Description
Hydrilla verticillata (Hydrilla) is a submersed plant (submersed aquatic vegetation).
Potentially Misidentified Species - Elodea nuttalli is regarded as a synonym of E. canadensis by some authors. Catling and Wojtas (1986) give criteria for separating it from E. canadensis, both are native.
Other Taxonomic Groupings - A genetic analysis (randon amplified polymorphic DNA) identified 5 clusters in world populations of H. verticillata: southeast Asian-United States dioecious, Indonesian dioecious, Australia, Korea-United States monoecious (Madeira et al. 1997). Both dioecious and monoecious populations occur in the United States The form found in Chesapeake Bay and adjacent states (NC-DE) is monoecious, and believed to be of Korean origin (Madeira et al. 1997; Steward et al. 1993).
Taxonomy
| Kingdom | Phylum | Class | Order | Family | Genus |
|---|---|---|---|---|---|
| Plantae | Magnoliophyta | Liliopsida | Hydrocharitales | Hydrocharitaceae | Hydrilla |
Synonyms
Invasion History
Chesapeake Bay Status
| First Record | Population | Range | Introduction | Residency | Source Region | Native Region | Vectors |
|---|---|---|---|---|---|---|---|
| 1982 | Established | Expanding | Introduced | Regular Resident | East Asia | East Asia | Ornamental(Aquatic Plant) |
History of Spread
Hydrilla verticillata (Hydrilla) is native to Asia, Australia, and probably Africa, and has been widely introduced to warmer parts of the world, including Europe north to Ireland, Poland, Australia, New Guinea, New Zealand, Africa, and Panama (Cook and Luond 1982; Pieterse 1981) (Cook and Luond 1982; Pieterse 1981). Its first appearance in North America was in FL in the early 1950's by an aquarium dealer who had plants shipped from Sri Lanka, found them unsatisfactory, and dumped them into a canal near Tampa Bay, where they grew very well (McCann et al. 1996). Plants from this location were later shipped to Miami, cultured, and sold by 1955. This dioecious strain of H. verticillata, which is exclusively female and reproduces only asexually. spread extensively in FL, the southeast United States (SC-TX) , CA (1977), and by 1996, WA and AZ (Langeland 1996; Pieterse 1981; Cook and Luond 1982).
Invasions of H. verticillata in the mid-Atlantic states (NC-CT) were of a genetically distinct monoecious, sexually reproducing strain, believed to be native to Korea (Langeland 1996; Madiera et al. 1997; Steward et al. 1993). These strains are believed to be adapted to colder climates than the dioecious forms (Langeland 1996). Monoecious H. verticillata appeared in Trap Pond DE in 1976, NC in 1980, the Potomac River (MD-VA-DC) in 1982, Susquehanna River and uppermost Chesapeake Bay MD in 1985 and Trap Pond DE (1976) (Langeland and Smith 1985; Orth et al. 1987; Steward et al. 1984). By 1996, populations were found in the Schuylkill River in Philadelphia, and in ponds in southeastern CT (Florida Caribbean Science Center 2001).
Chesapeake Bay records:
James River- By 1998, H. verticillata had become established in the Appomattox and Chickahominy Rivers on the Coastal Plain (probably tidal) (Florida Caribbean Science Center 2001).
York River- By 1998, H. verticillata had become established in the Anna River, a nontidal York tributary (Florida Caribbean Science Center 2001), and by 2003 it was established in the York and Pamunkey Rivers (Orth et al. 2004).
Rappahannock River- In 1998, H. verticillata was absent from the Rappahannock estuary, but by 2003, it was established in the tidal fresh river.
Potomac River - In August 1982, H. verticillata was discovered in three locations: near Alexandria VA, in the Kenilworth Aquatic Gardens on the Anacostia River DC, in the Chesapeake and Ohio Canal near Seneca Falls MD. The Alexandria colony may have resulted from experimental transplants of submersed vegetation in 1980, when plants obtained at Kenilworth Aquatic gardens were misidentified as the native Elodea canadensis (Steward et al. 1984). Rapid spread occurred in the Potomac since H. verticillata's introduction. By July 1983, it was found at Quantico VA, 27 nautical miles downriver from site of introduction (Steward et al. 1984). By 1986, it was blocking navigation in parts of the Potomac, and required a control program (Anonymous 1994). It reached its abundance peak in the Potomac in 1987, and has been declining together with other submerged aquatic vegetation (SAV), to the present (Orth et al. 1993; Phelps 1994). By 1992, it was found south to Aquia Creek VA. By 1995, it reached Potomac Creek VA about 10 km downstream of Aquia Creek, in oligohaline-low mesohaline region (Moore et al. 2000). Potential still exists for some downstream expansion in the Potomac (Batiuk et al. 1992).
