Drive up I-476 to the northeast corner of Pennsylvania and it’s easy to find pristine lakes and ponds—beautiful bodies of water that look the same today as they did decades ago.

Unfortunately, say Timothy Block and Ann Rhoads, most lakes closer to the city have fared less well: Punished with steady streams of pollution and choked with non-native, invasive plants, these distressed lakes show all too clearly how human activity—everything from boating to golfing to fertilizing lawns—can negatively impact waterways.

“In some parts of our state, particularly in Southeast Pennsylvania, many of our lakes are heavily invaded by non-native plant species,” says Block, director of botany at Penn’s Morris Arboretum. “But if you get up into the Poconos, we find that sometimes invasive plants are a problem and sometimes not—but it has a lot to do with the human manipulation of lakes.”

Block and Rhoads, a senior scientist at the Arboretum, recently completed a multi-year project where they visited ponds, streams and lakes across Eastern Pennsylvania to measure, for the first time, the extent to which non-native plant species have invaded Pennsylvania waters. Their work proved not only that lakes and ponds here are affected in varying degrees by these potentially devastating non-native plants, but also that the lakes most impacted are those that also have poor water quality.

The lesson? Ponds and lakes, Block and Rhoads say, suffer noticeably when humans push them too far.

“Lakes and ponds are curious things,” Block says. “They can really only do one thing. They can’t serve a wide range of purposes. If you want a place where you can water ski and race around in your boat, and you don’t want plants getting in your motor, that’s one use right there. And if you use a lake for that, it can’t also function as a good natural ecosystem.”
This is especially obvious, Block and Rhoads say, in urban and suburban lakes. These waterways, unlike rural waters, are much more likely to be exposed to harmful stormwater runoff, a nasty form of pollution that can carry high concentrations of nitrogen and phosphorous into surrounding waterways, with dire consequences.

These nutrients, though found naturally in most lakes and necessary for plants and wildlife to thrive, can be problematic at elevated levels because they can promote the overgrowth of aquatic plants, upsetting the natural balance of a pond’s ecosystem. Lawn fertilizers, commonly used in both residential areas and on golf courses, for instance, are a major source of nutrient runoff. In developed areas, the use of such fertilizers is much more common—and so too are invasive plants.

Block and Rhoads say invasive species, such as the widespread Eurasian milfoil, seem to thrive under these high-nutrient conditions. Some highly polluted lakes, as a result, see their native plants marginalized, or even completely crowded out, by the faster-growing invasive species.

How serious a problem that is, says Block, remains unknown.
What scientists do know, however, is that the unchecked spread of non-native plants can, at the very least, reduce biodiversity: Just two years after milfoil was introduced into the waters of Lake George in New York, for instance, the number of native plants there was more than cut in half.
“I guess one of the biggest problems with these plants is that we don’t know as much about the ecological impact they have as we do know about invasive animal species,” Block says. “So we have to wonder, when native species are displaced by non-native plant species, what happens to the animals that feed on those native plant species?”

Adds Rhoads: “It can shift an entire ecosystem when you tamper with native species.”