Water lettuce bloom engulfs Crescent Lake

Crescent Lake’s glinting water has been crowded out by water lettuce, spoiling the waterfront view for many visitors and residents of Historic Uptown, but for retired ecologist Jim Bays, the mats of invasive plant spreading across Crescent Lake are nothing new. He has watched water lettuce come and go there for roughly 20 years.

Bays has lived in Historic Uptown for two decades and spent his career working in ecology. Since 2021, he has also worked with Stewards of Urban Lakes, a volunteer organization focused on improving water quality and restoring natural habitat in St. Petersburg’s urban lakes.

“Our focus is on ecological improvements through habitat enhancement,” Bays said, along with cleanups to remove debris.

The organization holds monthly events at many of St. Petersburg’s lakes, including Crescent Lake, where volunteers regularly remove water lettuce. Recent weather conditions, however, have helped produce a particularly voluminous bloom that has made keeping up with the plant more difficult.

Water lettuce is considered invasive in Florida and thrives when there is little competition for space and sunlight. Bays noted that despite being invasive, studies of lake and marsh sediments indicate the plant has existed within Florida’s ecosystem for 11,000 years

“It’s perfectly adapted to grow and keep growing without stopping until it totally covers the water,” Bays said.

For people walking around Crescent Lake, the most obvious impact is aesthetic. Ecologically, a thick mat can crowd out native vegetation along the shoreline while preventing sunlight from reaching plants beneath the water.

As water lettuce spreads across the surface, it creates a canopy over the lake. That reduces the amount of light available to submerged vegetation, potentially changing the habitat below.

But Bays said the plant is not entirely without ecological value. Dense surface vegetation can limit the sunlight available to algae as well, which can help suppress harmful algal blooms. Crescent Lake struggled with those blooms before the city installed an aeration system in 2021.

Water lettuce also creates small habitats of its own. Insects congregate around the plants, providing another food source for birds, while crayfish and other aquatic invertebrates can live among the dangling root systems.

When kept under control, Bays said, that habitat can benefit some of the same wildlife that draws people to the city’s urban lakes.

The problem comes when water lettuce stops being one part of the ecosystem and begins dominating it.

Stewards of Urban Lakes volunteers remove the plants during monthly events, but the amount currently covering Crescent Lake requires more than volunteers pulling vegetation from the water. The primary method for removing large quantities is a floating harvester that moves across the lake collecting the plants. That process is already time-consuming, and Bays said equipment problems have further slowed the current cleanup. Much of the city’s equipment has recently been in “the shop” undergoing repairs, causing delays in clearing the lake.

Recent drought conditions followed by heavy rains also contributed to the amount now visible at Crescent Lake, according to Bays. Seasonal temperatures are another factor.

Water lettuce typically begins creeping back into the lake around May. Growth remains minimal during the cooler winter months, but accelerates as summer temperatures rise, making substantial blooms something residents can expect to see during Florida’s warmer months.

Bays expects it will take about another two weeks before the bulk of the current water lettuce is cleared.

Looking ahead to next summer, he hopes more manpower can help prevent the plant from accumulating as quickly. Bays plans to recruit additional volunteers next year, so Stewards of Urban Lakes can increase its cleanup schedule at Crescent Lake from once a month to multiple times per month during periods of heavy growth.