A reef seeded with oysters by the state of Maryland — about 130 oysters per square meter — removed 20 times more nitrogen pollution from stuff such as home lawn and farm fertilizer in one year than a nearby site that had not been seeded, according to a recently released study.
The upshot, said Lisa Kellogg, a researcher for the Virginia Institute of Marine Science who led the four-year study, is that oyster reefs could potentially remove nearly half of nitrogen pollution from that one river on Maryland’s Eastern Shore “if you took all the areas suitable for restoration and restored them.” A wider restoration could help clean the Chesapeake Bay, where the Choptank and other major rivers drain.
It is a huge deal, Kellogg said. Man-made nitrogen pollution is part of a one-two punch that creates oxygen-depleted dead zones that have bedeviled the bay. At one time, when oyster reefs were so mountainous and plentiful that European explorers complained about navigating around them, the Chesapeake was crystal clear.
Oyster reefs are more than just the rocks of ages. They are the ultimate mixed-use development, inhabited by more than 24,500 marine animals that are not oysters — mussels, clams and sea squirts, to name a few — that also filter nitrogen.
Excessive harvesting of oysters, combined with massive farm and urban pollution, depleted the bivalves, denuded reefs and clouded the water by the 1980s. About that time, two diseases, Dermo and MSX, came out of nowhere to decimate the stock of oysters in Maryland and Virginia.
Oysters in those two states are experiencing a modest recovery because of the restoration and farming known as aquaculture.
In Virginia, 236,200 bushels of oysters were harvested two years ago, up from 79,600 bushels in 2005. Maryland took in 121,000 bushels in 2011, nearly 95,000 more than 2005.
Kellogg and her fellow researchers wanted to show that oyster reefs could greatly improve water quality and are worth the millions of dollars being invested in their restoration.
They also hoped to build on previous studies that had depended on simulations to estimate nitrogen removal at oyster reefs. Their study, started in 2009, used actual measurements of nitrogen levels.
“There was a huge amount of interest in nitrogen removal,” said Jeff Cornwell, a researcher at the University of Maryland Center for Environmental Science who contributed to the study.
The researchers measured the amount of nitrogen gas production in water channels on a reef near the Emerson C. Harrington Bridge on the Choptank at Cambridge. They did the same at a site where restoration had not occurred.
The study was funded by NRG Energy.
“The rates we’ve seen in the Choptank for removal is the highest we’ve seen anywhere,” Cornwell said. “The . . . results are so promising. As we develop data sets we can calculate what oysters did when they were a larger population.”
Bivalves scarf up phytoplankton that consume nitrogen. Phytoplankton produce algae blooms that die, turn into a sticky black goo and suck oxygen — needed by pretty much every inhabitant of bay tributaries — from the water. Dead zones deplete the bay of life and rob predators such as eagles and osprey of food.
When oysters and other creatures eat phytoplankton, they poop it into dark areas of the reef where microorganisms feed. Those organisms convert the nitrogen to nitrogen gas that wafts into the atmosphere, Kellogg said.
“The good news about oysters is if we do it right and give them a chance . . . they produce huge numbers of eggs each year,” she said. “It’s a prolific species that can provide this habitat again, to the extent that we allow it.”
That message hasn’t fallen on deaf ears. This month, Virginia plans to embark on its largest state-funded oyster replenishment in history, focusing on sites in the James, York, and Rappahannock rivers and in the Chesapeake in Pocomoke and Tangier sounds, according to the Virginia Marine Resources Commission.
The state has already created huge sanctuaries in public waters where watermen are restricted to harvesting oysters on a rotating basis about every two years. In March, the Virginia Marine Police hit 10 people with 115 charges related to oyster violations.
Maryland has been even more aggressive about restoring oyster habitat, pouring $50 million into oyster recovery over the past 16 years. Oyster harvesting is forbidden by the state on a quarter of the bars where they grow; violations can draw fines of up to $25,000 and as many as 15 years in prison.
For years, Maryland scientists at the Horn Point Oyster Hatchery in Cambridge have put oysters in the mood to mate, bathing them in warm water tanks. When the vibe is right, the males spew sperm, the females spread eggs and, soon, larvae develop.
The larvae live on a steady diet of algae until they develop into spat — like teenagers ready to move on to shells of their own. Millions are taken by boat onto the Choptank and Harris Creek and dropped onto reefs.
Cornwell and Kellogg hope their work will encourage states to fulfill a mission to create 98 acres of reef a year — even though the diseases that kill the bivalves continue to be a threat.
“You have to leave them out, have them develop a resistance and have a self-sustaining population,” he said. “There has to be a jump-start to have the oysters come back in a more natural way.”
Cornwell paused for a few seconds, thinking about the risk. “If they’re all dead soon, you have to evaluate the value,” he said.