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Floating Wetlands: Five Lessons Over Eleven Years

The National Aquarium started experimenting with floating wetland technology in 2010. The goals were to reintroduce wetland habitat into the Inner Harbor and promote healthy water. What lessons have we learned?

  • Conservation

In 2010, the National Aquarium installed a 200-square-foot floating wetland in Baltimore's Inner Harbor. This was the first time in the United States this technology was introduced into a brackish tidal system. Prior to our initial efforts, floating wetlands were traditionally used in stormwater retention ponds. Over the past 11 years, the Aquarium's Conservation team has refined the floating wetland design to develop a model that best fits the specific needs of the Inner Harbor, evaluating its progress through scientific research.

These wetlands are part of the National Aquarium's work—and a broad citywide effort—to restore the harbor and improve its ecological health.

Based upon their ongoing work with and maintenance of the floating wetland, the Aquarium's Director of Field Conservation Charmaine Dahlenburg and Conservation Aide Langston Gash spoke with us to summarize five lessons learned from the floating wetland project over the past 11 years.

1. Not all floating wetlands are created equal.

The Aquarium's 2010 floating wetland, located in the water between Piers 3 and 4, faced overuse from Canada geese, an accumulation of invasive dark false mussels and a failed anchor system. It was retired in 2013. Another version, installed in 2012, was tested and retired a short time later due to similar challenges. Version three was installed in 2015 and remains in place today. It's being used for nutrient uptake studies but will soon be retired.

Using information gained from the first three floating wetlands, the Aquarium began rethinking the technology and developing a custom floating wetland unique to the needs of the Inner Harbor. In August 2017, this new floating wetland prototype—version four—was introduced.

This prototype is custom-designed with four key components our previous floating wetlands lacked: elevation changes to allow for a variety of high and low marsh shrubs and grasses; a center channel with moving water that mimics shallow-water habitat for native wildlife; an aeration system to mix the upper portion of the water column surrounding the prototype while also providing the mechanism to control where the prototype sits in the water column through a pontoon ballast system; and a structure made from materials designed to withstand the harbor's brackish water and Baltimore's four seasons.

"While this four-year-old, 400-square-foot prototype continues to perform well, our next challenge is figuring out how to scale it up to cover about 15,000 square feet while minimizing costs," says Charmaine.

2. The harbor experiences extreme water quality events.

Baltimore's Inner Harbor commonly experiences two types of water quality events—algal blooms and sulfur bacterial blooms—that can turn the water unusual colors, change the way it smells and negatively impact wildlife.

As part of the Maryland Department of Natural Resources' Eyes on the Bay project, the Aquarium continuously collects and reports core water quality parameters in the harbor. This 24/7 monitoring provides important, near real-time data about water quality events as they are happening.

In one instance, the Aquarium measured levels of chlorophyll from an algal bloom that were so high, Charmaine thought the equipment may be malfunctioning. For our area, a healthy level of chlorophyll in a body of water is 15 micrograms per liter. Fifty micrograms per liter is considered a significant amount of chlorophyll, while 100 micrograms per liter is considered extreme. On January 15, 2020, the Aquarium's devices, called sondes, recorded chlorophyll levels averaging 400 micrograms per liter. One reading reached 670 micrograms.

"When I contacted the manufacturer of the probe, they said they'd never seen readings that high," Charmaine recalls. "Over the history of the Eyes on the Bay program, of all the data collected across the state of Maryland, the Aquarium's sondes in the Inner Harbor have recorded the top 1,000 to 2,000 chlorophyll readings, which is significant."

Algal blooms are caused by excess nutrients, like nitrogen and phosphorus, entering the water through human activities. This rapid growth in algae creates a chain reaction leading to extended periods of low dissolved oxygen when the algae die and consume oxygen as they decompose. Unfortunately, aquatic animals struggle to survive in these conditions.

"If you see animals swimming at the surface of the harbor, it can be a red flag that they're struggling for oxygen due to an extreme water quality event," Langston explains.

3. Aeration is key.

The aeration system on the floating wetland prototype—which taps into the system used inside the Aquarium—offers stability for animals when dissolved oxygen levels are low during water quality events.

Charmaine says that as soon as the aeration system was turned on in 2017, the team noticed wildlife responding to it.

"Fish tend to accumulate around the aerators by the hundreds or thousands," she says. "It's clearly creating a microhabitat that's beneficial for wildlife."

To evaluate the aeration component of the wetland, the team is conducting a study to compare water quality data from the center channel of the floating wetland with data from another part of the harbor. While this research is continuing, the preliminary data is positive, showing cooler, more stable temperatures in the channel where dissolved oxygen levels are higher thanks to the aeration system.

4. The harbor is teeming with hidden life.

"Qualitatively, we know our floating wetland provides habitat for aquatic life; we can see the diversity," says Charmaine, mentioning the pumpkinseed sunfish, gizzard shad, snapping turtles and ghost anemones seen on and around the wetland recently. "However, we needed to find a way to quantitatively assess the Harbor's biodiversity."

Following Maryland Sea Grant's biodiversity and biofilm project, Charmaine, Langston and researchers from the Institute of Marine and Environmental Technology submerge acrylic disks in the Inner Harbor to analyze the communities that grow on them. These biofilms are the foundation of the food chain and attract other animals.

Each month, the team collects three disks from the harbor, records video of four random sections under a microscope and then views the video to count and identify the organisms. Samples scraped from the disks are further analyzed using DNA barcoding, a process conducted in partnership with volunteers at Baltimore Underground Science Space and IMET.

Langston, who was first involved with the Aquarium as an intern at IMET, reported that in 2017, a total of 248 different species were identified on the acrylic disks through DNA barcoding.

5. The harbor is restorable.

Charmaine says the Charles River Conservancy in Boston is a good model for Baltimore; the two cities and their waterways have key similarities, along with some differences.

Three freshwater rivers empty into Boston Harbor, part of Massachusetts Bay, which connects directly to the Atlantic Ocean. Baltimore's Inner Harbor, on the other hand, is further inland and more isolated. It connects to the Patapsco, which flows into the Chesapeake Bay, which then flows into the ocean.

In the 1980s, Boston was under a court order to upgrade its sewer system, as Baltimore has been since 2002. Boston upgraded its wastewater treatment plant to prevent raw sewage from entering their waterways; they also turned a city dump into a park and wildlife refuge, similar to Baltimore's Masonville Cove. A new, 43-mile-long park connects Boston's neighborhoods to its waterfront. And while swimming in Boston's harbor had been prohibited since the 1950s, it no longer is.

"We don't know what the next 10 years will bring here in Baltimore," Charmaine says. "We are digging deep into the beneficial science of floating wetland technology. Living shorelines and floating wetlands are certified for pollution mitigation in nontidal areas; we hope to prove that our prototype provides similar, measurable benefits in tidal areas."

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