The Chesapeake Bay's waters are warming, in some places more rapidly than the region's air temperatures, researchers from the University of Maryland say. If unchecked, scientists say, the trend could complicate costly, long-running efforts to restore the ailing estuary, worsen fish-suffocating dead zones and alter the food web on which the bay's fish and crabs depend.
Drawing on remote sensing by satellites, researchers at Maryland's Center for Environmental Science found that water temperatures have risen since the 1980s across more than 92 percent of the Chesapeake and its river tributaries. The study appeared in the October issue of the journal Remote Sensing of Environment.
The increase averaged nearly 1.2 degrees Fahrenheit per decade, according to Andrew J. Elmore, co-author of the study and a geologist at the center's Appalachian laboratory in Frostburg.
Baltimore and other parts of the bay showed up as "hot spots," with warmer water than surrounding areas. Elmore and his co-author, Haiyong Ding, a visiting scholar from China, say the heat anomalies appear to be linked to spreading urbanization and to warm-water discharges from power plants.
"In the case of power generation, we're deliberately increasing the temperature of water," Elmore said. "With urbanization, it's not intentional, but it's an effect."
As buildings and pavement cover the landscape, the geologist said, they increase the amount of rain running off into nearby streams, warming it as it flows across sun-baked surfaces.
The warming trend seen in the satellite data echoes findings of earlier studies. Last year, researchers with the U.S. Geological Survey reported that most of the streams feeding into the bay are warming, with an overall average temperature increase of 2.5 degrees since 1960.
"It's not a new finding that the bay is warming," said Donald F. Boesch, president of the University of Maryland's environmental science center. "But [this] brings it up to date."
By using satellites instead of temperature gauges in the water, Boesch said, "you can even see the bay warming from space."
When comparing the new findings with those of earlier studies, Elmore said, the satellite data indicate the warming has accelerated in the past few decades. That has implications for the bay's water quality as well as for the plants and creatures that can live in it, Elmore and other Maryland scientists say.
"Many of the changes we see in the bay are tied in with these changes in temperature," said Elmore. "What are we restoring the bay to?"
Maryland and the other five states in the bay watershed are under a federal "pollution diet" to reduce the algae-feeding nutrients from farms and urban areas that cause an oxygen-starved dead zone to form every year.
Rising water temperatures could make it harder to shrink that dead zone, or keep it from coming back, said Lora Harris, an ecologist with the University of Maryland's Chesapeake Biological Laboratory in Solomons.
Warmer water tends to hold less dissolved oxygen, she said, and it can aggravate the layering or "stratification" of the bay's waters, which makes it easier for dead zones to form.
Warming waters also could alter the types of algae and microscopic animals in the bay, Harris said. That might mean less to eat for fish and crabs.
"Menhaden, they love a juicy diatom," she said. "Small cyanobacteria, not so much."
"Eelgrass and soft clams are going to have an increasingly difficult time as the bay warms,'' Boesch said.
Climate-warmed water was not originally factored into the pollution reduction goals set five years ago by the Environmental Protection Agency for shrinking the bay's dead zone. But federal regulators are now factoring it into their reassessment of how much more needs to be done over the next decade, said Zoe Johnson, a climate change coordinator in the National Oceanic and Atmospheric Administration's Chesapeake Bay office.
The bay states and federal government agreed last year to adjust strategies for restoring and maintaining the bay's plants and creatures to take into account climate change impacts, such as warming temperatures, shifts in precipitation and rising sea level.
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"There can be winners and losers," Johnson said.
While declines in eelgrass could rob juvenile fish and crabs of places to hide from predators, other species, such as Southern flounder and red and black drum, might find a warmer bay more hospitable. But it is difficult to do more precise handicapping, she said, because average water temperatures are not as influential on the fortunes of plants and animals as the extreme hot or cold temperatures they might experience.
Boesch said warming is occurring gradually enough now that it is unlikely to seriously undercut efforts to clean up nutrient and sediment pollution in the bay. The states are committed to adopting measures by 2025 to shrink the dead zone, based on current computer models. But if climate change is not slowed, Boesch said, it could become harder to maintain water quality.
"It isn't hopeless," Harris said.
She suggested that the most useful aspect of the new study is the focus it has put on land-based sources of warmer water, reinforcing the need to curb stormwater runoff and power plant discharges.
"Now we have a much clearer picture of what's happening where," Harris said. "That does give us the ability to think about how to optimize investments in restoration."