Patuxent River - Hydrilla verticllata was recently found in the Patuxent near Jug Bay (Moore et al. 2000) .
Upper Bay - By 1985, H. verticillata was present on Susquehanna flats and Upper Bay. (Orth et al. 1987; Staver 1994). Hydrilla verticillata growth in the Upper Bay appears to be primarily in small patches of mixed vegetation, and extends south to Sassafras River (Orth et al. 1993; Posey et al. 1993; Staver 1994) and, by 2003, to the mid-upper Gunpowder and Middle Rivers on the Western Shore and the Chester River on the Eastern Shore (Orth et al. 2004).
History References - Anonymous 1994; Batiuk et al. 1992; Cook and Luond 1982; Florida Caribbean Science Center 2001; Hurley 1990; Langeland 1996; Langeland and Smith 1984; Madeira et al. 1997; McCann et al. 1996; Moore et al. 2000; Orth et al. 1987; Orth et al. 1993; Orth et al. 2004; Pieterse 1981; Phelps 1994; Posey et al. 1993; Staver 1994; Steward et al. 1984; Steward et al. 1993
Invasion Comments
Vector(s) of Introduction- Local boaters blame the experimental transplants for the spread of H. verticillata in the Potomac (Fofonoff 1997 personal observation), but by 1980, this plant was probably established in several locations in the Potomac drainage where imported ornamental plants were grown (eg. Kenilworth Aquatic Gardens, Lilypons Gardens) (Steward et al. 1984), so that invasion of the tidal river was inevitable.
Range Status - The range of H. verticillata is expanding, but its biomass has been declining in the Potomac River since 1987 (Orth 1993; Phelps 1994).
Ecology
Environmental Tolerances
| For Survival | For Reproduction | |||
|---|---|---|---|---|
| Minimum | Maximum | Minimum | Maximum | |
| Temperature (ºC) | 0.0 | 32.0 | 16.0 | 32.0 |
| Salinity (‰) | 0.0 | 12.0 | 0.0 | 12.0 |
| Oxygen | hypoxic | |||
| pH | 6.8000000000 | 10.0000000000 | ||
| Salinity Range | fresh-oligo |
Age and Growth
| Male | Female | |
|---|---|---|
| Minimum Adult Size (mm) | ||
| Typical Adult Size (mm) | ||
| Maximum Adult Size (mm) | 2000.0 | 2000.0 |
| Maximum Longevity (yrs) | 0.7 | 0.7 |
| Typical Longevity (yrs | 0.6 | 0.6 |
Reproduction
| Start | Peak | End | |
|---|---|---|---|
| Reproductive Season | |||
| Typical Number of Young Per Reproductive Event |
|||
| Sexuality Mode(s) | |||
| Mode(s) of Asexual Reproduction |
|||
| Fertilization Type(s) | |||
| More than One Reproduction Event per Year |
|||
| Reproductive Startegy | |||
| Egg/Seed Form |
Impacts
Economic Impacts in Chesapeake Bay
Hydrilla verticllata (Hydrilla) has had a wide range of positive and negative economic impacts in the Chesapeake Bay region, particularly in the Potomac River.
Aesthetic - Large rafts of H. verticillata are deposited by the tide in the Potomac ( Fofonoff 1995-1996 personal observation). Decomposition of these in summer is probably unpleasant.
