This week we will talk a little more about Hydrilla, specifically the severe environmental and ecological impacts of unregulated Hydrilla growth. Many of the potential impacts of Hydrilla have not been fully studied, however those that have been realized have had severe impacts. Follow along over the next few weeks as we discuss these impacts.
Environmental Impacts:
Dense Hydrilla infestations can cause severe fluctuations to water quality, specifically dissolved oxygen and pH. These wildly fluctuating conditions can be detrimental to habitat quality, and thus to the flora and fauna which live in our water bodies. Think of a water body and all of it’s processes as a balance. When that balance is in check, life in a water body is able to run smoothly. When things are out of balance, chaos can ensue. For example, many of you know that plants give off oxygen and use CO2 during the day. What many of you may not know however, is that at night, plants use oxygen and give off CO2. With normal levels of plant material in a body of water, the process of oxygen exchange can remain in balance. However, when voraciously growing Hydrilla is present, a large amount of oxygen can be consumed very quickly during low light conditions. When large Hydrilla infestations respire at night, oxygen levels can decline so rapidly that large fish kills can occur. Hydrilla also contributes to the increased release of nutrients from lake sediment. Increased nutrients thus result in algal blooms and die-offs. Large algal die-offs can also lead to fluctuating oxygen conditions. Again, let us refer back to the “balancing act” in which Hydrilla often interferes.
Ecological Impacts:
Hydrilla’s rapid and dense growth shades out all other plant species, including natives, reducing diversity to a single species. Dense mats of Hydrilla also alter habitat structure and food-web relationships for fish, which can lead to changes in fish population and community dynamics. For example, Largemouth bass (a favorite in Kerr Lake and Lake Gaston) are ambush predators that attack from cover. Increased Hydrilla cover can lead to larger numbers (not size) of bass but decrease other species that don’t favor ambush cover (white perch, striper, etc). Once Hydrilla reaches very dense levels it can interfere even with the hunting of prey by bass, so bass populations may increase initially, but decrease after hydrilla takes over a water body. The reduced hunting of prey for bass ultimately leads to stunted populations of many small individuals. Some fishermen and wildlife enthusiasts sometimes argue that hydrilla actually improves habitat for fish. It is true that some hydrilla cover (20 – 40 %) often provides increased cover for ambush hunting species, the plant hardly ever stays at levels where it is beneficial and instead, grows out of control until it diminishes nearly every resource but itself (see the above photo). It is important to remember that the management of hydrilla doesn’t mean the destruction of more beneficial plant species. By removing or managing Hydrilla in a water body, we provide an opportunity for more beneficial and less aggressive native species to grow at levels suitable for optimal habitat. Less hydrilla often means more plant diversity and more plant diversity equals a balanced habitat (see the “balance” theme again? I thought you would)
Although many birds and waterfowl feed on Hydrilla, generally bird populations will also decline in a heavily infested areas. Hydrilla is not a preferred food source for waterfowl and often doesn’t produce the same dietetic benefits that other native plants do. Hydrilla has also been linked with a fatal neurological disease primarily affecting bald eagles and American coots. Avian Vacuolar Myelinopathy, or AVM causes brain lesions in these species and ultimately leads to death. For more information on AVM, visit UGA’s Wilde Lab posted below in the web links for additional information.
Hydrilla is also associated with increased mosquito breeding area as it slows the rate of water flow in lakes. The stagnant conditions created by large mats of Hydrilla make the perfect breeding zone for large amounts of mosquitoes which can carry deadly human and animal diseases.
Hydrilla doesn’t just affect water quality, other plants and animals, but it also can have severe impacts for humas as well! Please stop back by next week when we will continue with the cultural and economic impacts of Hydrilla.
Web Links For Additional Information:
NC State University Aquatic Plant Management Web Site
AVM Information (UGA)
AERF Biology and Control of Aquatic Plants Manual
This column is prepared by Aquatics Extension Associate, Brett M. Hartis. You can reach him at (919)-515-5648 or email at bmhartis@ncsu.edu.