Linda Benedict | 5/5/2005 6:13:42 PM
During the last century, the Atchafalaya Basin has evolved into a highly altered and regulated floodway of the lower Mississippi River. The basin still supports a complex mosaic of aquatic and terrestrial habitats, but this unique ecosystem continues to be threatened.
Rising floodwaters deliver huge amounts of Mississippi River and Red River sediment to the basin floodplain, and many areas have been covered by several meters of sediment since 1900. This sediment deposition, in conjunction with water management and resource development projects, has reduced inflow from the Atchafalaya River, impaired water circulation and altered flow patterns on the floodplain, and reduced water quality throughout portions of the lower basin during late spring and summer.
Many of the water management projects planned by the U.S. Army Corps of Engineers and the Louisiana Department of Natural Resources focus on increased delivery and movement of low-sediment water across the floodplain during the flood pulse to improve water quality for the basin’s valuable crawfish and finfish resources. But, successive invasions of aggressive and competitively superior exotic aquatic plants over the past century threaten the integrity of the basin’s aquatic habitats and may affect the benefits of improved water management.
Non-native aquatic plants such as common salvinia (Salvinia minima), water lettuce (Pistia stratiotes), water-milfoil (Myriophyllum spicatum), Brazilian elodea (Egeria densa) and alligatorweed (Alternanthera philoxeroides) can all be found in the basin. Water hyacinth (Eichhornia crassipes) and hydrilla (Hydrilla verticillata) have been, by far, the most successful invaders (Figure 1) and have the greatest potential to affect the ecology of the basin’s aquatic systems.
Water Hyacinth, Hydrilla
Water hyacinth, apparently introduced in the United States in 1884, is a highly productive floating plant that can reproduce at an incredible rate under favorable conditions, doubling its surface area coverage every six to 15 days. Its aggressive growth can crowd out native floating plants such as frogbit (Limnobium spongia), and shading of the water surface by dense water hyacinth mats eliminates submerged native plants such as cabomba (Cabomba caroliniana) and coontail (Ceratophyllum demersum). Water hyacinth has become particularly problematic in Florida and Louisiana, resulting in extensive spraying and harvesting programs to maintain open waterways.
Sometime during the 1970s, hydrilla became established in the basin and has since become abundant in low-current habitats. Hydrilla has several characteristics that give it a competitive advantage over many native species. It can grow up to several centimeters per day, and high stem densities near the surface intercept most of the available sunlight, eliminating other plants. Hydrilla tolerates a wide range of pH, can photosynthesize at less than 1 percent of full sunlight, and can reproduce from fragmentation, seeds and underground tubers, which can survive extended periods out of water, ingestion by waterfowl and herbicides.
Hydrilla and water hyacinth now dominate the aquatic plant community in the basin, and expansion of these exotics has undoubtedly reduced the diversity and abundance of native aquatic plants. Dense stands of water hyacinth impair oxygenation of the underlying water column by reducing the air-water interface, minimizing wind-generated turbulence and virtually eliminating photosynthetic phytoplankton. By late summer, water hyacinth typically covers thousands of acres of basin aquatic habitats, causing extensive hypoxia, defined as levels of dissolved oxygen (DO) below 2 milligrams per liter. Habitats subjected to hypoxic DO conditions exhibit reduced abundance and diversity of zooplankton and fishes (Figure 2), with most invertebrates (larval insects and mysid shrimp) restricted to the feathery roots suspended under the water hyacinth plants.
Dense beds of hydrilla can have similar effects on DO concentrations. In the summer and fall, hydrilla beds are inhabitable only by organisms adapted to low or highly fluctuating DO levels. Even in shallow (1 meter deep) hydrilla beds, DO concentrations near the bottom often remain below 2 milligrams per liter throughout the day. Although daytime DO levels in the hydrilla canopy may be high, this habitat becomes hypoxic at night as plant respiration increases (Figure 3). These DO concentrations reduce the abundance and diversity of vegetation-dwelling invertebrates and juvenile fishes, and dense stands of hydrilla are virtually inaccessible to sportfish such as largemouth bass, which are unable to locate suitable prey.
Non-native plants cause tremendous problems in aquatic ecosystems such as the Atchafalaya Basin, but their eradication presents a huge, if not insurmountable, task. Water hyacinth can be collected and crushed by large floating harvesters, but much of the basin is relatively inaccessible and difficult for the harvesters to maneuver through because of submerged cypress stumps. Disposal in non-aquatic areas would be necessary so that decomposing plants would not contribute to DO problems.
Mechanical control is similarly not feasible for hydrilla, partially because small cuttings resulting from any harvesting operation have the ability to sprout roots and create new plants. Although the Aquatic Weed Section of the Louisiana Department of Wildlife and Fisheries spends several hundred thousand dollars annually on a 2-4, D spraying program for water hyacinth, there is no effective chemical control for hydrilla in an ecosystem the size of the basin.
A major impediment to effective control is the annual flood pulse, which can carry water hyacinth plants and hydrilla cuttings to new basin habitats, creating new problem areas each year. If winter conditions are mild and the flood pulse is of typical magnitude, there is virtually no lake, bayou or canal in the basin that is not susceptible to colonization.
Biological control has been successful for some exotic plants, such as alligatorweed, which has been adequately controlled with a flea beetle and a stem-boring moth. Two weevils have been imported as control agents for water hyacinth, and although well-grazed leaves are evident on most plants by late summer, these weevils do not appear to be impairing the density or distribution of water hyacinth in the basin.
A stem weevil, two leaf-mining flies and an aquatic moth have been released in Florida to control hydrilla, but significant control by these insects is not evident. Grass carp have effectively eliminated hydrilla in Texas and Louisiana lakes, but the number of fish required for the Atchafalaya Basin would be prohibitive, and the effect on native plants and sport fisheries make this option inadvisable.
Drawdown can be an effective control method for hydrilla, and a multi-year drawdown program is being implemented by the Department of Wildlife and Fisheries to try to control hydrilla in Lake Henderson; unfortunately, drawdown of the rest of the basin is not possible.
Fisheries research efforts at the LSU AgCenter focus on documenting the effects of these exotic plants on water quality and the abundance of basin organisms. For example, AgCenter researchers are investigating the effects of hydrilla infestation on food habits and growth of juvenile largemouth bass and the distribution of invertebrates in relation to water quality and position in the hydrilla beds, such as the edge or middle. These studies will provide information to better manage plant beds (dredge or lay down fiber mats to create more vegetation “edge,” if it is better habitat) for fish and anglers.
In the absence of new biological control discoveries, it is apparent that the ecological effects caused by exotic aquatic plants will continue to alter water quality, food web structure, fisheries production and navigation in the Atchafalaya Basin. The evidence is overwhelming that exotic plants have the capacity to significantly affect the ecology of most aquatic systems in Louisiana, and the basin provides an exceptional example of a floodplain river ecosystem altered by the successful invasion of these aggressive aquatic weeds.