Nature-Based Solutions to the Harbor's Nitrogen Problem

Plants and bacteria growing on the floating wetland draw excess nitrogen from the water—and attract native species that contribute to the effort, too.

  • Conservation
Three years after we installed our newest floating wetland in the Inner Harbor, we continue to evaluate the prototype’s materials, observe native species on and around it, and measure its impact on water quality.

Talking about the health of Baltimore’s Inner Harbor might bring to mind the bobbing bottles, bags and other plastic trash we can easily see. This stuff is, of course, a problem—but factors that determine water quality are mostly things we can’t see, like levels of dissolved oxygen, and acidity or basicity. In the Inner Harbor, one major water quality issue is an excessive amount of nitrogen. Nitrogen is an important element that all living organisms rely on, but too much of a good thing is … not good.

Excess nitrogen in the harbor has a few sources. It comes from polluted stormwater runoff, produced when heavy rainfall races over streets, sidewalks and other impervious surfaces and into storm drains, streams and the harbor itself, carrying pet waste, fertilizer, litter and other debris with it. It also comes from Baltimore’s aging sewage and stormwater systems, which were separated in 1905 after the Great Baltimore Fire. Today, 115 years later, both systems have cracked pipes, which allow stormwater to enter the sewage system and vice versa. When heavy storms overwhelm these pipes, nitrogen-laden raw sewage sometimes discharges into the Jones Falls, a stream that empties into the harbor.

Because plants feed on nitrogen, too much of it in the water fuels a repeating cycle of algal blooms and die-offs that then fuel bacterial blooms. This hurts fishes, turtles, crabs, birds—every animal that lives in the harbor or relies on it.

While preventing excess nitrogen from getting into the harbor in the first place is key, much of the National Aquarium’s conservation work on our campus is aimed squarely at removing it once it’s there—part of our overarching efforts to protect waterways and wildlife.

Evaluating the Wetland

The Aquarium’s newest floating wetland prototype has been in the Inner Harbor between Piers 3 and 4 for three years now; it was installed and planted with grasses in August 2017. In 2019, the waterfront campus project, which will expand and build upon the prototype, was supported with a $3 million gift from CFG Bank, and this fall, the Aquarium received grants from the Baltimore Office of Sustainability and 11th Hour Racing to support ongoing water quality research around the floating wetland.

Members Make It Happen

Did you know Aquarium members support important conservation projects like our floating wetlands? We are grateful to all our supporters—including you!

"Floating wetlands remove nitrogen from the water in two ways—there are biofilms, or beneficial bacteria, growing on the plastic base material itself, and native tidal marsh plants growing within that base,” explains General Curator of Living Exhibits Jack Cover. “These plants are growing hydroponically, in a soil-less plastic material similar to a giant floating Brillo pad. Their roots take up nutrients directly from the water. With the harbor’s high nitrogen content, they’re basically sitting in fertilizer."

"The strategy here is to use these plants to remove excess nitrogen from the water before it can fuel an algal or bacterial bloom.”

Over the past three years, Jack has worked with Director of Field Conservation Charmaine Dahlenburg and other Aquarium team members to inspect and evaluate the prototype. They are determining if the materials, adjustable pontoons, aeration system and native plants can withstand the harbor’s water quality fluctuations as well as exposure to intense sunlight and heat in summer; snow, ice and cold in winter; and waves and wind year-round.

So far, they have.

“We know plastics don’t easily break down; in this case, we’re using that to our advantage,” Jack explains. “The wetland is designed to require minimal upkeep. We replant any bare patches in spring, do occasional weeding and change out our fouled airlift pipes with clean ones twice a year. Our airlift pipes—which move water from under the floating wetland and circulate water down the wetland’s shallow channel—provide the perfect environment for filter-feeding barnacles, dark false mussels and hydroids to grow. They also get clogged with plastic bags. Because the aerators cause floating trash to collect in a few locations, keeping the wetland free of trash is our most time-consuming task, and that’s mostly aesthetic.”

A blue crab, held by Director of Field Conservation Charmaine Dahlenburg, is one of the native species drawn to the floating wetland prototype.

Attracting Native Species

As intended, the prototype attracts native species. Fishes, reptiles, crustaceans, mollusks and birds have all been observed on and around it, seeking food and shelter.

  • Small mummichogs and banded killifish use the prototype’s shallow channel for laying eggs, protecting newly hatched fry and avoiding predators. Pumpkinseed sunfish and largemouth bass forage for food there, too. American eels, Atlantic silversides and larger predatory fish like white perch and striped bass have been documented in and around the wetland.
  • Northern water snakes, common in natural salt marshes, are frequent visitors, feeding on small fishes and eels. Young snapping turtles and red-eared slider turtles have been seen swimming in the channel and hiding in the grasses.
  • Blue crabs use the island for protection during molting; they also gather around the prototype when dissolved oxygen in the harbor drops to low levels. (The wetland’s aeration system adds oxygen to the water.) We’ve also seen white-fingered mud crabs and grass shrimp, both small native species. Tiny crustaceans, called amphipods, are abundant and provide food to small fishes.
  • Populations of native Eastern oysters and hooked mussels are surviving and slowly growing on the submerged edges of the prototype, while non-native dark false mussels are thriving.
  • Black-crowned night herons and a great blue heron have been seen hunting for fish on the wetland, while mallard ducks and Canada geese have used it as a nesting site. Cormorants are often seen diving for fish nearby.

