The Rivanna Rambler

November 6, 2008

 

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Last week, we learned about the aquatic weed, hydrilla, an herbaceous, perennial freshwater herb originally imported from southeast Asian for aquariums and water gardens – and a plant that has taken over millions of acres of shallow standing and moving water in the United States.  Unfortunately, it has taken root  in our watershed, particularly in the South Fork Rivanna Reservoir, but it has also been found in tributaries like the North Fork as well as the river itself downstream from the reservoir.
Like many plants that are characterized as invasive, it is not unattractive.  It has delicate leaves organized in whorls of three to eight around a central stalk that rises to the surface in water that can be from a foot to fifteen feet deep.  The leaves are 2 to 4 millimeters wide and can be from six to 20 millimeters long – and what distinguishes hydrilla from the native pondweed (elodea), as well as a another look-alike transplant, the Brazilian waterweed, is the presence of many sharp teeth along the margin of the leaves.

Invasive species take hold in our local habitat through a variety of means.  Some terrestrial plants out-compete natives by emerging earlier in the season, or being resistant to the evolutionary system of ecological checks and balances, or by reproducing in a scale that overwhelms local species.  Hydrilla has its own set of tricks for flourishing in the places it is introduced, whether by boats traveling from one waterway to another, or on the wind, or using other aquatic species to hitchhike a ride.  Once in a waterbody, it spreads out and forms dense canopies that can shade out native vegetation – and while these areas can provide protective nursery grounds for juvenile fish, it is not a major food source for aquatic species.  Fishermen on our reservoir report seeing larger fish taking cover in the hydrilla – but it is hard to get to them without snagging a lure full of heavy weed.

Hydrilla also alters water quality by raising the pH and decreasing oxygen as well as increasing temperature under the mats of weed.  In slow moving water, the stands that are rooted to the bottom can catch sediment and accelerate the process of filling in and creating islands, as has been seen along many stretches of the South Fork Rivanna Reservoir.

And these clever hydrilla have more than one way of reproducing. The plants, such as those often found in the mid-Atlantic, can host both male and female flowers, making for an easy date between ovary and sperm.  Plants also produce rhizomes that root horizontally in bottom soils, sending up new shoots opportunistically.  If that weren’t enough, the plants also produce turions, small tuber-like growths that grow between stem and leaf and breaking free in the fall and floating till they fetch up in another spot, ready to take root in the spring.  With all these reproductive mechanisms at work, it’s easy to see how simply mowing down the plants, as is done in some areas to clear docks or swimming areas, is only a partial solution – and how eradication, as well as prevention of its spread, is next to impossible.

Though chemical agents can kill the plants, many localities have used grass carp – also an imported species – to control hydrilla, though this has its own risks. Grass carp will out-compete native fish species – and are only recommended in enclosed water bodies.  Our reservoir is only a temporary impoundment of water flowing from the headwaters on its way downstream to the James and the Chesapeake Bay … clearly not a candidate for using these aquatic weed chompers that could easily escape downstream.

As the community continues to look at ways to preserve the South Fork Rivanna Reservoir – for storage of drinking water, as well as a recreational amenity – we’re going to have to address this aquatic plant that has made itself at home along large stretches of the reservoir – and seems to be expanding rapidly.  While some say that our human population is at risk of outpacing the resources capacity of this county and this watershed, there is irony that the spread of other species is also hard to keep in check.  We would do well to look for the parallels, both in the biology as well as our solutions.