Aquatic Invasive Species

Kings Bay, Florida, is a large spring-water system that houses a large population of manatees.  Recently the area has been infested by mats of Lyngbya Wollei algae.  While some theorized that the cause was pollution, Jason Evans et al. from the University of Florida hypothesized that introduced plants such as hydrilla, water hyacinth, and milfoil changed the water quality and assisted the algae’s proliferation.

However, scientific data on the history of these weeds in King’s Bay is scant, so Evans turned to citizen observers.  The citizens answered that outbreaks of hydrilla and water hyacinth were once encouraged, as they were believed to improve water quality.  Recently, scientists have begun treatments against the weeds, including artificial changes to the water quality.  Citizens believe that this tampering with the water is the primary cause of Lyngbya’s surge.  Evans concludes that the water weeds absorb the same pollutive nutrients as the algae, and in fact could act to control it.

Source: Ecological Restoration Vol. 25, No. 3 (2007)

Hydrilla verticillata is a highly invasive aquatic plant that has seriously affected water flow and use in the United States. Brought from Asia to decorate aquariums, hydrilla was introduced when owners discarded their plants into canals in central Florida. Since its introduction in the 1950s, hydrilla has taken over costal waterways from Maine to Texas and lakes and reservoirs in several central states and California. The University of Florida and the Institute of Food and Agricultural Sciences (IFAS) Center for Aquatic and Invasive Plants made a short video demonstrating the pervasiveness of hydrilla.

Several characteristics of hydrilla make it the ideal aquatic invader. First, hydrilla reproduces by means of vegetative propagation. A new individual can grow from a small fragment of stem without the use of spores or seeds. Therefore, boat motors unintentionally contribute to its spread by chopping the plants into many pieces, which creates new individuals. Hydrilla can also spread from one body of water to another as stem fragments on boat trailers or motors. Second, hydrilla grows in deep water with little sunlight where few other plants can survive. The population then invades shallower water, blocking sunlight to native plants with its dense growth. Both characteristics make it extremely difficult to remove.

Many states have attempted to eradicate hydrilla using aquatic herbicides and predator species. However, deep tubers pose a large problem to removal, as they can remain dormant and undetected for years before sprouting. A 2003 New York Times article describes Texans’ attempts to gain approval for the use of grass carp, another potentially invasive species, to fight hydrilla. Thousands of sterile grass carp have already been released into Lake Austin.

Hydrilla overpopulation causes a variety of problems in a body of water. Its tangle of sprouts hinders boating, swimming, and fishing, and it outcompetes natural vegetation. The thick growth has even been blamed in drownings. An interesting question relates to why the plants have had their level of invasive success. Does hydrilla spread well due to its innate qualities, or was there a particular vulnerability in the invaded areas? Or can its success be mostly attributed to humans’ accidental help, such as when boaters break the stems into pieces and transport them to new bodies of water? Finding answers to these questions may be crucial in eradicating hydrilla.