Chesapeake Bay Foundation scientists put reef balls laden with oyster spat into the Severn River on Wednesday — not to grow oysters, but to see if they might break up dead zones of low oxygen.
It’s an experiment. Can man made oyster reefs create enough turbulence in the water to improve oxygen levels in the water column?
The deepest areas of a waterway lose oxygen first because the waters are heaviest due to higher salinity content and cooler temperatures. Layers of water nearer the surface tend to have higher dissolved oxygen needed for aquatic life to survive.
Marine life will not survive in low oxygen conditions. But if the water column is mixed it helps distribute what oxygen is left, improving conditions for fish and other species.
“We know historic oyster reefs grew up in to the water column. They looked more like Manhattan skylines than the flat oyster beds we think of today,” said CBF Maryland Fisheries Scientist Allison Colden. “This project will test whether man made oyster reefs with vertical structure agitate currents and break up dead zones.”
If the experiment proves successful, that the agitated water flow helps increase oxygen, it could provide even more reason to accelerate oyster restoration efforts in the bay region.
On Wednesday the researchers and crew of CBF’s oyster restoration vessel, Patricia Campbell, placed 120 of a planned 240 reef balls in an area called the Winchester Lump, a rise in the river bottom where a large oyster reef existed, about a half mile north of the Severn River Bridge.
Once all are in place, a device will monitor changes in currents affected by the reef balls and any changes in dissolved oxygen brought about by agitating the water column.
CBF’s senior naturalist John Page Williams and U.S. Naval Academy Oceanography professor Andrew Muller have documented hypoxia — dead zones — in the area of the Winchester Lump.
“This bottom can go bad if we get two weeks of hot stable weather in July,” Williams said.
Williams said his monitoring indicated hypoxia around the Winchester Lump three weeks ago, awfully early in the season.
Hypoxia happens throughout the Chesapeake Bay and its tributaries especially in summer when algae blooms created by excess nutrients die off and gobble up the oxygen in the water column.
“In all these tributaries the salty water at the bottom comes from the bay,” said University of Maryland Center for Environmental Studies scientist Larry Sanford said before boarding the Patricia Campbell to retrieve his monitoring device from the bottom of the Severn.
The high salinity at the bottom contributes to the layering in a body of water. Warm summer temperatures at the surface also contribute to the layering because deeper water is much cooler and heavier.
Huge oyster reefs used to contribute to mixing the layers, Williams and bay scientists believe.
Historically oyster reefs were sometimes tall enough to create navigation hazards.
During a 1608 voyage of exploration, Captain John Smith wrote that the oysters “lay as thick as stones” in the bay.
“The abundance of oysters is incredible,” wrote the Swiss nobleman Francis Louis Michel in 1701. “There are whole banks of them, so that the ships must avoid them.”
But overfishing beginning with the great boom of oystering after the Civil War depleted oyster populations. Over 75 percent of the great oyster reefs were removed between 1870 and the 1920s, according to a CBF report published in 2010.
Add to that the ravages of disease. MSX and Dermo wiped out about 80 percent of the oyster reefs that were left since the 1980s.
Oysters not only provided jobs for thousands of Marylanders and good eating too, but they helped clean the bay. Oysters are said to be able to clean 50 gallons of water a day just through eating and respiration.
That, and the potential to keep the state’s historic oyster business alive, has spurred the state to begin a concerted effort to replant oysters across bay waters including large targeted oyster sanctuaries like Harris Creek off the Choptank River near Tilghman Island.
Many other areas have been planted with reef balls. Those installed Wednesday were loaded with oyster spat at the CBF oyster restoration station at Discovery Village in Shady Side.
Previous oyster reefs built with the 250-pound reef balls have proved successful in re-establishing oysters for oysters sake, and create habitat for other species. But the question is, will the new reef in the Severn provide the additional benefit of improving oxygen?
With the balls installed atop the Winchester Lump researchers hope that will be high enough in the water column to help agitate the currents and mix up the heavier waters with lighter more oxygen bearing waters higher in the water column.
Naval Academy and UMCES scientists will monitor the reef area currents and oxygen using sensitive equipment placed in the middle of the grid of oyster reef balls placed in groups of four on the bottom.
Once the Patricia Campbell was in place on Wednesday the first task was to retrieve the same monitoring equipment from the bottom where it had been pre-experiment data.
Once all the reef balls are in place the monitoring equipment will be redeployed.
The experiment is part of the Chesapeake Oyster Alliance’s effort to add 10 billion new oysters to the Chesapeake by 2025.
The Alliance is a broad partnership designed to spark governmental action, public attention, and funding to accelerate ongoing oyster restoration efforts.