If we want to understand how a changing climate is impacting the Potomac River, researchers think we should look to the people, the drinking water supply — and the fish.
Dozens of experts gathered at the Griffith Water Treatment Plant in Fairfax, VA, recently to compare notes on the health of a river that’s a major tributary to the Chesapeake Bay and supplies drinking water to nearly 6 million people.
Much of the discussion during the one-day conference in September focused on the impact that more erratic weather patterns — namely, heavier rains with longer dry spells in between — is having on the river.
Jamie Bain Hedges, Fairfax Water’s general manager, noted that the utility is in the process of creating an additional water reservoir out of a nearby rock quarry to help protect against water shortages in the future.
“In our business, we don’t focus just on what’s going on today, but we also have to focus on decades of tomorrows,” she told attendees. “That’s why we’re working on the quarry next door and with folks [in this room].”
The Potomac Conservancy also has been taking notes on the hyper-local impacts of the global climate crisis. The nonprofit’s 2021 climate report described a 2019 rainfall that dumped a month’s worth of rain — more than 3 inches in about four hours — causing flash flooding that left some commuters standing on the roofs of their flooded cars.
“For many of our friends and neighbors, the fact that climate change is something happening now is starting to fall into sharp relief,” said Katie Blackman, vice president of programs and operations at the conservancy. “There is a growing concern about the local effects.”
With erratic weather, the overarching trends of change can be hard to pinpoint. That’s why Nathaniel “Than” Hitt, a fish biologist at the U.S. Geological Survey’s Eastern Ecological Science Center in Kearneysville, WV, prefers to focus on fish.
He sees the dynamics of the Potomac River’s fish population as an important indicator of which changes matter and how they will shape the river’s fisheries in the future.
When it comes to the impact of the climate on local fish populations — particularly higher, flashier flows of water — there are winners, and there are losers. Scientists predict that more extreme flows in the river will increase the populations of “opportunistic” species that adapt well to such changes and find food in a variety of environments.
That includes blue catfish and other “live fast, die young” species, as Hitt puts it. Whether the river is running high or very low, these species still find a way to thrive, eating what’s available. While many nonnative fish species fall into this category, native ones do, too. The banded killifish is a native, opportunistic fish that has exploded in abundance and is moving upriver.
The central stoneroller is another native species that capitalizes on using alae that it scrapes up from rivers. Its abundance has also grown in the Potomac River of late, taking advantage of available food sources.
Hitt uses this information about individual species and a scientific approach called life history theory to predict how species communities will change in the future. These changes can also be used to observe what’s happening in the Potomac River.
The research relies on juvenile fish count data that’s been regularly collected by the Maryland Department of Natural Resources since 1975. This data allows researchers to look at even the most subtle of changes over time and to draw conclusions about how fishing and water quality advocates can respond.
For many fish, “the problem is when rare events become common,” Hitt said.
That’s the case for brook trout in the Shenandoah River, which flows into the Potomac. The data show that if the region has a high flow of water in the winter once every five years, the brook trout population can be stable. Although the eggs they lay on gravelly, cold-water stream bottoms can be washed away by heavy flows and rains, the population can recover if they are periodic. But if the high flows occur every winter, the fish begin to struggle.
While opportunistic species tend to thrive in a changing river, species that like more stable environments do not. Hitt said that the changes over time in the increase of various opportunistic fish and the decrease of stability-preferring fish is a key indicator of the river’s ongoing change.
“This is evidence for a destabilized flow regime in the river — biological evidence,” he said. “These [survival] strategies have been stable for so long, to see them change over the course of our lifetime is something to take to heart.”
Hitt’s research also found some reasons to be encouraged over the state of the river and its fish. A recent paper found that karst groundwater in the region has a stabilizing effect on stream communities and benefits the fish that prefer those environments, such as the blacknose dace, fantail darter and Blue Ridge sculpin.
Protecting groundwater — like protecting drinking water for people — can increase the resilience of the system. In this case, it benefits the headwater streams of the Potomac and the many fish communities they support.
This article was originally published on BayJournal.com and is republished with permission.