Erosion controls may hinder bay’s recovery
Study: ‘Hardened’ waterfronts could limit food, shelter for crabs, fish
Waterfront property owners all around the Chesapeake Bay have bulkheaded and riprapped their shoreline to protect it from erosion.
It’s their legal right to keep their land from washing away, and over the years a growing share of the water’s edge has been “armored” with low wooden walls or large rocks.
But a six-year, federally funded research effort is finding that the bay’s increasingly hardened shoreline could be hindering the estuary’s recovery from decades of pollu- tion. It may be limiting where crabs, fish and terrapins can find food and shelter. It also may be aiding the rapid spread of an invasive marsh grass and helping to sustain the population of stinging nettles, a summertime nuisance for swimmers.
“Anyone familiar with the Chesapeake has seen over the years how much we’ve modified the shoreline,” said Robert Magnien, a research director with the National Oceanic and Atmospheric Administration, which funded the $4.5 million study.
Along heavily developed Baltimore-area waterways such as the Middle, Magothy and Severn rivers, for example, half or more of the shore has been lined with walls or riprap.
“This is all right in front of our faces,” Magnien said, “but in many ways we really haven’t come to grips with it in a scientific, comprehensive fashion.”
One bulkhead or revetment alone may
not have a noticeable effect, he said, but researchers from eight universities and government agencies are studying the cumulative impacts of extensive shoreline armoring. By sampling 50 different places around the bay, the research team has been looking at how shoreline affects the health of the estuary, its fish and other creatures.
It will be another year or so before all the data have been compiled and analyzed, Magnien said, but one finding that has emerged already is that underwater grasses — a key indicator of bay health — begin to be affected when as little as 5 percent of the shoreline is armored.
The bay’s “submerged aquatic vegetation,” as scientists call the grasses, provide food for ducks and geese, and vital habitat for blue crabs, juvenile striped bass and other fish. The grasses dwindled in the 1970s and 1980s, victims of the same nutrient and sediment pollution that has fed algae blooms and a fish-stressing “dead” zone every summer.
Maryland and other bay states have labored for more than 30 years to clean up the water, and the grasses have waxed and waned, affected partly by weather conditions. Nowthey’re on the upswing, covering some 76,000 acres of bay and river bottom. But that’s less than half what the states and federal government set for a goal, and a far cry from what historically grew in the Chesapeake.
“When you have an estuary that’s impacted by sediment and nutrients that knock back the [grasses], then shoreline hardening’s impact may not be that signficant,” Magnien said. “But as we ratchet back that pollution, we expect [grasses] to come back. Wouldn’t we want to know whether or not that’s going to happen in face of all that shoreline hardening?”
The results to date suggest subtle but signficant impacts. On the Severn River last week, biologists with the Maryland Department of Natural Resources found healthy patches of grass growing on the bottom off a riprapped stretch of shoreline, and along a nearby expanse of “natural” waterfront, where trees had toppled into the water from an eroding bluff. The water was clearer in both areas, likely because bay grasses can filter out visibility-clouding sediment.
But the DNR biologists found the underwater lawn off the natural shoreline was twice as large as the patch near the riprapped shore. It was denser, too, and had at least one additional type of vegetation, redhead grass, in addition to milfoil and pondweed.
As biologist Brooke Landry snorkeled nearby to survey the grasses off the unmodified shoreline, a blue crab swam in and out of the green underwater jungle.
“There are lots of fish here,” Landry observed.
There are several reasons why grass beds may not be as robust near armored shorelines, scientists say. Earlier studies have shown that waves tend to bounce off bulkheads and riprap, and that turbulence can uproot grasses growing in the shallows.
Scientists with the Smithsonian Environmental Research Center in Edgewater working on the project say they’ve found that land use associated with a hardened shoreline also may be a factor. Farm fields and home lots with armored waterfront tend to have greater nutrient runoff from fertilizer or animal manure, which impedes the growth of bay grasses, they noted..
“We know what we do on the land is messing up water and” grass beds, said ecologist Donald Weller, one of several Smithsonian scientists involved with the study.
Researchers have found more fish and crabs off natural shorelines than armored ones. Senior Smithsonian scientist Denise Breitburg said 12 of the 16 fish species her team has been tracking were less abundant off hardened shorelines, including crabs, spot, croaker and juvenile white perch.
When Breitburg began the study, she said, she didn’t expect to see any difference, as fish and crabs move around in the water and their abundance depends on a variety of factors.
The finding of fewer aquatic creatures off hardened shorelines, she said, “shows there’s some real strong effects going on.”
Another unusual finding: Sea nettle larvae attach themselves to riprap in the water and can spend the winter there if deep enough to avoid freezing. The survivors emerge and morph into stinging white jellyfish each summer.
Hardened shorelines also can lead to the loss of tidal wetlands, scientists said, by preventing the formation of new marsh as sea level rises.
The new research indicates modifying the shore may help invasive phragmites to spread, degrading exisiting marsh. The tall, reedlike grass tends to grow in disturbed soil, including landscaped shoreline. It crowds out native wetland plants, and, while some birds and animals use it for shelter, it grows so densely it can be inhospitable to other wildlife.
“It’s a plant that just loves nutrients,” said Dennis Whigham, another Smithsonian ecologist who has been studying the shoreline-phragmites connection. “So when you create a disturbance and add nutrients, it creates the perfect storm.”
No one’s calling for an end to shore erosion controls. Waves and storms eat away at two-thirds of Maryland’s 7,000 miles of shoreline, in some cases threatening to undercut homes or roads. But the severity of land loss varies. Fifty-six percent of the state’s shoreline experiences only “slight” erosion of less than two feet a year, according to the Maryland Department of Natural Resources, while 13 percent disappears at a faster rate.
Waterfront property owners must apply to local and state governments for permits to protect their shoreline. Since 2008, state law has required construction of so-called “living shorelines,” which often involve planting marsh grasses, and building up the shore with sand and other natural materials
At least some living shoreline projects involve building a low sill of rocks at water’s edge or even just offshore. Those projects appear to be less harmful to grass beds, scientists say, than bulkheads or revetments.
But the scientists said they didn’t study the new shoreline structures enough to draw conclusions.
“I think we have a lot to learn about living shorelines,” concluded Thomas Jordan, a Smithsonian scientist and lead investigator on the project.
DNR program chief Lee Karrh said he hopes the study will influence how DNR manages shore erosion on state parks, natural resource areas and other waterfront public lands under its control.
Since the late 1960s, the state has provided nearly $65 million, mostly in low-interest loans, to help private and public property owners build 500 living shorelines and an equal number of structural projects covering nearly 76 miles of shoreline, said Bhaskaran Subramanian, shoreline conservation section chief at the Department of Natural Resources.
Subramanian said he tries to encourage landowners to create marsh or beach along their shoreline, rather than rocks or bulkhead.
The Maryland Department of the Environment issued 457 permits last year for shoreline work in open water and another 12 for work in tidal wetlands, according to spokesman Jay Apperson. Of those, 109 were for tidal marsh creation or beach nourishment — projects he characterized as “living shorelines.”.
Another 275 were for a variety of structures, including revetments, sills and modifications to new or existing bulkheads, Apperson said. Seven were for new bulkeads.
Magnien said the study suggests regulators ought to consider the cumulative impacts of shoreline projects.
“Right now, most of our management is parcel by parcel,” he said. “Should we be looking at this more on a regional basis?”