During a break between rain showers Monday, a crew of young people from the Washington Conservation Corps tended to a wide patch of Oregon grape, snowberry and hawthorne recently planted at Raab’s Lagoon. The project to replant the native foliage on the shoreline at the county-owned park is just one of a wide range of efforts currently being carried out by county, state and local organizations to improve the health of the Quartermaster Harbor.

A study recently completed by King County, however, sheds new light on one aspect of the ailing harbor — its dangerously low oxygen levels.

Scientists recently completed a four-year study on Quartermaster’s low oxygen levels, which threatens life in the harbor. The extensive study found that human influences on the harbor, such as septic system contamination and fertilizer run-off, likely don’t play as significant a role in the low oxygen levels as once thought. However, some familiar with the situation and the study say it’s as clear as ever that conservation efforts should continue, as any additional pollution to the harbor could be detrimental.

“You don’t want to get over the tipping point. You’re already really low on oxygen there,” said Cheryl Greengrove, a physical oceanographer and professor at the University of Washington-Tacoma who took part in the study. “It’s an area of concern that causes us to be more vigilant about what we put into the system.”

Earlier this year, King County released a final report on the Quartermaster Harbor Nitrogen Management Study, a four-year effort to determine whether nitrogen-loading is leading to the bay’s depleted oxygen levels and what role human activity plays in the equation.

The $893,000 study, funded by a $625,000 grant from the Environmental Protection Agency, as well as funds from King County, the University of Washington-Tacoma and the state Department of Ecology, started in 2009 but built on years’ worth of data collected by scientists and field researchers.

The study comes at a time of mounting concern about the health of Puget Sound and Quartermaster Harbor. In past years samples taken from the harbor have shown very low levels of dissolved oxygen, a potentially serious situation. Low oxygen levels have been implicated in the massive fish kills that have plagued Hood Canal and other water bodies. They can also threaten sea life that depends on oxygen, particularly creatures that can’t easily move to more oxygenated waters.

“You get a different type of ecosystem if you chronically have low oxygen levels in some areas,” said Curtis DeGasperi, a hydrologist in King County’s Department of Natural Resources and Parks and the lead scientist on the study.

Excess nitrogen has been identified as a source of pollution in parts of Puget Sound and a cause of low oxygen levels, DeGasperi said. The naturally occurring element creates a cycle that can suffocate a water body: It triggers the rapid growth of algae, which eventually decompose; as they do so, bacteria that break down the algae suck oxygen out of the water.

Dissolved oxygen levels below 5 milligrams per liter are considered unhealthy, and at 3 milligrams per liter, the underwater environment can be stressed. During the study, oxygen levels in Quartermaster were observed dipping below 1 milligram per liter. While the levels stayed low only for a time, and no impacts on marine life have been observed so far, researchers say the extreme lack of oxygen is a cause for concern.

“If it got worse, I think it would be bad news,” Greengrove said.

DeGasperi said the study confirmed one thing researchers already suspected. In Quartermaster Harbor, nitrogen levels directly affect oxygen levels. As algae requires nitrogen to grow, the more nitrogen that is present in the harbor, the more algae grows and the more oxygen levels are driven down.

“It’s something that’s been noticed in other parts of Puget Sound,” he said, “and it’s the prime reason why nitrogen is the nutrient people are most concerned about.”

The study also confirmed that the physical makeup of the Quartermaster Harbor only exacerbates the situation. Researcher found that the inner harbor, with a narrow mouth facing away from incoming ocean tides, takes 100 days to fully flush.

“Anything that you put in the inner harbor is going to stay there for a long time,” Greengrove said.

During the summer months, light easily penetrates the shallow and stagnant bay, creating a petri dish effect where oxygen-depleting algae thrive. In fact, Quartermaster has had the highest concentration in Puget Sound of the algae that causes paralytic shellfish poisoning. And last summer another harmful algae bloom was blamed for a small fish kill.

