For years, some researchers at the Virginia Institute of Marine Science sensed that the tidal wetlands in the York River, which flows past their Gloucester Point campus, were changing — but they couldn’t say exactly how.

To find out, a research team surveyed all of the wetlands along the York and its two tidal tributaries, the Pamunkey and Mattaponi rivers, using sophisticated GPS equipment. They compared their results, gathered during the last few years, with detailed maps developed in the late 1970s — when fledgling regulatory programs sought to stem human disturbance to tidal marshes.

They discovered the marshlands had been disappearing before their eyes.

Altogether, the river lost almost 9 percent of its tidal marshes in just a little more than 30 years — 1,794 acres since the late 1970s. That’s about a third of the dramatic acreage loss experienced at the Blackwater National Wildlife Refuge since the 1930s.

But, being spread over a larger area, the losses had gone largely unnoticed.

“It is always harder to get recognition of the impacts of cumulative losses, than a catastrophic loss,” said Molly Mitchell, of the VIMS Center for Coastal Resources Management, who analyzed the survey results. “A hundred square feet here, a hundred square feet there — none of them by themselves are really critical, either from an ecological standpoint or a human impacts standpoint. It’s when you start to add them up that it becomes more important.”

The findings confirm that the insidious increase in water levels of just a few millimeters per year that the Chesapeake area has experienced has added up to big losses in river systems.

Understanding marsh losses has often been problematic outside a few localized areas such as Blackwater, which has lost 5,000 acres. In part, that’s because even as marshes are lost, new marshes are being created. In low-lying areas, marshes can slowly migrate onto former upland areas as water levels rise. And, sediment deposited by rivers build up new areas that can become marshes.

That makes it hard to understand the true status, and trends, of the Bay’s vast marshlands.

The York River survey found that, indeed, new marshes were created in some places — 3,080 acres in all. But that was more than offset by losses, which totaled 4,875 acres.

The survey also found that marsh losses were not uniform. While all major marsh types declined in acreage, fringing marshes — long, narrow bands of wetlands along shorelines that are often only a few feet wide — were especially hard hit, suffering a net loss of nearly 30 percent. Though narrow, fringe marshes play important roles in many rivers, buffering shorelines from waves and filtering nutrients in runoff before they reach the river.

Large marshes are important for a variety of animals — in colonial times, they were a haven for black bears — as well as birds, but fringe marshes are thought to be especially important for many aquatic species, such as blue crabs, terrapins and juvenile fish. Those species are often restricted to the few feet of habitat along the edge of a marsh, so the interior area of larger marshes are off-limits, Mitchell said, “but a fringe marsh, they can really use most of it.”

Fringe marshes are likely to be the first lost in the York and other systems, she said. The land behind them often rises abruptly from the river, minimizing the potential for the marsh to migrate into the upland. And they are more likely than the larger expansive marshes to have hardened shorelines, either bulkheads or rip-rap, behind them. These shorelines not only restrict the movement of marshes, but also reflect wave energy back into the wetland.

Not only was there an outright loss of marsh, but the plant communities in marshes changed as water levels rose and saltwater began reaching farther upstream. Saltwater marshes have less plant diversity than brackish and freshwater marshes, and the introduction of the invasive reed grass, phragmites, has further reduced diversity. Some research suggests that phragmites gains a foothold in systems as they become stressed by sea level rise.

The project also raised concerns that low-salinity tidal marshes, which declined by slightly more than 10 percent during the study period, are also at great risk. Low-salinity marshes are typically the farthest upstream, and therefore have limited ability to migrate. Also, being upstream, they typically don’t face the same wave action as saltier marshes, which have tall, more deeply rooted plants on their exterior edges.

Low-salinity marshes, in contrast, have shorter plants on the edge, which offer less protection and can be more vulnerable as water levels rise and expose upstream areas to more wave action.

