Recent freezes aid in war on invasives
Texas freshwater anglers, boaters and others who enjoy and benefit from the state’s rivers, reservoirs and other inland waters received a couple of welcome, if frigid, holiday gifts to start 2018.
The two spates of extreme winter weather that gripped much of the state earlier this month, dropping air temperatures well below freezing and keeping them there for 48 consecutive hours or more in some areas, dealt a cold blow to invasive aquatic species such as water hyacinth and giant salvinia that torment Texas waterways and native aquatic life.
“It’s almost certainly going to have a negative effect on some invasives, and that’s welcomed, for sure,” John Findeisen, who leads invasive aquatic plant control efforts for Texas Parks and Wildlife Department’s inland fisheries division, said of recent freezes that reached as far south as the Rio Grande. “Almost all of these plants are tropical or sub-tropical in origin, so they’re not evolved to handle really severe cold. When we get these strong fronts that drop temperatures to freezing, you’ll see those plants at least go dormant. And if it gets cold enough, long enough, it can actually kill them.
“We’ll take all the help we can get.”
Texas inland waters need that help. Invasive aquatic vegetation is a large, growing and spreading problem in the state with the nonnatives causing significant ecological and economic damage.
Water hyacinth and giant salvinia, two of the most noxious, are likely to be the most affected by the recent cold weather. Both are free-floating plants with almost supernatural ability to reproduce, quickly developing into thick mats that carpet the surface of lakes, bayous, streams, ponds, canals and other water bodies. Danger to boats, too
Those surface-covering mats create problems for boaters, with the vegetation often growing in such dense concentrations that even the largest recreational vessels can’t plow through them. And the thick stands of floating hyacinth or salvinia also can create economic havoc by retarding movement of water through water supply systems and clogging or otherwise damaging pumps and other machinery used to supply water for human use.
But it is what the nonnative plants do to the aquatic ecosystems they invade that is the most devastating. When invasives such as hyacinth and salvinia flourish to levels where they cover the water surface, that carpet of vegetation triggers a cascade of events that dooms the native ecosystem.
The mat prevents sunlight from penetrating the water. Denied sunlight, submerged aquatic plants as well as phytoplankton lose the key ingredient for photosynthesis. They die, taking with them the oxygen photosynthesis produces — oxygen distributed into the water and used by all aquatic life. The mat also prevents wave action from injecting atmospheric oxygen into the water. And the bacterial process of breaking down the now-dead native submerged plants also sucks dissolved oxygen from the water. The result is hypoxia.
Aquatic life in affected water — fish, invertebrates such as crawfish, insects, snails and mussels — can’t survive without that oxygen. The mat of invasive plants quite literally smothers the life from the water.
“It’s just dead water,” Findeisen said. “Nothing alive under it.”
Water hyacinth, a South American plant introduced more than a century ago, is found throughout the eastern and southern regions of Texas in scores of waterways. Giant salvinia, another South American native unthinkingly loosed on Texas waters, was first documented in the state in the late 1990s. Since then, it has spread to more than 20 public reservoirs and scores of smaller waters, public and private. Both plants now cover tens of thousands of acres across the state. Caddo Lake hard hit
Over the last two years, TPWD’s aquatic vegetation staff have treated more than 35,000 acres — about 55 square miles — of salvinia with herbicides in an effort to keep the plant at bay in waters where it is established and wipe out new infestations if caught soon enough. More than 13,000 of those treated acres were on Caddo Lake, where the combination of salvinia and hyacinth has been a particularly vexing problem for this most iconic of Texas lakes.
In addition to TPWD’s efforts at treating the invasives, state river and water authorities, the U.S. Army Corps of Engineers and other government agencies and private groups have spent millions fighting the plants.
Part of that action includes operating a handful of “hatcheries” in which a weevil that preys only on salvinia is produced for release as a biological control. Those weevils — Cyrtobagous salviniae — have proven a useful tool, credited with collapsing some salvinia infestations and then keeping the plants at a much reduced level.
But the battle has been, at best, a holding action. Once salvinia or water hyacinth are established in a reservoir or other waterway, they have proven practically impossible to eradicate. Even drawdown of lakes, stranding the invasives, isn’t a solution.
“Water hyacinth seeds can survive, dormant, for 10 to 12 years,” Findeisen said. “It’s a constant battle to beat this stuff back.”
But the recent freezing weather will help. Cold spells welcome
Both water hyacinth and salvinia are vulnerable to severe damage or death from extended cold temperatures. When water temperature drops to about 26 to 27 degrees and remains there for 48 hours or more, some of the plants die. Others hang on but have their ability to regenerate retarded.
If temperatures are colder than the mid-20s, especially if they drop into the single digits or if ice forms on or around the plants, the damage is significantly higher.
In portions of northeast Texas, including areas where salvinia/hyacinthplagued reservoirs such as Caddo, Fork, Toledo Bend are located, temperatures during last week’s siege of frigid weather did drop into or near the single digits.
And even as far south as Lake Texana, Choke Canyon and others bedevilled by water hyacinth, temperatures dropped into the teens.
“That’s done damage to some of those plants,” Findeisen said.
Recognizance by TPWD staff following the freezes the first week of this month found significant damage to salvinia and water hyacinth in some water bodies. The floating plants, burned by the freezing temperatures and ice, were browned. Some were dead. But many still had green leaves on their lower regions, indicating they had, so far, survived, Findeisen said.
But the same freezing temperatures that damaged the invasive plants may also have taken a toll on the weevils that prey on them, he added.
“We’ll be out looking at that, trying to gauge weevil survival,” Findeisen said.
But even if the weevils took a hit, the overall result of the frigid temperatures almost certainly will be positive for the war against the invasive plants. Some of the invasive plants died. Almost all of them fell dormant, and their ability to regrow has been affected.
“It’s kind of like hitting the ‘reset’ button,” Findeisen said. “It gives us a chance to get a head start on control efforts before the growing season begins this spring. It can give us an advantage we wouldn’t otherwise have had.”
If so, all that miserably cold weather will have proven a great benefit to Texas’ inland waters, the native aquatic life in them and the millions of Texas anglers and boaters whose lives are enriched by healthy freshwater ecosystems.