This story originally appeared on Inside Climate Science and is part of the Climate Desk collaboration.

The quality of New York’s drinking water—and the complex system that delivers it to each New Yorker’s tap—has long been a source of pride for city officials and residents alike. Intricate aqueducts, powered largely by gravity, bring water from the Delaware and Catskills watersheds, as well as from the closer Croton Reservoirs, to the city.

Partly as a result of the New York City Department of Environmental Protection’s consistent efforts to protect these watersheds through the Long-Term Watershed Protection Plan, this network is considered the gold standard for urban water supplies.

But in recent years, three invasive species have emerged to threaten water quality at the city’s oldest reservoir, the Croton system.

Today, the Croton system, which was the first watershed to ever serve the city on a large scale, usually supplies around 10 percent of the city’s water, and can provide much more in times of drought upstate. Located east of the Hudson, the system covers 12 reservoirs and three controlled lakes, all of which flow into the New Croton Reservoir in Westchester County.

Though most of the city’s water supply remains unfiltered due to the quality of the watersheds upstate, the water from the Croton system began to occasionally violate standards on taste, odor and color in the 1990s. To address that, the city’s Department of Environmental Protection built the Croton Water Filtration Plant, which became operational in 2015.

Currently, the city’s water supply network, particularly the New Delaware Aqueduct, is undergoing some repairs to fix a decades-old leak which was releasing millions of gallons of water into the ground in upstate New York. As that project enters its final phase, the New Croton Reservoir will be responsible for much more of the city’s total water supply.

But over the course of the past five years, the three invasive species—a fast-growing aquatic plant, a fingernail-sized mollusk and a water flea—have made homes there.

Invasive species are non-native to the ecosystems in which they reside, and often pose significant threats to native plants and animals. They are usually introduced, either directly or indirectly, through human intervention, and it is often costly to get rid of them.

In the most recent fiscal year, the Department of Environmental Protection has spent nearly $100 million on source water protection across the watershed. Over $1.5 million of that budget was spent on ecosystem protection like the management of invasive species.

The Zebra Mussel: Hard to Stop

The zebra mussel, a small mollusk from Eurasia named for the stripes that line its shell, arrived in the Croton system in 2021. According to Meredith Taylor, a biologist at the city’s Department of Environmental Protection, zebra mussels were introduced when a bass tournament, hosted on a private lake within the watershed, brought boats in from across the country. These boats were not under purview of the department, and thus were not subjected to its strict decontamination protocol, which often requires a full steam cleaning of the vessel to ensure no organisms remain on the boat.

“There’s really no way to disrupt the spread once they’ve become established in a water body,” said Taylor. “The population this year was really the first year where we were starting to see large numbers building up.”

The mollusks were eventually washed into the reservoir due to extreme precipitation events. These zebra mussels are filter feeders, which means they take in water and feed on plankton within the water column. This reduces the food available to small fish and native mussels, thus disrupting the natural food web. They can also latch on to native mussels, surrounding them and hindering their ability to feed or reproduce, sometimes causing death.

Their ability to accumulate on surfaces can also clog water supply systems, which has happened at the Croton Reservoir.

“We are trying to understand where populations are building, and to what density, to estimate when we need to contract for cleaning services to come in and scrape them off of our infrastructure,” said Taylor. “We’ve seen some of our sampling pipes have become clogged, so those have had to be changed out.”

Their presence can also influence water chemistry. Zebra mussels feed on specific types of algae and discard others, releasing byproducts in their feces. This can be a breeding ground for cyanobacteria, which is a type of algae that, when found in high concentrations, can create a “bloom.”

These blooms sit on the surface of the water, impeding fishing and boating, often emitting an unpleasant odor which can translate to the water supply, and reducing the amount of oxygen in the water, which affects the survival of native fish in the water body. It can even carry harmful toxins.

The Department of Environmental Protection has increased their monitoring of cyanobacteria in the water since the arrival of the mussels, but little can be done when it comes to the impacts on the larger ecosystem of the reservoir.

Hydrilla Can Change an Ecosystem “Overnight”

Hydrilla, an aquatic plant known to spread across the surface of lakes, ponds and slow-moving rivers in a thick dense mat of vegetation, has also arrived in the Croton Reservoir, as well as downstream in the Croton River.

“We’re trying to eradicate that population. It’s not widespread in the region yet, and we’re trying to keep it out of the [Hudson] estuary,” said Taylor. “It probably wouldn’t easily establish within the main stem of the Hudson River, but it could establish in many of the tributaries up and down.”

Hydrilla, a plant commonly used in aquariums, is often introduced when they are dumped into watersheds. When hydrilla spreads across a water body, it blocks the sunlight for native aquatic plants, often causing them to die off and impacting the fish that use them as a food source. Boats also struggle to pass through it, especially if no measures are taken to reduce the spread. Hydrilla dies off during the fall, causing large masses of vegetation to decompose and leading to oxygen depletion in the water. This also impacts marine organisms.

“You can see an ecosystem change overnight—specifically with hydrilla. You’ll see a normal pond, and then at the end of the growing season, it’s completely full of hydrilla,” said Nicole White, founder of Little Bear Environmental Consulting, which often works with city and state departments to combat invasive species. “Nothing else can survive there. It’s choked out.”

White, in partnership with other organizations including the Department of Environmental Protection, worked to eradicate hydrilla in the Croton River from 2018 to 2022. Out of the 449 sites they initially sampled, hydrilla was present at 40 percent of them.

