satellites pass over Maine and the Maritimes, scanning the environment below and beaming the information back to Earth. Since 1986, electronic images have been accumulating like pictures in a family album. For scientists like Andrew Thomas, associate professor in the School of Marine Sciences (SMS), this image record has become a valuable research and teaching tool for describing the Gulf of Maine environment.

Thomas is a specialist in remote sensing technology and director of the Satellite Oceanography Data Laboratory (http://wavy.umeoce.maine.edu/sodl.htm) in SMS. He uses satellite images to understand how temperature and circulation patterns in the world's oceans change from season to season and year to year with consequences for marine ecosystems. Such changes can explain, at least in part, the constant ebb and flow of salt water species. His projects range from the North and South Pacific and the South China Sea to the Gulf of Maine.

Satellite data can fill in “one piece of the puzzle” for biologists who need to understand, for example, why lobster populations rise and fall or what causes blooms of harmful algae, says Thomas. While biologists may be prime users of his results, the images of temperature patterns in ocean waters have also intrigued fishermen and other citizens interested in how water temperatures change from year to year.

Penobscot Bay

Lately, Thomas and his colleagues have been taking aim at Penobscot Bay and the Maine coast. The research team includes the Island Institute in Rockland and the Bigelow Laboratory for Ocean Sciences in Boothbay Harbor. At UMaine, Huijie Xue, Neal Pettigrew, Bob Steneck and Joe Kelley, all faculty members in SMS, also work on the project. They have employed students who are learning to organize and analyze the data. The project is funded by a four-year grant from the National Oceanic and Atmospheric Administration (NOAA) to the Island Institute.

Penobscot Bay is the heart of Maine's lobster industry. It has one of the world's most productive lobster fisheries and accounts for about half of Maine's total lobster haul. Historically, parts of the bay are thought to have provided spawning beds for cod, although local cod populations were fished out earlier in this century.

“Of real interest to us is whether or not this piece of the coast is oceanographically behaving like the coast east and west of us, and whether our study years are average conditions,” says Thomas. “You can get a good handle on the big picture in space and time with satellite data.”

Thomas' lab receives images (called “scenes” in the trade) directly from the satellites each day using a tracking dish and ground station in Libby Hall. These data are supplemented by historical data supplied by scientists at the University of Rhode Island, where they adjust the satellite images to account for changes in satellite technology and atmospheric conditions. They also correlate the data with information from ships and buoys.

“For historical studies, looking for subtle changes over time, this is a very good data set," says Thomas. To date, Thomas has received all scenes from 1990 to 1995. By this Fall, he expects to have data for the decade starting in 1986.

Once the data arrive from URI, Thomas combines the daily scenes into monthly, seasonal and yearly averages, compares them against each other and looks for significant anomalies. The results to date haven't contained any big surprises. “The dominant seasonal pattern is that in winter, the coldest water is in-shore and there's a gradient to warmer water off-shore. In summer, this flips around. The coldest water is off-shore and the warmer water is near shore, due to the Eastern Maine Coastal Current. With the satellite data, we're looking at changes in the strength and spatial pattern of this, from year to year.”

Just as the Gulf Stream brings warm water north along the East Coast, the Eastern Maine Coastal Current brings cold water southwest along the Maine coast from the Bay of Fundy to Mt. Desert Island, where it often veers offshore toward the open Gulf.

Colder in 1992

The satellite images clearly reveal temperature differences from year to year. “1992 looked like a really cold year, and 1990 and 1994 looked fairly warm. For example, you can really see how much colder it was in 1992 off Owl's Head than it was in 1990. And you can see the positions of various frontal features and how strong they were,” says Thomas.

“It looks like the Eastern Maine Coastal Current was stronger and surface temperatures were colder, extending further to the west into the vicinity of Penobscot Bay in 1992 than in 1990,” he adds.

Such results can give scientists important clues about the forces at work. Temperature trends that are consistently up or down across the entire region, for example, suggest that large-scale regional, not local, factors are the cause. On the other hand, local weather or circulation factors can cause temperature trends to vary from place to place. “What goes on near-shore may not be what happens off-shore,” says Thomas, “and the same goes for areas to the east and west along the coast.”

The circulation pattern in Penobscot Bay is also starting to become apparent. “Penobscot Bay has two major channels. Oceanographically, the eastern channel behaves much more like the outer part of the bay than the western channel. The western channel is usually distinguished by a fairly strong frontal zone that seems to separate water off shore of Owl's Head with that closer to Vinalhaven.

“This is of interest because according to Bob Steneck (SMS professor studying lobster population trends), different lobster settlement patterns occur in each area. Surface temperature plays a major role in when these guys settle.”

Over the next two years, scientists will expand the temperature record and probe it to determine what patterns are most closely associated with ecological changes such as shifts in lobster or sea urchin populations. “It may not be absolute temperature that's the important thing for a particular species or particular behavior. It may be the relative position of a frontal zone or perhaps the timing within the season of a particular event or something like that. It may be where the coastal boundary of the Eastern Maine Coastal Current was that year,” Thomas explains.

Ultimately, because they are indicative of circulation and seasonal variability, the temperature patterns measured by satellites are an important scientific tool. Numerical modelers will use temperatures to improve their equations and test their results as they attempt to mimic natural systems more closely.

“It's a matter of making careful observations and being able to make some predictions based on a reasonably wide set of standard deviations. A case in point, we're getting much closer with the El Nino simulations in the Pacific. In the late 1980s, the models were waffling around, but with the latest El Nino (1997-98), a couple of the models did quite well with predicting how well it would hit certain areas.

“Our overall goal is to use satellite data to their fullest potential for ecological management. As we work toward better predictions of abundance of future fisheries and potential harvests, they provide a unique view of the environment. Satellite data won't answer all the questions, but by combining these data with in-situ measurements and numerical modeling, better answers to many of our monitoring and management questions can be found.”

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