Boating - By 1986, H. verticillata beds in the Potomac became dense enough to require cutting in order to maintain navigation channels. The vegetation is cut by a harvester ship, dumped on shore to dry, and then disposed of in a landfill. In 1993, 97.2 Mg (tonnes) was removed from 4.76 ha of riverbed. The program is administered by the Metropolitan Washington Council of Governments and sponsored by the Army Corps of Engineers, VA Department of Game and Inland Fisheries, and the MD Department of Natural Resources (Anonymous 1994). Regrowth of H. verticillata was rapid after harvesting, plants returned to pre-harvest biomass in ~23 days. Three harvests or more per summer may be required to permit boat passage. A mechanical harvest kills about 11-22% of the fish population in the harvested area, mostly due to entanglement in the removed vegetation (Serafy et al. 1994). A number of biological control insects have been released in southern states, but these are unsuited to the Chesapeake region climate. Some investigations of cold-tolerant H. verticillata herbivores were made by the Agricultural Research Service (United States Department of Agriculture) but no releases of biocontrol insects have been made (Coulson 1998 personal communication).
Health - Hydrilla verticillata growth and decomposition is a problem in drinking-water reservoirs in the DC area (Anon. 1994); Dense H. verticillata beds probably provide mosquito breeding habitat as noted with Myriophyllum spicatum (Eurasian Watermilfoil) (Stevenson and Confer 1978).
Fisheries - Benefits of H. verticillata to recreational fisheries are substantial but difficult to quantify. Hydrilla verticillata beds support extremely high densities (10-49 m-2) of fishes, mainly baitfishes and juveniles (Killgore et al. 1989; Serafy et al. 1994) which in turn support predators such as Micropterus salmoides (Largemouth Bass). Cutting of H. verticillata kills many small fishes (see 'boating').
It is hard to balance navigation and aesthetic costs versus fisheries benefits. The fisheries improvements from H. verticillata in the Potomac are similar to those which would from a regrowth of native submerged aquatic vegetation (SAV), but in the more turbid parts of the river, the latter would have been difficult to achieve even in the absence of H. verticillata.
References - Anonymous 1994; Coulson 1998 personal communication; Killgore et al. 1989; Serafy et al. 1994; Stevenson and Confer 1978
Economic Impacts Outside of Chesapeake Bay
Hydrilla verticillata (Hydrilla) is regarded as a serious weed worldwide in tropical-warm temperate regions, in rice fields, irrigation canals, fishponds and waterways (Cook and Luond 1982). It is established in 12 US states (USGS Florida Caribbean Science Center 2000). High uniform densities of Hydrilla verticillata can have adverse effects on lake fisheries by interfering with movement and vision of predatory fishes and by using up oxygen when large biomasses decompose (Engel 1995). In Chesapeake Bay, turbidity and winter die-off allow vegetation-free areas to persist (Carter et al. 1994) and seem to reduce these adverse effects.
On one FL lake (Orange Lake, north-central FL), in years when H. verticillata completely covered the lake, recreational activies worth 11 million dollars were lost. In 1994-95, the state of Florida spent ~ 14.5 million dollars on H. verticillata control (Langeland 1996). Over 1 million dollars is spent to control H. verticillata in irrigation canals in CA (Florida Caribbean Science Center 2000).
References - Carter et al. 1994; Cook and Luond 1982; Engel 1995; Florida Caribbean Science Center 2000; Langeland 1996
Ecological Impacts on Chesapeake Native Species
Many of the effects of Hydrilla verticillata (Hydrilla) occurred where it recolonized locations where submersed aquatic vegetation (SAV) had long ago been eliminated by pollution. Consequently, many of the impacts attributed to this plant are are difficult to distinguish from those of submersed aquatic vegetation (SAV) in general. However, H. verticillata has some unique features, including the denseness of its surface canopy and its capacity for growth under conditions too turbid for other species (Hurley 1990).
Competition- Hydrilla verticillata invaded Chesapeake Bay at a time when submersed aquatic vegetation (SAV) in the Bay tributaries had declined on an unprecedented scale. The spread of H. verticillata was part of a general resurgence of SAV in the Potomac River and elsewhere in the Bay (Carter and Rybicki 1986). Causes of this resurgence and the subsequent (1988-present) decline are uncertain and may include nutrients, river flow, turbidity, weather, filtering by the clam Corbicula fluminea (Asian Freshwater Clam), and herbivory and predation by birds (Carter et al. 1994a; Phelps 1994). Hydrilla verticillata quickly became a biomass dominant in tidal fresh and oligohaline waters; indication successful competition with other SAV species. However, other introduced and native plants increased as well. Hydrilla verticillata is unusually capable of net photosynthesis and growth under low light (Barko and Smart 1981), and so outcompetes most other SAV under turbid conditions (Carter et al. 1994a). Adverse effects on native SAV probably include shading and competition for space.