Join Symone Johnson, our manager of education programming, as she (figuratively) dives into the harbor to take a look at what lives there!

Another draw for native species is the oyster shell habitat being built up on the submerged concrete columns that support the footbridge between Piers 3 and 4.

Restaurants around the harbor give the Aquarium their oyster shells, which are used to create an oyster reef ecosystem. By providing these habitats for native species, we’re re-creating the natural food web that existed when the Inner Harbor was a salt marsh. This, too, is part of the nitrogen removal plan.

“Algae and biofilm communities, which colonize the floating wetland’s submerged surfaces and oyster shells, plus the critters these hard surfaces attract, like mussels, barnacles and worms, use nitrogen from the water to grow,” Jack explains. “The floating wetland and oyster shell habitat offer protection and food for small animals like white-fingered mud crabs, grass shrimp and killifish that feed on the biofilm communities and algae. Then a bigger fish like a white perch comes along and preys on the crabs, shrimp and killifish, and then a night heron or cormorant eats the perch. The nitrogen has been removed from the water by way of the food chain.”

While oysters generally do not fare well in low-salinity environments like the harbor, some oysters are growing slowly on the edge of the floating wetland. Also, hooked mussels, which are native to this area, and dark false mussels, which are not, both clean and filter water, and there’s a growing population of both on the oyster shell habitat and the edges of the wetland.

The Aquarium team collects core samples from the floating wetland to study nitrogen tissue content in the plant shoots and their root systems.

Measuring Impacts

We don’t just take these animals’ presence as proof that what we’re doing is working. To build upon what we’ve observed on the wetland, and to complement the continuous, 24/7 water quality monitoring the Aquarium has supported since May 2016, Aquarium team members are conducting and supporting research to better understand the harbor’s water quality.

This research is focused on seasonal water quality trends in the harbor, what triggers the cycle of algal and bacterial blooms, the role of water quality in biodiversity, and how the elements of the floating wetland contribute to harbor health.

With grant money from 11th Hour Racing and the Baltimore Office of Sustainability’s Critical Area Management Program (CAMP) Offset Fee Fund, and in collaboration with Koolhof Earth, a nonprofit that undertakes projects to reduce environmental impact to water resources, Charmaine is working with the University of Maryland Center for Environmental Science’s Chesapeake Biological Lab to examine how much nitrogen is removed by floating wetlands’ plastic material alone and how much can be attributed to the plants.

Samples are being taken from the Aquarium’s older floating wetland, which was installed in 2015: once in fall 2020 and twice in spring 2021.

During each visit, Charmaine explains, “plant shoots and their root systems are collected to sample nitrogen tissue content, and we measure dissolved and particulate nitrogen from the surrounding waters. We also extract intact cores from the wetland and perform experiments to measure exchanges of dissolved oxygen and nitrogen gas, since that shows us exactly how much nitrogen is being removed.”

One goal of the study is to better understand the benefits of floating wetland technology so that these wetlands can be certified as an official best management practice (BMP) for removing nitrogen from waterways—particularly in urban areas like Baltimore with its working port and harbor.

Parallel to this research, Charmaine is working with partners at the Institute of Marine and Environmental Technology (IMET), Baltimore Underground Science Space (BUGGS) and Maryland Sea Grant to publish a paper documenting the variety of species that live in the Inner Harbor and the impact water quality has on biodiversity.

“It will be the first of any report to provide a snapshot of what is living in the harbor,” she says. “The census was developed using a method of DNA barcoding to identify species present.”

Electric scooters dumped in the Inner Harbor are a growing problem the Aquarium is trying to help solve. Their batteries leak toxins that negatively impact water quality.

Removing (Unusual) Debris

Understanding the excess nitrogen problem, working through possible solutions and documenting results are endeavors that require patience and scientific know-how, but other harbor cleanup efforts demand brute strength and create big piles of metal and muck.

Jack and Charmaine recently joined forces with Dr. Eric Schott from IMET and partners at Blue Water Baltimore and Waterfront Partnership to address the electric scooters they could see resting on the bottom of the harbor when tides were low and the water was clear. Even in murkier conditions, a GoPro camera dropped below the surface revealed the extent of the problem.

“We know that the scooters’ batteries leak toxins that negatively impact water quality,” Jack explains.

After several messy, tiring hours using a grappling hook, inflatable boat and lots of muscle power during two separate efforts in September and October, the team hauled out a grand total of 29 e-scooters, one mini scooter, six bikes, two skateboards, a sign post, two sidewalk barriers, two lightning rods, a metal Baltimore City trash can, a net, various articles of clothing and a large, headless doll.

Baltimore City Department of Transportation is now developing strategies to address the large number of scooters that are ending up in the water.

“It’s not the responsibility of any one person or organization or agency to clean up the Inner Harbor,” says Jack. “If you want something done, you do it.”

And so, the National Aquarium does.

A Better Baltimore

Our work in and around the Inner Harbor is just one of the ways the National Aquarium partners with neighbors throughout Baltimore to protect our city’s waterways and wildlife.

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