Ultimately, they successfully eradicated hydrilla from three miles of downstream river using a very low concentration of herbicide for five seasons, but the impact of the hydrilla on the river’s ecosystem was so severe that at the end of the project, White had to replant many native aquatic plants in the Croton River.

According to Taylor, hydrilla still remains at the New Croton Reservoir. The Department of Environmental Protection is also using herbicide to change that.

The presence of hydrilla in the reservoir can have implications for not just the marine ecosystem, but also the water quality and the survival of local birds. Hydrilla is also known to harbor cyanobacteria, which can turn into a toxin–Aetokthonos hydrillicola. This type of harmful algae can kill waterfowl and bald eagles, and has the potential to harm human health.

“It’s a neurotoxin, so in places where Aetokthonos is found on hydrilla, lots of wildlife have died from brain lesions,” said White. “So fish have died, reptiles, like turtles, have died, waterfowl that eat the hydrilla, and then the predators of those waterfowl as well.”

Climate change leads to warmer weather in the fall and more extreme precipitation events which move sediments around in the water column. This creates better conditions for the spread of hydrilla, of cyanobacteria blooms, and, by proxy, of these toxins. Their presence has not been confirmed in New York state, though the Department of Environmental Protection is sampling for it.

The Water Flea, a Looming Threat

The fishhook water flea was found in the reservoir last year. The novelty of its arrival means that the Department of Environmental Protection officials have not seen adverse impacts on the marine ecosystem yet, and its wider implications for the food chain are unknown. The water flea can impact water quality by eating large quantities of zooplankton, which creates better conditions for the growth of harmful algae blooms. Taylor believes it was likely transferred through fishing gear because the water flea’s eggs can survive drying out for long periods of time.

Similarly to zebra mussels, it is virtually impossible to completely rid a water body of fishhook water fleas once they have established themselves. The most important thing to many scientists is often stopping their spread to new waterways—around 40 percent of New York’s freshwater is connected to canals, which makes it much easier for species to move around.

Without a change in policy, it is possible that more invasive species will make their way to the city’s reservoirs, impacting the ecosystems that support the quality of New Yorkers’ drinking water, and recreational fishing, in new ways. Efforts to stop that in the future will require the behavior of those who use the watershed to change.

“The biggest obstacle is that it’s not easily apparent to a recreational user that their actions can have really deep impacts. When you are fishing in one location and you’re not catching anything, and you decide to hop over to the next reservoir, you’re not thinking that that little action could have such huge ramifications,” said Taylor. “Even through our best outreach efforts to convey that message, I think it is a long shot.”

Scott George, a biologist for the United States Geological Survey, has been monitoring the spread of the round goby—a small invasive bottom-dwelling fish which often outcompetes native fish species—throughout New York’s waterways. The goby quickly arrived in the Hudson system in 2021, alarming scientists.

“What has surprised me is how unpredictable the course of the invasion has been,” said George. “When we started studying round goby in the central part of New York State–and in the Utica area–around 2016, we really didn’t see much evidence of expansion for a number of years.”

Once they are established, goby often reduce the populations of native nest-building fish, like smallmouth bass, because they eat their eggs, but they also provide an abundant food source for adult bass, complicating their impact on the larger ecosystem.

Ultimately, they have an important impact on the native species living in a waterway, and cannot be eradicated. Biologists like George focus on closely monitoring invasive species like the round goby to measure their impact, while exalting the importance of early detection and rapid response for species like hydrilla, which is responsive to herbicides.

“The whole premise here is that the more quickly you can identify a problematic invasive species, whether it’s a plant, an animal, a fish, the more quickly you can identify that it’s colonized a new area, the higher the probability that it can either be managed or possibly even eradicated completely,” said George.

Understanding the impact of an invasive species on any given water body is difficult. In New York, many of the state’s water bodies often host multiple invasive species, obscuring the impact of any one animal or plant.

Meg Modley is the aquatic invasive species coordinator for the Champlain Lake Water Basin Program. The basin encompasses waters across Vermont, New York and the region of Quebec in Canada, which all drain into the Lake Champlain watershed.

In recent years, the Champlain Lake Water Basin Program, in partnership with the US Army Corps of Engineers, has been working to design a method to prevent the spread of invasive species through the canal that connects the Hudson River to Lake Champlain.

“We’d lift watercraft recreational equipment up and over a height of land, decontaminate them with high pressure hot water and put them on the other side at the watershed divide,” said Modley, describing one of the possible outcomes of the plan.

The basin program itself has also already installed a number of boat cleaning facilities along the watershed, which has encouraged recreational boaters to inspect their boats for invasive species. For Modley, a US policy that prevents their spread, as well as the allocation of federal funding to that end, would be very welcome. Politicians, in her view, need to better understand the cost of mitigating and eradicating these species from water bodies where they have established.

Modley has also seen the arrival of the fishhook water flea in the watershed, and monitored its substantial impact on the presence of plankton in the water. She fears the potential for the further spread of hydrilla and round goby, as well as an invasive aquatic plant called the Japanese knotweed, to Lake Champlain, particularly as storms become more frequent due to climate change.

“When we get these more intensive rain events and storms, we get a lot of disturbance, a lot of redistribution of sediments,” said Modley. “And wherever we have disturbance, invasive species tend to dominate or colonize first.”

As New York winters grow more mild, some scientists also worry that some invasive species brought in through means like aquarium dumping will unexpectedly be able to survive winter in these waterways.

“There’s been concern, because we don’t know what many of the species are capable of,” said White.