Hydrilla verticillata has not displaced other SAV species completely, probably because of its high germination temperature (16 C); compared to 13 C for native Vallisneria americana (Wild Celery). (Carter et al. 1994b). In May and June, in the Potomac River H. verticillata forms dense prostrate growths, not developing a dense surface canopy until July, so other species [e.g. the introduced Myriophyllum spicatum(Eurasian Water Milfoil), and Potamogeton crispus (Curly Pondweed), and the native Vallisneria americana (Wild Celery)] can rise above the H. verticillata in May and June (Killgore et al. 1989; Carter et al. 1994b). In field studies in the upper Bay (Havre de Grace MD), H. verticillata also showed delayed growth, but dominated the SAV biomass by August, particularly at depths of 1 m compared with 0.5 or 1.5 m. Turbulence may have been a problem for H. verticillata in the more exposed offshore (1.5 m) beds, where it may have been more fragile than V. americana or M. spicatum (Staver 1994).
Hydrilla verticillata is listed as highly invasive in DE, MD, and VA (Cooley 1993; Delaware Natural Heritage Program 1998; Virginia Department of Conservation and Recreation 1999).
Habitat Change - Rafting and deposition of H. verticillata on intertidal mudflats in the Potomac may be adversely affecting rare and/or poorly known plants of that habitat [e.g. Elatine americana (American Waterwort); Bacopa innominata (Tropical Waterhyssop); Eriocaulon parkeri (Estuary Pipewort); Cardamine longii (Long's Bitter Cress)] in the Potomac (Strong 1995).
Hydrilla verticillata beds frequently develop a very dense canopy which provides cover to a variety of organisms. Nearly monspecific H. verticillata beds at a site in the tidal fresh Potomac had higher densities of epifaunal invertebrates (mostly hydrobiid snails and insect larvae) than a nearby site with mixed vegetation in July. However, these differences were not present in August (Thorp et al. 1997).
Habitat modification by H. verticillata in the Potomac favored cover-oriented native and introduced fishes, at the expense of open-water pelagic species, all of which are native. The native species favoring H. verticillata stands include Notropis hudsonius (Spottail Shiner), Lepomis gibbosus (Pumpkinseed), Perca flavescens (Yellow Perch), Fundulus diaphanus (Banded Killifish). The open-water species, displaced by patches of H. verticillata, include: Brevoortia tyrannus (Atlantic Menhaden), Menidia beryllina (Inland Silverside ), Morone americana,(White Perch), Dorosoma cepedianum (Gizzard Shad) Alosa aestivalis (Blueback Herring) (Killgore et al. 1989; Serafy et al. 1993).
Hydrilla verticillata patches on the Susquehanna flats had higher invertebrate densities (Acarina; Amphipoda, Oligochaeta) than unvegetated plots, probably because of cover as well as higher rates of particle deposition and greater sediment stability (Posey et al. 1993).
Hydrilla verticillata also alters the physical and chemical enviroment. Patches of H. verticillata in the Potomac River had altered temperature stratification, and altered tidal flow patterns compared to unvegetated areas. Tidal current velocities were greatly reduced within the patch, creating differences in water level between the patch and channel, as water entered the patch later, and left earlier than in unvegetated areas (Rybicki et al. 1997). Dense patches on the Susquehanna flats had elevated dissolved oxygen (DO) and pH levels of 9.5-10 for periods of several hours, during periods of elevated photosynthesis. Fishes avoid these conditions in the lab but in the field remain in H. verticillata patches during periods of high pH and supersaturated dissolved oxygen (DO) (Serafy et al. 1993). Decomposition of large biomasses of dead H. verticillata can reduce oxygen concentrations (Pieterse 1981). Because H. verticillata forms a denser canopy than most other submersed plants, these physical and chemical effects may greater than those produced by most native species.
