National Research Program


Potomac River

Benefits from an exotic species of submerged aquatic vegetation (SAV)

An exotic species of aquatic vegetation, Hydrilla verticillata, was seen as a severe invasive nuisance in the 1980s when the fast-growing Asian plant began spreading in the Potomac River. However, recent data analyses indicate that fears of some of the adverse affects of hydrilla appear to have been unfounded.

Found to produce vegetation masses dense enough to impede boat traffic and water sports in some areas, it was feared that hydrilla's spread would eliminate native SAV, and have an adverse affect on waterfowl, such as black duck, and on fish, and crabs in the tidal Potomac. Meanwhile, efforts began in the 1980's to restore the Potomac River and Chesapeake Bay by reducing nitrogen loads entering the river system and Bay. USGS scientists wanted to understand how hydrilla was affecting the benefits sought for by management investments. Using information from annual field surveys and aerial photographs, USGS scientists created a data base to document which species of vegetation were found in different sections of the Potomac River system. They recorded the percentage of total coverage and biomass of each species attained annually and discovered that, hydrilla was always more than 40% of the total abundance of vegetation but did not eliminate other species over the 17-year period of study. They found the percent of native SAV increased over time. Results indicated that as nitrogen concentration decreased, diversity of vegetation increased. In addition, they compared abundance of waterfowl between periods when there was no SAV and periods when exotic species of SAV were dominant. They found a positive response by waterfowl to vegetation communities dominated by exotic SAV.

Graph of waterfowl counts and submersed acquatic vegetation, Potomac River[Waterfowl counts are from the National Audubon Society Christmas Bird Count Surveys. Submerged aquatic vegetation (SAV) coverage for the upper and lower tidal river segments is divided into three time periods. Myriophyllum spicatum was present, but SAV coverage was unknown for 1959-1965.]

This long-term, quantitative study of aquatic plant biodiversity in an estuary where millions of dollars are spent annually to reduce nutrient input demonstrates that exotics are not always harmful to an ecosystem. The findings also support current federal and state management strategies to improve water clarity and reduce nutrient loads.

References:
Rybicki, N. B., and Landwehr, J.M., 2007, Long-term changes in abundance and diversity of macrophyte and waterfowl populations in an estuary With exotic macrophytes and improving water quality: Limnology and Oceanography, v. 52, no. 3, p. 1195-1207. (pdf format, 1200 KB - published by the American Society of Limnology and Oceanography with open access)

Rybicki, N.B., Yoon, S.N., Schenk, E.R., and Baldizar, J.B., 2007, The Distribution of Submersed Aquatic Vegetation in the Fresh and Oligohaline Tidal Potomac River, 2004 , U.S. Geological Survey Open-File Report 2007-1198. (pdf format, 1380 KB)

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This study was supported in part by USGS Chesapeake Bay studies, part of the federal-state Chesapeake Bay Program. For additional information, see above references, contact Nancy Rybicki, nrybicki@usgs.gov

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