Chesapeake Now: Underwater Grasses
Aug 01, 2014 09:00AM ● Published by Cate Reynolds
Restoring One of the Chesapeake Bay’s Most Precious Resources
by Lisa A. Lewis
Somewhere in the vast expanse of the Chesapeake Bay, juvenile blue crabs seek refuge from predators in a dense bed of underwater grasses. Although the crabs’ behavior may not seem significant, there is a definite connection between their choice of habitat and their potential ability to survive. In fact, the blue crabs may escape detection and elude capture, thanks to an often overlooked—yet vital—resource: underwater grasses.
Concealed beneath the shallow waters of the Chesapeake Bay—gently swaying rhythmically with the tide—lies a veritable wonderland of underwater grasses. Although each species varies in appearance and responds differently to factors such as weather and growing conditions, these underwater grasses, also known as submerged aquatic vegetation (SAV), are critical to the Chesapeake Bay ecosystem. Indeed, SAV is closely monitored by scientists because its health is dependent on good water quality, and, therefore, its abundance is an excellent barometer of the overall health of the Bay.
Each year, thanks to the generous support of its partners, the Virginia Institute of Marine Science (VIMS) maps SAV beds in the Chesapeake Bay using aerial photography. The aerial surveys, which are conducted from late spring to early fall, estimate the annual acreage of SAV. (VIMS began tracking SAV in 1978.) SAV abundance had been declining up until the early 1970s, and since that time, there was a slow but steady increase from 1984 until 2002. During the following years, there have been fluctuations in abundance, so researchers were very encouraged to observe an increase in acreage in 2013. According to the most recent report (the 2013 Seagrass Report), which was released in April 2014, the abundance of SAV in the Chesapeake Bay increased 24 percent between 2012 and 2013, reversing the downward trend of the previous three years. (The report is released the following spring.) This figure reflects an increase from 48,195 acres to 59,927 acres.
“It’s really encouraging that we’re seeing recovery in some areas, especially given the fact that this is the first rebound we’ve seen in three years,” says Lee Karrh, a biologist for the Maryland Department of Natural Resources (DNR) and chair of the Chesapeake Bay Program SAV Workgroup. “But it’s important to put the data in context. Abundance is still down, and we’re still below our goal of 185,000 acres. In 1972, Tropical Storm Agnes really wiped out SAV beds—not only in the Susquehanna Flats but throughout a lot of the upper Bay. However, SAV had been declining even before that. It slowly recovered over the next 40 years, peaking on the Flats in 2009 and surpassing our restoration goal. The data from the 2013 Seagrass Report is promising, and it’s always great to see an increase in SAV abundance. But we definitely need to keep going in that direction.”
VIMS scientists attribute the increase to the rapid expansion of widgeongrass in the saltier waters of the mid-Bay, from the Pocomoke Sound to the Honga River, south of Cambridge, Maryland, and an increase in the acreage of the Susquehanna Flats. There was also a moderate recovery of eelgrass in the shallow, salty waters, where the hot summers of 2005 and 2010 caused significant diebacks.
“The expansion of widgeongrass in the mid-Bay’s saltier waters was one of the driving factors behind the overall rise in bay-grass abundance,” said Professor Robert “JJ” Orth, head of the Seagrass Monitoring and Restoration Program at VIMS, in a press release. “While widgeongrass is a boom-and-bust species—notorious for being incredibly abundant one year and entirely absent the next—its growth is nevertheless great to see.”
A New Era
The aerial surveys conducted in 2013 were especially significant—not just because of the increase in SAV but also because they introduced a change in the way the data is reported. For the first time, SAV abundance was mapped in four different salinity zones rather than by geographic zone, which was how it had been reported since 1984. Since each salinity zone is home to specific underwater grass communities that respond differently to various conditions, reporting SAV by salinity zones makes it easier for scientists to observe trends in different communities over time. Mapping SAV by salinity zones instead of geographic zone makes more sense from an ecological perspective.