The chemical environment in sediments was altered in stands dominated by H. verticillata, and/or Myriophyllum spicatum (Eurasian Watermilfoil) compared to stands dominated by the native Vallisneria americana (Wild Celery) in the Susquehanna Flats (MD), in upper most Chesapeake Bay. In H. verticillata/M. spicatum patches, sediments were less oxidized, porewater inorganic PO4 concentrations were higher, and PO4/Fe ratios were lower than in V. americana patches. Hydrilla's and Myriophyllum's roots are relatively shallow compared to those of V. americana. These two exotic plants extract most of their nutirents from the water column, while V. americana obtains most of its nutirents from the sediments (Wigand et al. 1997). Consequently, species replacement has implications for nutrient cycling. However, other native plants (e.g. Elodea canadensis, Canadian Waterweed) resemble H. verticillata in growth form and could also have similar behavior with regard to roots and nutrient use.
FoodPrey - Hydrilla verticillata also provides a food source for birds including Fulica americana (American Coots), ducks, and geese (Esler 1989; Esler 1990; Hench et al. 1994). The resurgence of submerged aquatic vegetation (SAV) in the Potomac and Upper Bay, dominated by H. verticillata, has probably greatly increased the food supply for overwintering birds (Hurley 1990).
References - Barko and Smart 1981; Carter and Rybicki 1986; Carter et al. 1994a; Carter et al. 1994b; Cooley 1993; Delaware Natural Heritage Program 1998; Esler 1989; Esler 1990; Hench et al. 1994; Hurley 1990; Killgore et al. 1989;Phelps 1994; Pieterse 1981; Posey et al. 1993; Rybicki et al. 1997; Serafy et al. 1993; Staver 1994; Thorp et al. 1997; Virginia Department of Conservation and Recreation 1999; Wigand et al. 1997
Ecological Impacts on Other Chesapeake Non-Native Species
Hydrilla verticllata (Hydrilla) has affected other introduced sumerged plant species through competition, and has affected non-indigenous fishes through alteration of habitat. This plant has also affected herbivorous fishes and waterfowl by providing an abundant new food source.
Competition - The introduced submerged aquatic vegetation (SAV) species Myriophyllum spicatum (Eurasian Watermilfoil), Egeria densa (Brazilian Waterweed), Potamogeton crispus (Curly Pondweed) and Najas minor (Minor Naiad) are probably adversely affected by Hydrilla verticillata. M. spicatum, in particular, is less capable of growth at low light levels (Barko and Smart 1981) and so is vulnerable to shading by H. verticllata. However, M. spicatum germinates at 13 C (Barko and Smart 1981) and P. crispus begins spring growth at temperatures above 4 C (Wehrmeister and Stuckey 1992), so these species can rise above the prostrate growths of H. verticllata in spring (Carter et al. 1994b).
Habitat Change - Among fish species using H. verticillata stands in the Potomac are several introduced species,Carassius auratus (Goldfish), Lepomis macrochirus (Bluegill), and Micropterus salmoides (Largemouth Bass). Predatory species such as M. salmoides frequently patrol the edges of H. verticillata beds. (Killgore et al. 1989; Serafy et al. 1993). However, when dense growths fill most of all of a body of water, fish yields may decline because of hypoxia due to decay, because of reduced populations of planktivores, and because of impaired movement and vision of predators (Engel 1995; Serafy et al. 1993). In the Potomac, the seasonal dieback of H. verticillata, and the presence of channels too deep for submerged aquatic vegetation (SAV) growth, limit these adverse effects (Killgore et al. 1989).
Corbicula fluminea (Asian Freshwater Clam) had increased survival in H. verticillata patches on the Susquehanna flats, probably because of reduced otter and duck predation due to cover. The survival of the deeper-burrowing, introduced clam Rangia cuneata (Wedge Clam) was not affected by the presence of H. verticillata, although R. cuneata in H. verticillata patches had reduced body weights, perhaps because of increased deposition of particulates, resulting in finer, gill-clogging sediment (Posey et al. 1993).
Food/Prey - Cyprinus carpio (Common Carp) are known to eat H. verticillata. (McCrady 1990). Sterile triploid Ctenopharyngodon idella (Grass Carp) have been introduced for H. verticillata control in reservoirs around the DC area (Jenkins and Burkhead 1993). They were considered for H. verticillata control in the Potomac but rejected, since they were likely to disperse (Anon. 1994). Two leaf-mining flies (Hydriella pakistanae, Hydrellia balciunsi) and two weevils, Bagous affinis, Bagous hydrillae have been released for H. verticillata control in FL (McCann et al. 1996), but we have no information on their release or ability to survive in the Chesapeake Bay region.
