Despite record high river flows that sent huge amounts of sediment and nutrients into the Bay last year, the amount of underwater grasses — considered by many to be the best overall indicator of the Chesapeake’s health — increased.
The expansion came as something of a surprise. Not only did it reverse two straight years of declines, but it did so under conditions generally considered to be bad for the grasses.
Grasses — also known as submerged aquatic vegetation, or SAV — depend on clear water. High flows typically flush in huge amounts of sediment, which cloud the water, and large quantities of nutrients, which spur sun-blocking algae blooms.
“I find it fascinating that in spite of last year’s large runoff events, the SAV generally seemed to hold their own,” said Bob Orth, of the Virginia Institute of Marine Science who conducts an annual grass bed aerial survey funded by the Bay Program.
Last year, the survey found 63,400 acres of grass beds in the Chesapeake and tidal portions of its tributaries. While that was only about one-tenth of the roughly 600,000 acres of SAV thought to have once filled the Bay, it was an increase of about 3,500 acres, or 5.8 percent, from 1995. It was also 66 percent more than the low mark of 38,135 acres observed in 1984, though less than the recent high of 73,081 acres spotted in 1993.
Grasses are considered critical to the Bay because they provide important nursery areas, food and habitat for a wide range of Bay species, from blue crabs to fish to waterfowl.
The survey found increases in all major areas of the Bay:
- In the upper Bay, roughly from the Chester and the Magothy rivers north, grasses increased by about 5 percent.
- In the middle of the Bay, from the upper Bay south to the Pocomoke and just north of the Rappahannock (including the Potomac River), grasses increased by 8 percent.
- In the lower Bay, grasses were up by 2 percent.
Within the three large regions, the Bay is divided into 45 smaller tributary segments that generally reflect differrent salinity regions. Of those, 31 had some grasses in 1996, of which 22 had increases and 9 had decreases. Within those segments, the picture of SAV health grows murkier as some recorded dramatic increases in places while having sharp declines in beds only a few miles away.
In the upper section of the Potomac River near Washington, for example, much of the SAV, including a large bed near the Woodrow Wilson Bridge, disappeared last year. But this was offset by areas just south of the bridge in that segment.
Overall in the Potomac, the amount of grasses increased, even though the river took the brunt of last year’s flood events. “What’s interesting is we haven’t seen a general overall decline in the upper Potomac,” Orth said. The Potomac recorded its highest-ever flows in 1996, averaging 22.3 billion gallons a day, 2.3 times higher than normal. (By comparison, flows out the Susquehanna, the Bay’s largest tributary, were only 1.6 times higher than normal).
The timing of last year’s peak flows may have helped beds. While flows were higher than normal all year, the greatest amounts came in January — before grasses started — and in September, after the most critical part of the growing season.
Perhaps because of that timing, some rivers that had increases — such as on Maryland’s Western Shore — had improved water quality last summer despite overall high flows, according to monitoring data.
“Water quality doesn’t know the rules about how it’s supposed to respond to flow, nobody told it,” said Peter Bergstrom, a biologist with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office, and chair of the Bay Program’s Submerged Aquatic Vegetation Workgroup. “It looks like in at least three of the Western Shore tributaries, the Patapsco, the Magothy and the South, conditions actually got better in 1996 compared with 1995 — exactly the opposite of what you would expect.”
In those tributaries, scientists were pleased by the continued rebound of redhead grass, one of several particularly important species of SAV because they thrive during the summer, providing habitat at a critical time for juvenile fish. Redhead beds continued to expand in the Severn River, where a large bed was first observed near Aisquith Creek in 1994. Beds also continue to thrive in the Upper Patuxent.
In coming weeks and months, scientists will compare grass beds observed in the more than 1,500 aerial photos taken during last year’s survey to water quality information for those areas in an attempt to explain some of the changes.
The greatest declines were observed in Eastern Shore tributaries, from the Chester River north and in the Manokin and Big Annemessex rivers. All of those rivers also showed declines in water quality, with high concentrations of sediment or algae in the water.
Making the water quality link is not always obvious, though. Some creeks feeding into Maryland’s Gunpowder River showed dramatic increases in SAV, while water quality data indicated that overall river conditions declined from 1995 to 1996.
That could be because the water samples were taken too far downstream from the grass beds and did not reflect the conditions in the bed. Or, it could mean that samples — typically taken every two to four weeks — may have missed swings in water quality that affected the growth of the plants.
“We’re not out there day in and day out,” Orth noted. “The plants are. Over a critical time period, if the water quality is good, they’re going to go bonkers and take off and grow.”
And, if the conditions are bad during the critical spring period when they undergo their most rapid growth, the plants may die.
“These plants are very, very sensitive to small changers in water quality,” Orth said. “They don’t store a lot of energy. It’s not like a bear hibernating which, if it has a bad year, can live off its fat.”
Plant biology also plays an important role. Some areas of the Bay have water quality that should support SAV, but no grasses. That could be because there are no nearby beds to supply seeds. Likewise, some areas that have grasses but only marginal water quality may succeed because they have an ample supply of seeds, and just enough “good” water conditions during critical growing times, Orth said.
That could explain the appearance last year of a new bed in the lower James in an area that does not appear to meet the water quality requirements for grasses, but is near another bed.
Orth and Bergstrom cautioned that grass beds sometimes appear to reflect “lag effects” from year to year, which means that some delayed impacts from the 1996 high flows may not be seen until this year. “One year at a time may just be too short to relate SAV to water quality,” Bergstrom said.
Grasses declined sharply for years after Hurricane Agnes hit the Chesapeake with record flooding in June 1972. Grasses hit their lowest level in 1984 and gradually increase during the relatively dry years in the late 1980s until a series of unusually high flow years hit in 1993, 1994 and 1996.
Protection and restoration of their beds is one of the Bay Program’s top priorities. Not only do they provide habitat, but, if present in large beds, they can improve local water quality by absorbing nutrients, filtering sediment and pumping oxygen into the water. The Bay Program has an “interim” goal of restoring 114,000 acres of grasses to the Chesapeake by 2005.