Michael Hall Member Name
Staff Environmental Scientist
Invasive aquatic plants are a problem all over the world. Transported either by accident or intentionally, some plants are extraordinarily successful in their new home. As a result, they clog water intake pipes, entangle boat propellers, crowd out native plants, take up oxygen needed by fish, and depress property values.
Taking a close look at management of one such plant – Hydrilla verticillata or water thyme — helps show some of the trends in this field.
Hydrilla was brought to the United States from its home in Asia as an attractive aquarium plant. It was first found in Florida waters in the 1950s and is now common throughout the southeast.
Delineation: the right mix of traditional and leading-edge technology
Managing Hydrilla, as with many such invasive plants, starts with getting a clear idea of the problem at hand. This involves delineation – finding out the location of the target species, its distribution and population density, and how it impacts that area.
Some ways to do this are new, including the growing application of aerial drones, which can survey vast areas of a body of water quickly, at low cost, and in a way that’s safe for team members.
Computer based models help us track likely locations to be surveyed, given what is known about the depth of the water, turbidity, temperature, and other factors. These models also help us track research results, including information received from the public. They allow us to focus our research on areas where the target species is likely to be found, so we do not waste time looking in areas where it is unlikely to be.
Other plant monitoring methods are more traditional, low tech and less quantitative– such as the “rake toss.” This means taking the metal heads of two ordinary gardening rakes, fastening them back to back, and tying them to a cord. This device is thrown from a boat, dragged along the bottom of the water body, and then pulled back to surface.
Studying the plants collected by the rakes is a good way to determine the target species presence or absence, and the population density. The rake toss method also provides general data about aquatic plant community density and distribution that can be used in the management plan. Rake tosses can be done on a coordinate grid pattern, or along an underwater contour lines that do not exceed the species’ known maximum depth, or whatever system meets the research goals.
But there is no substitute for boots on the ground – or in boats – particularly if those boots are on local feet. We find that if we make local stakeholders aware of the problems posed by species such as Hydrilla, we get tremendous cooperation and support.
Typically, we contact local municipalities, hunters’ and anglers’ associations, property owner associations, marina, and boat launch operators – anyone with an interest in the quality of water in local lakes, reservoirs, and watercourses.
Sometimes this involves actual in-person meetings, but we also use public relations outreach teams to contact local news outlets, build informative websites, and create a presence on social media. We even put up fliers — resembling old-style “wanted” posters — at high-traffic places such as boat launch ramps and docks. This helps the public accurately identify the species of concern.
We ask members of the public to collect samples of plants that they believe are the target species, and deposit them in special containers we place at marinas and boat launches. We also urge them to take photos of plants they see and upload or email them. We ask them to note details such as whether they found the plant in the water or on land, the exact location, the weather and other factors.
Public engagement also involves educating the public on how they can avoid aiding the spread of the invasive species to other parts of the same water body, or other lakes and watercourses.
Management plan depends on the project objectives
Once we have the species delineated, the next stage is usually to develop a plan for managing the problem, which depends on what our client wants done, and their target goals.
Sometimes the organizations we work for are recreation-oriented, and they want to make sure that the water body is in good condition for anglers, and those who depend on anglers’ business such as hotels, lodges, and marinas. This can include putting a high priority on keeping plants clear from boat launches.
In other cases, we may be working for a power or water utility that wants to be sure its water intakes are not blocked, and that entrained plants do not damage turbines, valves, or pumps in the system.
In cases where the plant species has a strong foothold, it’s rare that we’re asked to eliminate a species entirely from that water body. This is partly because of the cost, and because re-introduction of that species is all too likely. Hydrilla is particularly difficult to eradicate because it can be propagated by fragmentation of the plants when they get chopped up by a boat’s propeller. As well, the Hydrilla reproductive tubers can lie dormant in the sediment for years before growing.
Instead, the objective of the management plan is to use the best tools and data available to control the damage and mitigate the spread of the target species.