References - Anonymous 1994; Barko and Smart 1981; Carter et al. 1994b; Engel 1995; Jenkins and Burkhead 1993; Killgore et al. 1989; McCann et al. 1996; McCrady 1990; Posey et al. 1993; Serafy et al. 1993; Wehrmeister and Stuckey 1992
References
Anonymous (1994) It's harvest season in the Potomac, Water Environment - Technology 6: 24-25Barko, John W.; Smart, R. Michael (1981) Comparative influences of light and temperature on the growth and metabolism of selected submersed freshwater macrophytes, Ecological Monographs 51: 219-235
Batiuk, Richard A.; Orth, Robert J.; Moore, Kenneth A.; Dennison, William C.; Stevenson, J. Court; Staver, Lorie W.; Carter, Virginia; Rybicki, Nancy (1992) Chesapeake Bay submerged aquatic vegetation, habitat requirements and restoration targets: a technical synthesis, In: (Eds.) . , Annapolis, MD. Pp.
Carter, Virginia; Rybicki, N. B. (1994) Invasions and declines of submersed macrophytes in the tidal Potomac River and estuary, the Currituck Sound-Back Bay system, and the Pamlico River estuary, Lake and Reservoir Management 10: 39-48
Carter, Virginia; Rybicki, N. B.; Landwehr, Jarate M.; Turtora, Michael (1994) Role of weather and water quality in population dynamics of submersed macrophytes in the tidal Potomac River, Estuaries 17: 417-426
Carter, Virginia; Rybicki, Nancy (1986) Resurgence of submersed aquatic macrophytes in the tidal Potomac River, Maryland, Virginia, and the District of Columbia, Estuaries 9: 368-375
Catling, Paul M.; Wojtas, Walter (1986) The waterweeds (Elodea and Egeria) in Canada, Canadian Journal of Botany 64: 1525-1541
Cook, C. D. K.; Luond, Ruth (1982) A revision of the genus Hydrilla (Hydrocharitaceae), Aquatic Botany 13: 485-504
Cooley, Gene (1993) Invasive exotic plants that threaten native species and natural habitats in Maryland., , Annapolis MD. Pp.
1998 Non-native plant species in Delaware. http://www.dnrec.statede.us/iw/weeds.htm
Engel, Sandy (1995) Eurasian watermilfoil as a fishery management tool, Fisheries 20: 20-27
Esler, Daniel (1989) An assessment of American coot herbivory of Hydrilla., journal of Wildlife Management 53: 1147-1149
Esler, Daniel (1990) Avian responses to Hydrilla invasion, Wilson Bulletin 102: 427-440
1996 Nonindigenous Aquatic Species Database. http://nas.er.usgs.gov/
Haller, William T.; Sutton, D. L; Barlowe, W. C. (1974) Effects of salinity on the growth of several aquatic macrophytes, Ecology 55: 891-894
Hench, John E.; Gibbs, Ron; Hench, Jayne S. (1994) Some observations on Hydrilla and wintering waterfowl in Montogomery County, Maryland, The Maryland Naturalist 38: 3-9
Hurley, Linda M. (1990) Field guide to the submersed aquatic vegetation of Chesapeake Bay., , Annapolis, MD. Pp.
Jenkins, Robert E.; Burkhead, Noel M. (1993) Freshwater fishes of Virginia., , Bethesda, MD. Pp.
Killgore, K. Jack; Morgan, Raymond P. II; Rybicki, Nancy B. (1989) Distribution and abundance of fishes associated with submersed aquatic plants in the Potomac River, North American Journal of Fisheries Management 9: 101-111
Lal, Chaman; Gopal, Brij (1993) Production and germination of seeds in Hydrilla verticillata, Aquatic Botany 45: 257-261
Langeland, K. A.; Smith, C. B. (1984) Hydrilla produces viable seed in North Carolina lakes, Aquatics 6: 20-21
Langeland, Kenneth A. (1996) Hydrilla verticillata (L. F.) Royle (Hydrocharitaceae), 'the perfect aquatic weed', Castanea 61: 293-304
Madeira, Paul T.; Van, Thai K.; Steward, Kerry K.; Schnell, Raymond J. (1997) Random amplified polymorphic DNA analysis of the phenetic relationships among world-wide accessions of Hydrilla verticillata, Aquatic Botany 59: 217-236
McCrady, Ellen Joy (1990) Interactions between the invasive freshwater clam, Corbicula fluminea, and its fish predators in Lake Fairfield, Texas, , Arlington, Texas. Pp.
