Thick weed growth in Florida bass lakes sucks up oxygen, repels sport fish

Some of Florida’s top bass lakes have areas where the weeds grow so thick they look like you could walk across the top of them. A recently published study of two of those lakes shows what the plant-choked regions mean for sport fish like bass and crappie.

Back in 2006 and 2007, Aaron Bunch slogged through dense stands of pickerelweed, cattails and other emergent species in lakes Kissimmee and Istokpoga for what remains some of the hardest field work of his career.

“We’re talking about knee-to-waist deep thick detritus muck,” Bunch said. “You often see alligators all around you. That’s just the norm. You get comfortable with the wildlife out there.”

Bunch, at the time a graduate research assistant at the University of Florida’s School of Forest Resources and Conservation, was in that mess to gather data on water quality and fish diversity. The resulting studies, one of which was recently published in the journal Lake and Reservoir Management, showed a clear pattern. The denser the plants, the lower the dissolved oxygen. And the lower the dissolved oxygen, the fewer the sport fish species.

The data could be valuable for lakes faced with areas where healthy, diverse plant growth gives way to thick plots of a single species. For fishery managers in those lakes, the takeaway is clear:

“It’s important to have a complex mosaic of plants with lots of edge habitat,” said Bunch, now a tidal rivers project leader with Virginia Department of Game and Inland Fisheries

Pickerelweed completely covering the shoreline (Credit: Aaron Bunch)

The dense, single-species stands are an issue in lakes with stabilized levels brought on by water regulation in the state. These are shallow floodplain lakes that once experienced natural flood cycles that would occasionally sweep through, kill off plants and perform a little housekeeping by moving plants and other organic material up onto the landscape. Without those floods, the plants grow unabated while organic matter builds up below, decaying in a microbial process that sucks up dissolved oxygen.

“These lakes are stabilized, so there’s really no period of time where the floodplain is inundated and the plants die off,” Bunch said.

Managers that want more sport-fish-friendly habitat can break up these vast stands of plants. That creates edge habitat for fish like bass to haunt, while also opening up the nearshore habitats to open water. That kind of connectivity can get water moving amongst the plants, ideally freshening things up and boosting dissolved oxygen levels.

A tussock habitat — a floating island of dense plant growth (Credit: Aaron Bunch)

Herbicides are one option for clearing space, but that leaves the dead plants behind, where they’ll decay and continue to deplete dissolved oxygen. Another method involves machinery that physically removes the plants.

“You have these giant mechanical contraptions that go through and actually chop out these areas,” Bunch said. The plant material is then either piled up on the shorelines or hauled away by the truckload. “It’s a large-scale operation to remove all this biomass from these lakes.”

That could be important work for supporting fish diversity in Florida’s stabilized lakes. But the study also mentions that, while not great for bass, areas of thick plant growth are good habitat for birds, amphibians and reptiles. After all, those alligators that watched Bunch work need a place to live, too.