WILMINGTON — Researchers and city officials still don’t know what caused last month’s fish kill event at Greenfield Lake but data collected post-kill has revealed problematic conditions in the lake.
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Michael Mallin with the UNCW Aquatic Ecology Lab told Port City Daily Monday his team has yet to pinpoint the cause of hundreds of fish floating up dead over the second weekend in May. The lab regularly collects data from the lake and helps the city understand water conditions.
Mallin’s team and Rob Clark, water quality programs manager at Cape Fear River Watch, found the northwest portion of the lake had low levels of dissolved oxygen, or the amount of oxygen available to aquatic organisms, in the days after the fish kill.
Healthy water should generally have dissolved oxygen concentrations around 80% to 120%; Greenfield Lake was measured at 5% after the fish kill. It’s determined to be an “anoxic” condition.
“So the thing that got me is — why did the lake go anoxic?” Mallin said.
Mallin’s theory is a harmful algal bloom caused the fish kill. Blooms occur naturally but can materialize when excess nutrients enter a water source, often from runoff of fertilizer, manure, wastewater and stormwater.
The fast-growing bloom can harm fish populations in a few ways, most often by sucking up so much dissolved oxygen to the point of suffocation for other organisms. Algal blooms can also damage fish gills, impairing its ability to breathe. Some blooms emit toxins upon decay that not only damage the lake ecosystem but humans and other animals that come into contact with it.
Mallin said two of the lab’s PhD students visited the lake while the fish kill was going on and reported eye-stinging and throat-tightening.
“If you ever experience those symptoms, get off the lake,” Mallin said. “That’s a sign of toxicity.”
However, the lab did not find evidence of toxicity, though their testing was limited. Additionally, the dead fish — manually scooped up by City of Wilmington staff and disposed of — were not biopsied for toxins.
Though tracing the residue of an algal bloom has proven difficult, Mallin did discover two sources of contamination in the lake which could affect its overall health.
The lab sampled from across the lake, but Mallin said one branch that extends up to Carolina Beach Road offered surprising data.
“There were super high fecal bacteria counts and also the suspended solids in the water was, like, 100 milligrams per liter — usually, it’s never more than five to 10 maximum,” Mallin said.
He explained high fecal coliform bacteria itself may not be pathogenic, or disease-causing, but often indicates high levels of pathogenic bacteria.
“That’s a big problem for humans,” Mallin said.
The source of fecal matter remains a mystery, though Mallin has discussed potential scenarios with Dave Mayes, the City of Wilmington’s director of public services. Per Mallin, Mayes suspected runoff traced back to Starway Village, an affordable housing project under construction a mile away, could have contributed to high turbidity in the area, though not necessarily the fecal coliform increase.
Turbidity is the measure of clarity in a liquid; the more particles — not necessarily bacteria, but also other organisms and sediment — the more turbid.
“They were dewatering the area there so they could build, you know, without it caving in or something,” Mallin said. “So he thought maybe it was due to that.”
Internal emails obtained by Port City Daily show a city compliance officer traced high turbidity in Greenfield Lake back to the 2346 Carolina Beach Road site.
“Judging by the photos sent to our office, appropriate measures are not in place to control turbidity,” New Hanover County engineering specialist Beth Furr wrote to Starway’s developers.
Furr then warned a fine would be imminent if Starway’s discharge issue was not addressed.
The project manager, John Reise with Progress Carolina LLC, ensured turbid water would not be “discharged off site again.” He said a dam broke during pumping the water that led to the discharge being routed to an offsite drainage system; the water would be kept onsite moving forward.
Neither the City of Wilmington nor Reise responded to PCD’s request for comment by press.
According to Mallin, Mayes also posited a trailer park in the area recently experiencing sewer issues could also have contributed to the sediment plume found in the lake.
PCD reached out to the Cape Fear Public Utility Authority to verify Mallin’s statement.
“CFPUA staff have been called a few times to investigate reports of sewer overflow at a mobile home park on Bell Street,” CFPUA spokesperson Vaughn Hagerty said. “Each time, the problem has turned out to be a backup on the private property owners’ side, not in CFPUA’s collection system. The most recent call in our records occurred in November 2023.”
PCD asked if the backup could have contributed to increased sediment in Greenfield Lake; Hagerty said the utility authority does not have enough information to make a determination.
The higher the turbidity in a body of water, the harder it is for light to penetrate, making photosynthesis harder for plants while promoting regrowth of pathogens in the water. Increased sediment can also clog fish gills, and high amounts of organic matter (algae, fecal coliform) or inorganic matter (sand, phosphorus) or a combination of the two, could contribute to low oxygen levels in the lake.
Still, Mallin thinks an algal bloom is the most likely cause, as the lake has been experiencing smaller harmful algal blooms over the last two months, though none strong enough to kill hundreds of fish.
Clark also pointed out Greenfield Lake is the prime location for rapid algae growth.
“It’s not surprising — this is Greenfield Lake,” Clark said. “It has issues with blooms in general, like even the non-harmful blooms.”
Clark pointed out the lake is shallow and slow moving, prime conditions for blooms that exacerbate with rising temperatures, with April to October considered bloom season. Still, Clark said in the three years he’s been with Cape Fear River Watch, he’s never seen an event this severe, though he heard the lake suffered a fish kill during Hurricane Florence.
Strong storms can bring in large quantities of sediment to bodies of water, increasing turbidity and contributing to anoxic conditions. This makes lakes in hurricane-vulnerable conditions more susceptible.
Additionally, Greenfield Lake’s watershed is 2,500 acres, densely populated and more urbanized than other areas of the city, which comes with more impervious surfaces and increased runoff. Every time it rains, stormwater picks up nutrients and sediment on those surfaces and carries them to the lake.
The city, which owns the lake, employs several methods to remove excess nutrients from the lake. A pilot project is underway in Squash Branch — one of the highest areas of nutrients nitrogen and phosphorus — to dredge those elements out of the water. It is also experimenting with mechanical removal through a harvesting boat called a Conver, as well as testing an algaecide called Oxymycin.
UNCW, Cape Fear River Watch and the city have partnered on an EPA grant to retrofit WIllard Pond in order to capture more nutrients in the Jumping Run tributary, though it has has yet to be funded.
The public can also reduce runoff contamination by cleaning up after pets and being cognizant of the environment in their fertilizer use.
Increased temperatures and storms due to climate change, along with planning practices used in the city, have the potential to keep escalating algal blooms.
“We are absolutely amplifying and affecting the amount of blooms that are happening on the lake through how we develop, how much development we have, how we choose to develop,” Clark said.
This includes installing stormwater infrastructure that filters nutrients or reducing the development of impervious surfaces.
Clark added the water also poses an environmental justice issue.
“The surrounding area around the lake there’s a large population of underserved communities, people of color, and they use this lake as a recreational source. So what does that say when the lake is toxic and there’s fish dying in it?” Clark said.
Clark and Mallin said their respective organizations along with the city and North Carolina Department of Environmental Quality are planning a meeting where they discuss mitigation efforts to prevent as many algal blooms as possible.
Tips or comments? Email journalist Brenna Flanagan at brenna@localdailymedia.com.
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