Bombay Hook National Wildlife Refuge on Delaware Bay.

Bombay Hook National Wildlife Refuge on Delaware Bay. JAMES O'GUINN, FLICKR

Domino Effect: The Myriad Impacts of Warming on an East Coast Estuary

Delaware Bay provides a case study in how warming oceans, more severe storms, and sea-level rise are impacting estuaries around the world. The effects — from loss of wetlands to steep declines in shorebird populations — cascade throughout the ecosystem.

The 140,000 acres of tidal wetlands in Delaware Bay sustain hundreds of aquatic and terrestrial species, including the second-largest population of shorebirds in North America. Yet, as sea level increases — now rising at about 1.2-inches per decade and expected to dramatically accelerate this century — this habitat is vanishing. 

“The Delaware estuary is like the Mississippi mouth — naturally muddy and wetland rich,” says Danielle Kreeger, science director for the Partnership for the Delaware Estuary. “Most of your signature fauna depend on the marshes for breeding, nursery, and feeding. We’re losing an acre a day now, but we’re probably going to see a colossal blowout in the next 10 to 20 years.”

Delaware Bay, one of the largest and richest estuaries in the United States, is a case study in how warming oceans, associated storms and sea-level rise are eroding wetlands, damaging water quality, and unraveling terrestrial and near-shore aquatic ecosystems in many parts of the world. In Delaware Bay, the victims include such interdependent organisms as trees, marsh grasses, fish that use the wetlands for spawning and nursery habitat, oysters, mussels, crustaceans, shorebirds, marsh birds, and waterfowl.

As Kreeger and other researchers have observed, rising seas spreading over relatively flat estuaries turn low-lying marshes into open water and transform higher-elevation wetlands — those rising up to roughly three feet above current sea levels — into saltier marshes that will increasingly be subject to Delaware Bay’s daily tide swings of six feet. 

Sea levels in Delaware Bay are now rising at a rate of roughly 3.5 millimeters (0.14 inches) per year.  Kreeger says that by 2100, when that rate is expected to double or triple, Delaware Bay could lose 90 percent of its wetlands to rising seas, wreaking havoc with a complex ecosystem.

“Not only are salt marshes being fractured and ‘pushed’ further upland, but salt water is now intruding farther into the creeks, leading to the demise of the trees,” biologists Robert Loveland and Mark Botton wrote in the book “Changing Global Perspectives on Horseshoe Crab Biology, Conservation and Management.” “The very beach itself is being eroded, overwashed and moved inland; areas of peat — remnants of earlier salt marsh vegetation are being exposed for the first time in thousands of years.” The pair noted that as far back as the early 1980s they began to see dying forests near the banks of small creeks collected by Delaware Bay.

One of the first bird victims was the black rail, historically abundant throughout Delaware and Chesapeake bays. “In the 1990s it was doing well,” says Gregory Shriver, who teaches wildlife ecology at the University of Delaware. “But revisits in the past five years have shown a 90-percent reduction.” The marsh habitat the birds require has been inundated.

With sea-level rise, super-high tides inundate the nest of saltmarsh sparrows, causing most of the young to perish.

Shriver and his colleagues also have documented a four-percent annual decline in clapper rails. Clapper rails can tolerate saline conditions, but as lower marshes are flooded they’re forced up into brackish and freshwater marshes — habitat of king rails. The two species hybridize. So king rails are in danger of genetic swamping, and both species are being pushed up against human development. 

The saltmarsh sparrow is in desperate trouble in Delaware Bay and throughout its range, from Maine to Virginia. Shriver’s team has documented an annual 9-percent population decline over the past 17 years. Females weave cup-shaped nests a few inches above the high-tide mark but below the top layer of saltmeadow cordgrass, the better to hide clutches from predators. Young fledge in 24 to 26 days, but that’s about the duration of the lunar cycle. So with sea-level rise, the super-high tides that come with the new and full moons now inundate nests. Eggs can float and survive, but hatchlings are rarely strong enough to make it to higher ground. 

The problems faced by swamp sparrows and other marsh-nesting birds are hardly confined to Delaware Bay, as rising seas increasingly inundate wetlands worldwide. “Based on our findings, we predict a collapse of the global population of saltmarsh sparrows (A. caudacutus) within the next 50 years and suggest that immediate conservation action is needed to prevent extinction,” Maureen Correll, Shriver, and fellow researchers wrote in the August 19, 2016 issue of Conservation Biology.

Imperiled least terns are also taking a hit. “They nest right on the beach,” says Shriver. “Nesting success drops as you get more overwash and increased flooding.” 

Delaware Bay’s beaches also provide the world’s most important spawning habitat for horseshoe crabs. But these beaches are being eroded by a rising sea. The State of Delaware has responded by adding sand. But New Jersey has relied more on metal and concrete structures that repel waves, creating backwash that carries sand seaward. 

Loveland and Botton write that a comparison of aerial photographs from the 1930s with contemporary satellite imagery clearly shows “a landward movement of the shoreline along the New Jersey coastline of Delaware Bay.” The construction of bulkheads, groins, and jetties also has eroded sections of the shoreline and reduced the suitability of beaches for horseshoe crabs and shorebirds, which feed on horseshoe crab eggs. 

