ALBANY — When boaters show up this summer to Great Sacandaga Lake in the lower Adirondacks they are likely to be met at public launch sites by stewards asking to check for alien plants or animals.

The stewards, college students, will be looking for aquatic invasive species that have been found so far in about one-quarter of the lakes surveyed in New York’s northern mountains.

They will also ask to check boats leaving the lake, which last fall was the first inland waterway in New York where the spiny water flea was found. They want to keep that small crustacean, native to Eurasia, from spreading to other American lakes and rivers.

“When we move from one waterway to another, we’ve just got go be mindful of what’s hitchhiking,” said Hilary Smith, director of the Adirondack Park Invasive Plant Program. “We need to include cleaning our boat and gear as part of the sport.”

Using hundreds of volunteers, the program has monitored 216 Adirondack lakes, finding 53 with one or more harmful nonnative plants like Eurasian water milfoil, curly leaf pondweed or water chestnut.

“My sense is we’re going to find more uninvaded lakes than invaded,” Smith said.

While there are roughly 3,000 lakes and ponds in the Adirondacks, Smith said many are remote or private. Those are less likely to have invasive species, which the Nature Conservancy calls a national and global threat to freshwater resources. Without natural checks or predators, the aliens reproduce and spread fast, threatening native species.

Founded by the Nature Conservancy and state agencies in 1998 and now involving more than 30 organizations, the regional partnership has also helped remove about seven tons of invasive land-based plants like Japanese knotweed at 33 campgrounds in the Forest Preserve and along 275 miles of highways.

The stewards program began with students from Paul Smiths College on Lake Placid, Upper Saranac lake and Upper St. Regis Lake, Smith said. It has expanded to include Long Lake, Raquette Lake, Lake Champlain and Buck Pond campground.

On Lake George in the eastern Adirondacks, stewards will be at boat ramps for the fourth straight summer, said Emily DeBolt, program coordinator for the Lake George Association. Last year, between Memorial Day and Labor Day, four stewards and an assistant coordinator inspected 2,964 boats, removed and bagged plant or animal material in 113 samples, and found invasive plants or animals in 61.

“We have three things some other lakes nearby don’t have — Eurasian water milfoil, zebra mussels, curly leaf pondweed. We’re not interested in exporting anything we have,” DeBolt said. They also don’t want new invasive species like the spiny water flea on a boat from nearby Great Sacandaga Lake, she said.

Boaters are asked what waterway they last visited, whether they washed the boat, drained the bilge and let it dry in between. Most agreed to inspections.

“It’s completely voluntary,” DeBolt said. “If someone says no, they back off.”

They have no police powers. If they find an invasive species — or suspect they’ve found one — they’ll ask the owner if they can remove it. The invasive species can be pulled or washed off on dry land, just so it doesn’t get into the water.

The stewards try to hit the busiest launches on the busiest days between Memorial Day and Labor Day.