The Heat is on: Warming Water Threatens Aquatic Life in Chesapeake Bay Region

Warming water is threatening to undo decades of efforts aimed at improving aquatic habitat in the Chesapeake region, from headwater streams to the open water of the Bay itself.

The increasing water temperatures, which threaten species like brook trout and striped bass, are already offsetting some of the habitat benefits of the multibillion-dollar Bay restoration effort, a new report warns. Worse, some actions taken to reduce pollution are actually contributing to warmer, more stressful, stream conditions for fish.

"We're behind the eight ball right now in considering this in our major policies," said Rich Batiuk, a former senior science official with the state-federal Bay Program partnership, who helped organize a 2022 workshop focused on the region's rising water temperatures.

Batiuk was a leading architect of the current Bay cleanup strategy for reducing nutrient and sediment pollution to clear the Bay's water and shrink its oxygen-starved "dead zone." The resulting water quality improvements were intended to boost aquatic life. But some of the assumptions underlying that effort didn't account for the negative impact that rising water temperatures would have on fish, crabs and even worms and algae.

Now, a report from the Bay Program's Scientific and Technical Advisory Committee, stemming from last year's workshop, warns that those changes could undermine progress toward Bay Program goals for "fisheries management, habitat restoration, water quality improvements, and protecting healthy watersheds."

Just one example: A 2013 study found that meeting all Bay nutrient reduction goals would increase habitat for Atlantic sturgeon, an endangered species, by 13 percent. But an increase in water temperature of 1.8 degrees would reduce available sturgeon habitat by 65 percent.

Batiuk and other scientists, as well as government and nonprofit organization representatives who participated in the workshop, stressed the urgent need to save areas that can still be protected from rising temperatures while mitigating harm to places where changes are inevitable. "We've got to be thinking about temperatures in the same way we talk about nitrogen, phosphorus and sediment," Batiuk said.

The failure to account for such impacts has resulted in the widespread use of nutrient control devices, such as detention ponds, which increase stream temperatures. At the same time, actions that would help reduce the threats from warming water - such as planting trees along streams and in urban areas - are far off track.

"We are behind on our goals that may most help this," said Rebecca Hanmer, a former Bay Program director who helped organize the workshop. "To say that we have to take rising water temperatures into account is not to say that's a new goal for the Bay Program. It's a new reality."

For aquatic life, the heat is on

In the Chesapeake, the average summer water temperature has increased about 1.8 degrees since 1995 - driven primarily by warming air temperatures.

Across the watershed, a U.S. Geological Survey analysis found that stream temperatures increased 2.52 degrees on average from 1960 to 2020. That increase stems not only from warming air, but land use changes that warm stream temperatures.

These increases have already impacted aquatic life. Nearly a century ago, when pioneering crab biologist Reginald Truitt was working with the Bay's blue crabs, the crustaceans spent nearly five months burrowed into mud as they hibernated during cold months.

Typically, the crabs would begin digging in by early December and remain until late April. Today, the crabs usually don't bed down until mid to late December, and they emerge by late March or early April.

"They've probably shaved three weeks at either end," said Tom Miller, who heads the University of Maryland Center for Environmental Science's Chesapeake Biological Laboratory. "So their winter has gone from five months, down conservatively to four months, possibly to as little as three-and-a-half months."

Such changes could have implications for the winter survey of hibernating blue crabs, which assesses their population each year. It could also factor into future harvest management.

Meanwhile, eelgrass beds, one of the Bay's most important underwater habitats, have been declining for decades. Scientists fear that the heat-sensitive plant, which is the only underwater grass species in many portions of the Lower Bay, will largely disappear in coming decades, dramatically reducing important habitat for juvenile blue crabs and fish, waterfowl and other species.

In the headwaters, brook trout, which typically do not tolerate water warmer than 68 degrees, also have been declining for decades. Other cold-water dependent species, such as the checkered sculpin, a small fish, are facing similar habitat losses. Many less-studied fish, mussels and amphibians may also be at risk.

As water warms, it holds less oxygen, creating problems for species like sturgeon, which require high oxygen concentrations. In the summer, low-oxygen levels in bottom areas of the Bay force striped bass toward warmer surface waters, which are stressful to the fish and cause increased mortality when the fish are handled.

Warmer temperatures also increase the toxicity of some heavy metals and other chemicals, promote the growth of bacteria and harmful algal blooms, and spread pathogens that can infect fish.

Scientists are also concerned that warming temperatures could disrupt predator-prey relationships that have existed for millennia. That could be especially worrisome for migratory fish, which evolved to spawn in specific habitats at certain times so their young can take advantage of abundant food. As temperatures change, food sources may no longer be available at the right place or right time.

"Those rising temperatures could affect spawning, other prey relationships, nonnative species or pathogens [and] diseases," said Stephen Faulkner of the USGS Eastern Ecological Science Center in Leestown, WV. "There's a whole suite of potential indirect effects related to rising temperature."

