Plants hitching a ride

Growing up, I spent my summers exploring the lakes of New York’s Adirondack Mountains (I recently wrote about one Adirondack lake in particular here). I loved to swim, but I did not love swimming through patches of plants growing up from a lakebed – the feeling of their tendrils swaying in the wake of my passing, clinging to my skin as if they wanted to grab my body and pull me down into the depths of the water, was enough to send me thrashing back to the lakeshore.

Eurasian water-milfoil, an aquatic plant native to Eurasia and northern Africa that has spread across much of North America, grew in such abundance in Upper Saranac Lake (a lake that I swam in many times as a child) that a local foundation raised $1.5 million to begin removing the plant from the lake in the early 2000s. Invasive aquatic plants often thrive and proliferate in their new environments so much that they crowd out native plants, degrade fish habitat, and clog waterways, preventing them from being used for boating or swimming.

The control effort in Upper Saranac Lake – which involves sending divers into the water to pluck milfoil by hand – has been largely successful at reducing the amount of the plant in the lake (in recent years, divers have collected roughly one-fortieth the amount of milfoil harvested at the beginning of the project), but if the removal stops, Eurasian water-milfoil could quickly rebound – meaning maintenance dives will have to continue, and someone will have to keep paying for them, indefinitely.

Dealing with invasive species in aquatic environments is expensive and time-consuming, and, as is the case with Eurasian water-milfoil in Upper Saranac Lake, they often cannot be completely eliminated. Perhaps the best offense, then, is a good defense – if invasive aquatic plants aren’t allowed into lakes to begin with, then no one has to dive down to the lakebed to remove them later.

One way for aquatic plants to spread among lakes is to hitch a ride on boats or trailers. If the bits of plant matter that get wrapped around a boat propeller or caught in the wheel wells of a trailer can survive out of water long enough to reach the next lake a hapless boat-owner visits, the plant might be able to spread to that lake. (This is why many states require boaters to wash their boats between lake visits.)

Research recently reported in the journal Hydrobiologia shows that some plant-parts are particularly adept at surviving dry spells. A team of scientists working in northern Wisconsin collected stems from two aquatic plants, Eurasian water-milfoil and curly-leaf pondweed, as well as buds from the pondweed, allowed them to dry outdoors (in order to simulate the conditions plants caught on a boat or trailer might experience), then placed them back in tubs of water to see if they were still capable of growth and, presumably, establishing themselves in a new lake.

Single plant stems were able to grow after up to 12 to 18 hours of drying, and stems that were coiled, as if twirled around a boat propeller, grew after up to 48 hours out of the water. The curly-leaf pondweed buds were able to survive for the longest – some sprouted after 28 days on land.

“The high cost and difficulty of eradicating introduced invasive species makes preventing secondary spread a management priority,” the authors write. Knowledge of how aquatic plants are able to spread between lakes, and how long they can survive out of the water, can help lake managers develop guidelines for cleaning boats and, hopefully, eliminate the need to send divers down to weed invasive plants from the beds of any more lakes.

Native aquatic plants are an important part of lake ecosystems, but invasive species, like Eurasian water-milfoil in North American lakes, can grow so much that they take over, killing other plants and preventing boating and swimming. 

(Image by dhobern via Flickr)