“Those low herbaceous plants don’t break wave energy,” Mitchell said. “They don’t have an issue with wave energy up there. So the structure of the two types of marshes is really different.”

Tidal marshes are one of the defining features of the Bay landscape, in part because the Delmarva Peninsula creates a sheltered system that protects them from ocean wave energy, allowing them to flourish. As a result, the Bay is ringed by nearly 300,000 acres of tidal marshes — about two-fifths of all tidal marshes found on the Atlantic Coast.

Those marshes absorb nutrients, filter water, buffer shorelines from erosion, and serve as food for waterfowl and habitat for myriad aquatic and terrestrial species.

With a disproportionately high rate of sea level rise in the region, many of those marshes — and the species that depend upon them — are especially vulnerable.

Exactly what the marsh losses mean for the Bay is hard to say. It’s clear that marshes play an important role in buffering adjacent land from waves, and recent studies suggest the loss of fringe marshes could hurt species such as diamondback terrapins. But quantifying their exact value to fisheries and for services such as nutrient removal remains elusive, said Carl Hershner, director of the of VIMS Center for Coastal Resources Management.

For instance, freshwater marshes are thought to be important for anadromous fish spawning — but it’s difficult to say how important. Similarly, limited work suggests fringe marshes remove nitrogen coming from upland areas, but it’s hard to say that happens in all — or even most — settings.

“We can say with some degree of confidence that we aren’t going to have nearly as many wetlands as we do now,” Hershner said, “but what that implies for the Bay is a whole lot harder to demonstrate.”

But some clues about the impact of tidal wetland losses may come from marsh birds. The Chesapeake historically has been an important area for marsh birds because of its disproportionately high amount of tidal wetlands, said Michael Wilson, an ornithologists at the Center for Conservation Biology, based in Williamsburg, VA. But that role will likely diminish in the coming century as the region faces a faster-than-average rate of sea level rise.

In the York River, king rails, which like freshwater tidal marshes, are already suffering steep declines, and the small population of coastal plain swamp sparrows, which live in a narrow band of vegetation along freshwater tidal marshes, are likely to vanish, Wilson said.

That mirrors the problems experienced by marsh birds in general, which have faced declining habitats for the last century — a trend expected to accelerate with sea level rise. Estimates by Wilson and colleagues show that by the end of this century, some rare species such as the clapper rail, Virginia rail, seaside sparrow and marsh wren could lose 70–80 percent of their populations within the Bay if water levels rise 1–2 meters as expected.

Some species, such as black rails and saltmarsh sparrows — which are already struggling in the region — are even worse off. They will likely be lost from the Chesapeake.

Populations of black rails, the smallest species of rail, have already declined sharply in recently years and are considered endangered in both Virginia and Maryland. “I don’t see a good future for them here in the Bay because of sea level rise and salt marsh loss and salt marsh transformation to low marshes,” Wilson said. “That’s a species we could lose in our lifetime.”

As dire as they are, Wilson said the marsh bird predictions are likely conservative — ornithologists often see marsh birds disappear even faster than they expect. Part of the reason, Wilson said, could be that areas thought to be “safe” are actually becoming inundated more frequently than thought.

“There’s areas out there that we believe have habitat and look fine on paper,” he said. “But those areas are getting drowned out from an increase in the number of extreme high tide events. There’s this silent killer that’s ruining habitat by disrupting the breeding in those marshes. Birds are nesting, but their nests are getting drowned out repeatedly.”

VIMS scientists hope to ultimately expand the survey techniques used on the York to other rivers to get a better handle on what’s happening statewide. Their results may also offer some insights for management, Mitchell said. For instance, it might become more important to be more protective of existing marshes. “Reducing anthropogenic impacts might be even more important than we think it is,” she said. “It is something you can control.”

And alternative shoreline protection technologies, such as living shorelines, could get even more emphasis, especially near vulnerable fringe marshes. “They might be more important than we realize because they protect a resource that is being disproportionately impacted,” Mitchell said.