More frequent heat waves

Creatures living in the water are not only facing warmer temperatures in general, but also aquatic heat waves that push temperatures into abnormally high territory for days on end.

Researchers at Virginia Institute of Marine Science found that from 1986 to 2010, the Chesapeake averaged four to five heat waves of at least five days' duration per year. Since then, there have typically been six to eight events per year, and their frequency and duration are expected to continue to grow.

Increasing heat waves are showing up in rivers and streams as well. "Back in 1996, the average number of heat wave days that a site would experience in a year was about 15," said Spencer Tassone, a doctorate candidate at the University of Virginia, who participated in a study of 70 stream sites nationally, though none in the Bay watershed. "In 2021, that value was 31 days."

Most of the events occur in the summer, when stream dwellers are already stressed by low water levels and are crowded into smaller spaces. Even if not directly deadly, heat waves may make fish and other stream life more vulnerable to other stresses and can reduce spawning.

While overall temperatures are edging up over time, Tassone said heat waves often determine whether creatures disappear from a stream. "It's really these heat waves that are extreme in nature, but short-lived, that have a disproportionate impact on the organisms," he said.

Managing for heat

Temperature is already bringing a "regime shift" to the Chesapeake, in which the mix of species is significantly altered from what persisted for decades, even centuries.

"We may be looking at a very different ecosystem and Chesapeake Bay in the future," said Julie Reicher-Nguyen of the National Oceanic and Atmospheric Administration Chesapeake Bay Office. "It may look more like estuaries in the Carolinas than what the Chesapeake Bay looks like now."

Some southern species, such as white shrimp and red drum are already moving into the Bay in increasing numbers. Shrimp are even starting to support a commercial fishery in Virginia's portion of the Bay.

There is no crystal ball that predicts how such changes will play out. Papers prepared for the workshop suggest that blue crabs and some forage species, such as bay anchovy and menhaden, may benefit as warmer temperatures increase productivity and expand their range.

For blue crabs, fewer severe winters would reduce mortality during hibernation. But other changes could counter that. Growing numbers of red drum could increase predation on blue crabs, and eelgrass losses will remove an important refuge for young blue crabs.

Likewise, oysters may benefit from longer growing seasons, but warmer temperatures may also promote diseases that have plagued their populations.

Some species, such as striped bass and flounder, may experience both positive and negative impacts from warming waters at different life stages. For instance, warmer temperatures might increase growth rates for small striped bass, but heat-sensitive adults will be more stressed by warmer summer conditions.

Bay water temperatures are driven largely by increasing air temperatures. With little ability to control that, management could focus on mitigating impacts by reducing other stresses, the workshop report said. For instance, near-shore habitats could be improved by limiting the hardening of shorelines, promoting the use of "living shorelines" to reduce erosion, and restoring forest buffers along the water's edge.

The report suggests that large-scale restoration projects, which mix living shorelines, oyster reefs and underwater grass beds, could help mitigate conditions for a variety of species when conditions get bad. "Doing restoration on a large scale to enhance fish habitat can help create refuges to climate change," said Bruce Vogt, ecosystem science and synthesis manager with the NOAA Chesapeake Bay Office.

Meanwhile, continued efforts to reduce nutrients could improve water clarity, which helps underwater grasses, controls harmful algal blooms and improves oxygen conditions.

Warmer water could also require some changes to fishing - and fishing management. Adult striped bass are more likely to die when handled during warm temperatures. Educational efforts, which the Maryland Department of Natural Resources is already working on, could encourage anglers to refrain from fishing during stressful conditions.

That type of program could be refined and expanded to other areas, and possibly for other species, the report suggests.

And because heat waves in particular create high-stress, lethal situations, it calls for exploring the creation of a heat wave warning system to alert anglers when they may want to reduce or avoid handling fish.

"If we're able to detect heat waves when they're happening in waters beyond the optimal temperature range for certain fish, then maybe that could help adjust what people do," Reicher-Nguyen said.

Sizing up stream opportunities

Stream temperatures in many parts of the watershed have risen more than temperatures in the Bay, and at a faster rate than air temperatures, data show. But the workshop report notes that those increases are not necessarily baked in.

While climate drives overall temperature increases, land cover continues to have a major influence on stream temperatures. Generally, forested areas moderate the impact of rising temperatures, while developed landscapes magnify the warming.

Forests can cool local air temperature by several degrees as trees evaporate water through their leaves. Shade from trees can also moderate ground temperatures, keeping sunlight from heating water as it runs off the land. Larger forests also promote the infiltration of rain into groundwater, where it is further cooled.

That's why cold-water streams - the habitat for brook trout and other temperature-sensitive species - tend to be heavily forested. A recent Maryland Department of Natural Resources study found that brook trout typically require at least 70 percent of a watershed to be forested and brown trout 52 percent. No trout are found when a watershed is less than 46 percent forested.