With an increase in storms, horseshoe crabs are having trouble spawning even on good sand. “They come onto the beaches to spawn when it’s calm,” says David Smith of the U.S. Geological Survey. “They don’t like rough weather.”

The scarcity of horseshoe crabs has diminished populations of shorebirds — none more so than the red knot which, as a result, was listed as threatened by the U.S. Fish and Wildlife Service in 2014. These birds migrate 9,000 miles between Arctic breeding grounds and wintering habitat in South America. On a given day, 90 percent of the nation’s red knots can be refueling on Delaware Bay beaches. “For red knots to access horseshoe crab eggs there have to be enough crabs arriving in sequential waves to dig up previously buried eggs,” says Gregory Breese of the U.S. Fish and Wildlife Service. Currently there seem to be enough crabs for surviving birds, but numbers of both red knots and crabs are depressed.

Red knots with horseshoe crabs. The birds feed on the crabs' eggs.

Red knots with horseshoe crabs. The birds feed on the crabs' eggs. GREOGY BREESE, U.S. FISH AND WILDLIFE SERVICE

“It’s really about how much quality beach habitat there is — the depth of sand,” Kreeger explains. “For horseshoe crabs to lay eggs sand needs to be at least two or three feet deep. A lot of beaches have not only lost width but also depth.” 

Coupled with loss of horseshoe-crab spawning habitat has been intense human exploitation, most recently by conch and eel fishermen who discovered that chopped horseshoe crabs make good bait. With better regulations, the crabs have at least stabilized, but the regulations were supposed to increase the population. “We haven’t seen that,” remarks Breese.

Because of a warming climate, red knots sometimes start their southern migration too late to catch Delaware Bay’s main horse-crab spawn. What’s more, red-knot recruitment is limited by predation of their hatchlings. Traditionally, lemming irruptions in the Arctic every three or five years have sated predators such as snowy owls, long-tailed jaegers, and Arctic foxes. But now those irruptions aren’t happening, and climate change is the suspected cause.

Meanwhile, a warming Arctic is causing the insects red knot hatchlings depend on to emerge earlier. Insect emergence is driven by temperature, but shorebird migration is driven by length of daylight. So red knots are arriving after the insects peak. A recent study published in Science found that juvenile red knots migrating from the northern Taimyr Peninsula of Russia to West Africa were underweight and had shorter bills. “The threat of extinction is more than real,” one of the researchers, Eldar Rahimberdiev, told the Washington Post in May 2016. 

Climate change benefits some species, but those benefits invariably come with costs. In the past, Delaware Bay’s oysters couldn’t live in the intertidal zone because it froze in winter. Now they’re surging into that niche; and they have longer growing and reproductive seasons. But the warmer, saltier water favors the oyster drill, a snail that bores holes into the oyster and sucks out its innards. Also favored by warmer water are two oyster protozoan diseases — Dermo and MSX. Dermo had been confined to Chesapeake Bay during the 1980s but by the mid-1990s had reached epidemic levels in Delaware Bay. Now it’s established in Maine.

Tests are underway with planting saltmarsh mallow and switchgrass, which could help protect beaches and marshes.

Atlantic menhaden, which patrol near-shore waters in enormous schools, maintain the bay’s water quality by filter feeding. But larvae mature in tidal creeks and marshes, and this nursery habitat is vanishing.

Oysters maintain water quality in the same way. But Kreeger has estimated that ribbed mussels, which get no respect or press because they’re commercially worthless, filter six times more water than oysters. Unfortunately, they live at the heads of marshes. “When marshes are healthy blue crabs can’t prey on ribbed mussels because they pack in so tightly they form an impenetrable wall,” Kreeger says. “But with all this marsh erosion the mussels break up and the blue crabs just go nuts.” 

The bay’s blue mussels are failing, too. They spawn in spring so their spat can set up on structures, but the water gets lethally hot. Richard Wong, a fisheries scientist with the Delaware Department of Natural Resources and Environmental Control, says that 20 years ago blue mussels covered pilings in Delaware Bay, but now he doesn’t see many.

“Because of the Clean Water Act [endangered] Atlantic sturgeon have been doing better in Delaware Bay,” says Kreeger. “But in the last five or ten years we’ve seen some regression. Early spring warmups add nutrients to the water column. We could get algal blooms and a drop in oxygen right at the time young sturgeon need it most.”

Can anything be done to slow the unraveling of coastal ecosystems? While significantly reducing greenhouse gas emissions appears unlikely in the foreseeable future, it may be possible to mitigate some of the effects of climate change. Tests are underway with plantings of saltmarsh mallow and switchgrass — species that can deal with some soil salinity. That could provide genuine protection for beaches, marshes, and seaside residents, notes Shriver.

And agricultural land and forests killed by high salinity could be transitioned to new marsh with public acquisition and management. Agencies like the Natural Resources Conservation Service — which assists farmers, ranchers, and forest landowners — could compensate landowners to improve their land for fish and wildlife

Corrections, May 22, 2017: An earlier version of this article incorrectly identified the mussels found in Delaware Bay; they are ribbed mussels, not red mussels. It is also incorrectly stated the number of acres of wetlands in Delaware Bay; there are 140,000 acres of tidal wetlands in the bay.