“This data is a reminder of just how resilient the Bay’s underwater grasses can be,” said Nick DiPasquale, director of the Chesapeake Bay Program, in a press release. “I am heartened by the news of increases in all of the salinity zones of the Bay, from Newport News to the Susquehanna Flats. Such visible signs of improvement in the ecosystem should encourage everyone’s commitment to strong restoration efforts throughout the entire watershed.”
Benefits of SAV
Scientists estimate that there are at least 17 species of SAV in the Chesapeake Bay. Like plants that grow on land, SAV has leaves, roots, conducting tissues, flowers, and seeds and makes its own food through photosynthesis. However, underwater grasses lack strong, supportive stems that are necessary to survive on land and are supported by the buoyancy of the water. Since these unique plants must be submerged by water at all times, they can only grow in shallow water—sometimes as shallow as three feet deep or less, especially if the water is murky—so that sufficient sunlight can reach them. In addition, SAV cannot grow next to the shore because it would be exposed at low tide, causing it to dry out and possibly be uprooted by the motion of the waves.
SAV plays a critical role in the Chesapeake Bay ecosystem. It serves as food for waterfowl and as foraging grounds for fish. It also provides habitat for juvenile and adult fish and shellfish and offers shelter from predators. The two species that drove the increase in SAV abundance in 2013, eelgrass and widgeongrass, are especially beneficial to wildlife. Eelgrass provides habitat for juvenile blue crabs, seahorses, pipefish, and speckled sea trout. In addition, eelgrass serves as food for Brant geese, Canada geese, wigeons, redhead ducks, and black ducks. Like eelgrass, widgeongrass is also an important food source for waterfowl. In addition, SAV absorbs nutrients, produces oxygen, improves water clarity, settles suspended sediments, stabilizes bottom sediments, absorbs wave energy, and reduces shoreline erosion.
“SAV is so important to the Chesapeake Bay because it provides a number of benefits, or ecosystem services,” says Dr. Christopher Patrick, an ecologist at the Smithsonian Environmental Research Center (SERC), who works in the Ecological Modeling Lab headed by Dr. Donald Weller. “These processes are beneficial to the ecosystem and also good for the food web.”
Threats to SAV
Since SAV is so beneficial, it is imperative to take steps to reduce threats to its survival. One of the primary factors that adversely affects the health and abundance of SAV beds is pollution. Sediments from runoff cloud the water, and nutrients, such as nitrogen and phosphorus, fuel algae growth in the water column and on the plants. This prevents sunlight from reaching the plants, causing SAV beds to die. Healthy grass beds absorb some nutrient pollutants, which improves water quality. But it’s a delicate balance: Too much pollution is detrimental, so the ultimate goal is to reduce the amount of pollution that enters the Bay.
According to the DNR, nutrients come from a variety of sources, both natural and as a result of human activities. Wastewater treatment plants, agriculture, and runoff from the land during storms all cause pollution to enter the Chesapeake Bay. In addition, water temperature and weather conditions, such as storms and droughts, also affect the growth and survival of SAV.
The Chesapeake Bay Program and its partners are dedicated to improving water quality, protecting existing grass beds, raising awareness, and promoting outreach and education in order to restore SAV in the Chesapeake Bay. Fortunately, there are several ways to support the resurgence of SAV, and the good news is that everyone can play a role. From upgrading wastewater treatment plants with pollution-reducing technologies to implementing best-management practices to using rain barrels, everyone can serve as strong advocates for the Bay.
“Our long-term goal is to continue to see SAV expansion and once again reach historic levels,” Patrick says. “We can accomplish this by reducing nutrient pollutants and sediment runoff, which, in turn, will improve water quality and increase SAV abundance. We’ve known that clear water leads to an increase in SAV for a long time. We just need to continue to work toward that goal.”
“Restoring SAV is an integral part of cleaning up the Bay,” adds Karrh. “Not only will it make the Bay healthier and more aesthetically pleasing, but it will also attract more wildlife, offer more access and opportunities for recreation, and boost the fishery industry. But in order to reach our goals and restore SAV, everyone must work together. Even little changes, such as driving less often and not using fertilizer, can have an impact. If we work together and make good choices, we can make a difference and protect the Bay for future generations.”