On Vashon, concerns over nitrogen in the harbor and its contribution to low oxygen levels have been one driver behind local conservation projects.

The Vashon-Maury Island Land Trust, for instance, has worked with owners of properties near Judd Creek to keep livestock manure, a source of nitrogen, out of the stream, which empties into the harbor. And this month King County will put on a manure management workshop aimed at keeping manure out of groundwater, which also ends up in the harbor.

County-sponsored projects such as the the native plant planting at Raab’s Lagoon and an even larger one underway at Dockton Park can also work to improve the situation, as foliage with deep roots can consume nitrogen before it makes it into the harbor.

And while a large-scale effort to get septic systems in Vashon’s Marine Recovery Area (MRA) inspected and fixed is aimed at reducing septic contamination in the harbor, officials say it could also work to reduce nitrogen outputs from failing systems.

Water quality in the harbor has already improved because of the effort, according to Darrell Rodgers, manager of community environmental health in the King County’s health department. The state recently opened up more than 120 acres of shellfish beds in the harbor that had been closed due to contamination, he said. The state also recently announced a low-interest septic loan program has expanded to include more than 2,000 parcels on the island.

“The goal is to clean up Puget Sound and clean up Quartermaster Harbor,” Rodgers said. “We’ve got different tools in our belt.”

When it comes to nitrogen, however, the recent study found that the largest contributors in the harbor may not be human causes, but natural ones. DeGasperi said that testing and modeling completed as part of the study revealed that most of the nitrogen in the harbor is naturally occurring and actually enters the harbor from the Puget Sound and the Pacific Ocean. And in the inner harbor, he said, natural processes that occur in sediment deposited historically on the harbor floor — possibly by human activity — also play a greater role than once thought.

“That’s a big driver of the oxygen problem in the inner harbor,” DeGasperi said.

He said researchers originally hoped to gather more specific data on the smaller amounts of nitrogen that could be traced to other sources — septic systems, manure, fertilizer and even alder trees — perhaps providing more helpful information to islanders. But doing so proved to be too complex and time consuming, DeGasperi said.

“We were overly ambitious in thinking we could accomplish that in the time frame we had and given the resources we had,” he said.

Nevertheless, DeGasperi said he expects human contributions to the low oxygen situations observed in the harbor are minimal, and he efforts to reduce nitrogen inputs into the harbor may have little effect in the end.

“Those kinds of subtle changes, we wouldn’t be able to measure them,” DeGasperi said.

Tom Dean, who has paid attention to the issue as director of the land trust, said he was surprised at the results of the study and wanted further information.

However, he said he believes the land trust’s efforts around the harbor and Judd Creek are still important. Most conservation projects have wide effects on the surrounding ecosystem, he said, with water quality in the harbor being just one consideration.

For instance, Dean said, the unnaturally abundant alder trees on Vashon release nitrogen into the harbor. Replacing them with other trees can help address that issue and also creates a more healthy, balanced forest.

He added that he felt releasing less nitrogen into the harbor could only be a good thing.

“My glass is always half full,” he said. “There are things we can do to improve the situation, even if there’s no magic fix.”

Greg Rabourn, Vashon’s basin steward for King County, also said projects such as the one at Raab’s Lagoon have wide goals. Native trees do consume nitrogen, but perhaps more importantly, they improve soil, create habitat for animals and a build a healthier shoreline in the harbor, something desperately needed to aid in the recovery of salmon.

“The primary focus is to restore natural functions,” Rabourn said. “Nature will generally solve the problem as we restore the natural processes. Something like this is just a no-brainer.”

Greengrove expressed more optimism about conservation efforts than DeGasperi, saying she believes any project that reduces nitrogen in the harbor by any amount is important. She noted that questions about the smaller nitrogen contributors remain, and it’s likely the work will be picked up in the future.

“We build our knowledge base this way, but we don’t always get the answer to questions we want without further study,” she said.