Orth, Robert J.; Nowak, Judith F.; Anderson, Gary F.; Whiting, Jennifer R. (1993) Distribution of submerged aquatic vegetation in the Chesapeake Bay and Tributaries and Chincoteague Bay - 1992, , Annapolis, MD. Pp.
Orth, Robert J.; Simons, Jim; Capelli, Judith; Hindman, Larry; Hodges, Stephen; Moore, Kenneth; Rybicki, Nancy (1987) Distribution of submerged aquatic vegetation in the Chesapeake Bay and tributaries- 1985, , Washington DC. Pp.
Phelps, Harriette L. (1994) The Asiatic clam (Corbicula fluminea) invasion and system-level ecological change in the Potomac River estuary near Washington, D.C., Estuaries 17: 614-621
Pieterse, A. H. (1981) Hydrilla verticillata- a review., Abstracts of Tropical Agriculture 7: 9-34
Posey, Martin H.; Wigand, Cathleen; Stevenson, J. C. (1993) Effects of an introduced aquatic plant, Hydrilla verticillata, on benthic communities in the Upper Chesapeake Bay, Estuarine, Coastal and Shelf Science 37: 539-555
Reed, Clyde F. (1977) History and distribution of Eurasian watermilfoil in United States and Canada, Phytologia 36: 417-436
Resource Management Inc. (1993) National list of plant species that occur in wetlands., , Minneapolis.. Pp.
Rybicki, Nancy B.; Jenter, Harry L.; Carter, Virginia; Baltzer, Robert A. (1997) Observations of tidal flux between a submersed aquatic plant stand and the adjacent channel in the Potomac River near Washington D. C., Limnology and Oceanography 42: 307-317
Serafy, J. E.; Harrell, R. M. (1993) Behavioral responses of fishes to increasing pH and dissolved oxygen: field and laboratory observations, Freshwater Biology 30: 53-61
Serafy, Joseph E.; Harrell, Reginal M.; Hurley, Linda M. (1994) Mechanical removal of Hydrilla in the Potomac River, Maryland: Local impacts on vegetation and associated fishes, Journal of Freshwater Ecology 9: 135-143
Staver, Lorie W. (1994) The impacts of the exotic species Hydrilla verticillata on the shallows in Chesapeake Bay., In: Correll, D. L.(Eds.) Towards a Sustainable Coastal Watershed: The Chesapeake Experiment. Proceedings of a Conference. , Baltimore, MD. Pp. 364-378
Stevenson, J. Court; Confer, Nedra M. (1978) Summary of available information on Chesapeake Bay submersed vegetation, , Annapolis MD. Pp.
Steward, K. K. (1993) Seed production in monecious and diecious populations of Hydrilla, Aquatic Botany 46: 169-183
Steward, Kerry K.; Van, Tahi K.; Carter, Virginia; Pieterse, Arnold H. (1984) Hydrilla invades Washington, D.C. and the Potomac, American Journal of Botany 71: 162-163
Thorp, Angela G.; Jones, R. Christian; Kelso, Donald R. (1997) A comparison of water-column macroinvertebrate communities in beds of differing aquatic vegetation in the tidal freshwater Potomac River, Estuaries 20: 86-95
Twilley, Robert R.; Barko, John W. (1990) The growth of submersed macrophytes under experimental salinity and light conditions, Estuaries 13: 311-321
1999 Invasive Alien Plant Species of Virginia. http://www.state.va.us/~dcr/dnh/invlist.htm
Wehrmeister, John R.; Stuckey, Ronald L. (1992) Life history of Potamogeton crispus, The Michigan Botanist 31: 3-16
Wigand, Cathleen; Stevenson, J. Court; Cornwell, Jeffry C. (1997) Effects of different submersed macrophytes on sediment biogeochemistry, Aquatic Botany 56: 233-244