Turning forests into farmland or—even worse—housing tracts, parking lots or shopping centers is bad news for a host of Bay dwellers, from tiny juvenile crabs living in rivers to some of the birds flying overhead.
While development in the watershed has long been pegged as one of the Bay’s many woes, new research by a team of scientists working around the Chesapeake provides some of the clearest links yet to its impacts on estuarine habitat.
Researchers are still analyzing piles of data they’ve collected in recent years, but some trends are already clear. Among their findings:
- Levels of toxic PCBs routinely begin reaching unsafe levels in some fish when as little as 10 percent of a watershed is developed.
- Sensitive marsh-dwelling birds begin to disappear when development comes within 500 meters of a wetland.
- Juvenile blue crabs disappear in rivers which drain either developed lands or agricultural fields.
- Fish communities become less diverse as watershed development increases.
The goal of the research, funded by the EPA’s Estuarine and Great Lakes Program, is to find ecological indicators, such as the presence or abundance of certain species, that can help scientists, managers and policy makers document trends in ecosystems which, in turn, can guide management activities.
Such indicators might be particularly valuable in helping to anticipate—and perhaps minimize—the impacts of more people pressing into sensitive coastal ecosystems not only around the Bay, but around the nation.
“Coastal areas are easily one of the most heavily impacted areas in the world,” said Peter Marra, a senior scientist with the Smithsonian Environment Research Center’s Avian Ecology Lab. “Whether you are driving through the meadowlands of New Jersey, or through Maryland, you often have concrete, tar or pavement going right into the marshes.”
The U.S. Commission on Ocean Policy recently noted that 37 million people and 19 million homes were added to the nation’s coastal areas in the past three decades, stressing the ability of those areas to maintain healthy ecosystems. In the Chesapeake watershed, the Bay Program estimates that forests—the most beneficial land use for water quality—are lost at the rate of 100 acres a day, with the greatest losses occurring near the Bay.
While the Chesapeake’s overriding water quality concern is related to excess nutrients, which lead to reduced water clarity and low dissolved oxygen levels, the new research shows that other factors are also important in affecting the abundance of fish, shellfish, birds and other resources.
“Almost all of our indicators were related in one way or another to development, and development itself doesn’t necessarily mean nutrients,” said Dennis Whigham, a SERC plant ecologist and one of the lead scientists for the project. “It may mean a whole lot of things.”
Nutrient levels were often higher in agricultural watersheds, the scientists said. But developed watersheds typically had the greatest impacts on aquatic and bird communities.
Part of the reason, the scientists suggested, is that agricultural nutrients are so dominant in many areas that they cause high “background” levels everywhere, as nutrient-laden water is moved up and down estuarine rivers and creeks with the ebb and flow of the tides.
“Development seems to be really important,” Whigham said. “We can see development signals in a lot of these potential indicators, even over the strong agricultural signals.”
The studies have found that some of the potential indicator species they examined only respond negatively to land use changes in areas near the water, while others are affected by changes throughout the watershed.
Similar efforts have successfully linked development to habitat degradation and aquatic community impacts in freshwater streams. Maryland’s Biological Stream Survey, for instance, has been able to closely match the health of streams to watershed land uses. Data from the survey can successfully predict the disappearance of sensitive species, such as brook trout, when a certain amount of the watershed is developed.
Making that link in estuarine systems was thought to be more challenging, if not impossible. Estuaries are zones where fresh and salt water mix, so their inhabitants typically are tolerant of swings in environmental conditions, while many freshwater dwellers are adapted to more stable conditions—and therefore can be more vulnerable to change.
“We were very pessimistic that we would be able to see much,” Whigham said. But the scientists were surprised at clear links between land use and aquatic life when they began examining a series of small estuarine watersheds around the Bay.
For instance, surveys conducted for the project showed that juvenile blue crabs were most abundant in watersheds where forests were the dominant land use. Young crabs also showed strong preferences for areas that had large wetlands, especially when extensive amounts of shorelines were covered by marshes. But juvenile crabs were largely absent from developed or agricultural watersheds even when wetlands were present, the surveys revealed.