In many areas, conserving existing forests and planting new streamside trees can mitigate the impacts of warming air and even reduce water temperatures in smaller headwater streams.

"We've had a really strong negative impact on temperatures in the watershed for decades," said Matt Ehrhart, director of watershed restoration at the Stroud Water Research Center in Pennsylvania. "And we have the opportunity to really play offense, I think, for many of our smaller streams, and make improvements, not just try to defend against the impacts of greater temperature change."

Unfortunately, those efforts are lagging throughout the Bay region. Tree cover is generally declining in the watershed, and efforts to plant streamside buffers are far below Bay Program goals - just 169 miles were planted in 2020, far less than the 900-miles-a-year goal.

As forests are lost and land developed, the hardened surfaces that replace trees warm rainfall and speed that heated water into streams.

A study in the Anacostia watershed found that runoff after summer storms could increase stream temperatures 5–7 degrees.

Urban areas with large amounts of roofs, pavement and other impervious covers have little opportunity to help stormwater soak into the ground where it can be cooled. While new stormwater best management practices, or BMPs, promote infiltration, many include drains to reduce the potential for flooding. That minimizes the time the water spends in the ground.

"It's not like the water is spending days and weeks underground before the runoff emerges," said Tom Schueler of the nonprofit Chesapeake Stormwater Network. "BMPs are not refrigerators."

In many urban areas, he said, probably the best that can be hoped for is "to do no harm" when it comes to temperature. That, in itself, would be an improvement because the region is rapidly installing BMPs that add to the problem. Generally, these are ponds that capture rain running off heated surfaces, allowing it to be further warmed by sunlight before being released into streams. Examples are stormwater detention ponds and lagoons at animal feeding operations.

An analysis prepared for the temperature workshop suggested that such "heater" practices are being installed in the Bay region at three times the rate of "cooler" practices.

Planting more trees in developed landscapes can help reduce stream temperatures by cooling the air and shading pavement. But Bay Program goals to increase urban tree cover are also off track. Instead of adding 2,400 acres by 2025, the region has actually lost 12 times that amount since 2013.

Still, workshop participants said there are opportunities to improve urban waterways. For instance, restoration efforts could target streams in developed areas that have the potential to serve as refuges for fish and other species, especially during heat waves.

The report called for more work to better identify where tree planting can have the most impact in both rural and urban settings.

It also pointed to another potential source of help for agricultural areas without much tree cover. Some farm practices, such as those that promote infiltration to nurture healthy soil, might help offset warming air temperatures by diverting rainfall into cooling groundwater rather than sending it directly into streams. But that needs more research, the report said.

Treating temperature as pollution

All of the states in the watershed have regulations aimed at protecting aquatic life from the impacts of heated water, but those mostly deal with the discharge of water from industrial or commercial use, typically power plants.

In the era of climate change, though, Maryland is using its water quality standards to protect trout streams from rising temperatures - an approach that might be adapted by other states.

Maryland's policies already call for preserving forest buffers along designated cold-water streams. But it is planning to apply a new regulatory tool as well.

The state has 170 miles of cold-water streams that are considered impaired because they exceed 68 degrees much of the time. So, it's treating temperature as a pollutant and writing cleanup plans - total maximum daily loads - to help cool warming waters.

The Maryland Department of the Environment is establishing pilot TMDLs in four watersheds, located in urban, suburban and rural landscapes in Baltimore, Carroll and Frederick counties. This will trigger the development of action plans for reducing heat pollution.

Using computer models, officials say they can estimate the amount and width of forested stream buffers, forest canopy and other green infrastructure improvements that would be needed to meet stream temperature goals. Municipalities will also have to make sure that permit decisions that could influence stream temperatures are consistent with the TMDLs.

Lessons learned from those areas could be applied to other waterways, said Lee Currey, director of MDE's Water and Science Administration.

"These are the ones that are most restorable in various settings, so we can put together a TMDL, then learn from that and take that knowledge and put it toward the other ones," Currey said. "The idea is to cool the stream down."

But the time frames for meeting water quality goals will be "challenging," especially in more urbanized areas, Currey said. "You're talking, once trees are planted, five to 10 years out before you start seeing something that will provide the shade cover."

Managing rising water temperatures is, indeed, a long-term problem - but one the region needs to start addressing now, the report emphasizes. More water quality monitoring is needed to identify areas where the warming can be reversed, and more research is needed to understand the full impact of temperature changes on aquatic life.

Whether through regulatory efforts like those in Maryland or voluntary efforts that target tree planting and habitat restoration, the fate of many species long associated with the Bay watershed rests with decisions and actions being made now.

"Essentially, we're asking the Bay Program to add a new lens onto how we look at just about everything - every decision we're making out there - from runoff control practices to education and communication," Batiuk said. "It all needs to be looked at in the context of rising temperatures."