“The whole search here is for indicators that are telling you that something isn’t right in River City,” Whigham said. “And small blue crabs is probably a pretty strong one to pay attention to.”
Another startling result was the discovery that PCBs in white perch were high enough to trigger EPA consumption advisories when as little as 10 percent of a watershed was developed. By the time development reached 35 percent, PCB concentrations were so high that no consumption was recommended.
While the chemicals, which have been banned for three decades, appear to be continuing to slowly leak out of developed areas, agricultural and forest watersheds had little influence on PCB concentrations in white perch.
“This is a tool that can be used as an indicator of risk for areas that have not been sampled yet for PCBs,” said Ryan King, a biologist at Baylor University who worked at the project while at SERC. “You can do this analysis and say a certain area is likely to have PCBs.”
Studies that scored the health of fish populations by their size and the diversity of species present in a system revealed lower scores in developed and agricultural watersheds than in those dominated by forests. High scores were also associated with areas that had minimal shoreline alteration and abundant subtidal habitats, such as woody debris.
“Obviously, there is a response from watershed development or agricultural and nutrient inputs,” said Donna Bilkovic, a researcher at the Virginia Institute of Marine Science. “But aside from that, there is a structure loss.” As shorelines are cleared of trees, which otherwise would die and fill in the water, the nearshore habitat loses its complexity, and fish have fewer places that offer shelter. “That is where they feed and hide,” Bilkovic said.
Water clarity, an important component for underwater grass beds, also fared poorly in watersheds with more development. The reason, said Charles Gallegos, a SERC plankton ecologist, seems to be that there are more fine particles in the water in those watersheds. Fine particles are bad for clarity because they float in the water, blocking sunlight for longer periods of time than heavier particles, which sink to the bottom.
Exactly why there are more fine particles in developed watersheds is uncertain, Gallegos said. Part of the reason could be that more fine clays, which tend not to be trapped by stormwater detention ponds, wash in from urban stormwater systems. Or, it could be that small organic particles are flushed in from stormwater and wastewater treatment plants. Those tiny organic particles are in turn broken down by small plankton and bacteria, rather than consumed by larger zooplankton, resulting in lots of small, light-blocking organisms in the water.
“The urban signal is so strong,” Gallegos said, “it caught me by surprise that it was as statistically clear as it was.”
The amount of development in a watershed was also strongly related to the abundance of phragmites, an invasive plant that degrades wetland habitats. Studies show that the amount of phragmites in wetlands increases with the amount of development in the watershed.
And researchers found that wetland bird communities were adversely impacted when just 14 percent of the land within a 500-meter buffer around the marsh was developed. [See “Development near wetlands creates tide of birds moving out,” on page 6.]
“It looks like it doesn’t take much development to see a change in a potential indicator in the estuary,” Whigham said. “We see a lot of things responding to land use.”
While scientists are in the process of publishing their first handful of papers based on the research, they are still shifting through data to find more linkages between land use and biological impacts.
Still, many questions remain. While land use changes are strongly correlated with impacts, it remains unclear exactly what factors are causing the problems. It could be changes in hydrology from development, or an increase in chemical pollutants, or a host of other factors—water in urban watersheds often tends to have lower oxygen levels, for example. “As far as causal linkages, that’s where it starts to get kind of dicey,” King said.
A pattern that seems to emerge, King said, is that while development affects stream and estuary health, the location of the development is also important. The further the development is from a stream or shore, the less its impact is felt by aquatic life downstream.
Shoreline impacts had clear impacts of fish and marsh birds. And, PCB levels in fish seem to decline when commercial development in a watershed was located farther away from a stream, King said. “So the whole watershed matters, but what is close to the stream matters even more,” he said. “That seems to be a pattern that is emerging.”
At some point, though, a watershed will hit a threshold where, no matter how well-buffered a stream or shoreline is, the aquatic health of the system will degrade. Exactly what that threshold is for estuarine watersheds still isn’t clear. But, Whigham said, “all of these threshold things we have done so far indicate that most of the land around here is already over the threshold.”