Habitat: Freshwater lakes, ponds, rivers, impoundments and
canals.
Map indicates recorded presence in at least one site within the drainage (USGS
Hydrologic Unit 8),
but does not necessarily imply occurrence throughout that drainage.
Nonindigenous Occurrences:
East of Mississippi River
- The most abundant aquatic plant in
Florida public waters (Schardt 1994), seventy
percent of Florida's freshwater drainage basins
contain waterbodies infested with hydrilla (FLDEP 1988-94 and other data). Less
common throughout Alabama, although widespread
in the Mobile Delta and northern portion of the Mobile Bay (Zolcynski 1997);
common at Coffeeville, Aliceville and Oak Mountain reservoirs of central Alabama
(D. Powell, Alabama Power Company, pers. comm. 1996), and well established at
Guntersville, and other northern impoundments on the Tennessee River (Bates
and Smith 1994). Extending along the Tombigbee River from Aliceville Reservoir,
AL into eastern Mississippi (E. Dibble,
Mississippi State Universiy, pers. com. 1998). Also, restablished in the Tennessee-Tombigbee
Waterway, at Beckner boat ramp and in the old bendway, (D. Franks, Mississippi
Dept. Wildlife, Fisheries and Parks, pers. comm. 2000). Reported in southeastern
Tennessee from the middle Tennessee River
drainage, where herbicide and natural decline in Lake Chickamauga have precluded
recent collection; yet, still found downstream, as scattered stands, in Nickajack
Reservoir (Tennessee Valley Authority 1990, D. Webb, Tennessee Valley Authority,
pers. com. 1997). Know primarily in the southwestern drainages of Georgia;
problematic at Lake Seminole for over a decade (Eubanks 1996). Occurring, otherwise,
at a few smaller waterbodes in Georgia's upper Ocmulgee drainage and at the
Strom Thurmond Reservoir on the upper Savannah River (L. Ager, Georgia Department
of Natural Resources, pers. com. 1998). Expanding over 32,000 acres in Lakes
Marion and Moultrie, South Carolina following
the 1989 hurricane, Hugo (Roach et al 1993). Problematic at six additional reservoirs
in South Carolina (S. de Kozlowski, South Carolina Department of Natural Resources,
pers. comm. 1997), within the Seneca, Saluda, Wateree, Four Hole Swamp, and
Cooper drainages. Covering approximately 5,800 acres in North
Carolina, primarily at eastern sites, including reservoirs in the
upper Neuse River drainage, yet reported from as far west as Buncombe county,
in the western mountains (NCDWR 1996). Established in the Potomac, Rappahannock,
Anna, Chickahominy, and Appomattox Rivers of Virginia's
coastal plain; extending into piedmont Virginia, at several reservoirs (E. Steinkoenig
and J. Kauffman, Virginia Game and Inland Fisheries; J. Tate, Virginia Department
of Agriculture and Consumer Services; pers. comm. 1998). Although reduced in
abundance, continuing to dominate beds of submerged vegetation in the tidal
freshwater reaches of the Potomac River on theVirginia/Maryland
border (Orth et al. 1996). Established in Maryland
at marsh creeks and rivers on the western and northeastern shores of the Chesapeake
Bay, including the Pautuxent River, where it has become the most abundant plant
species (M. Naylor, Maryland Dept. of Natural Resources, pers. comm. 2000; Orth
et al. 1996; Posey et al. 1993). Also common at reservoirs in mid-Maryland,
especially those draining the Patuxent River, and in the Ohio drainage of far
western Maryland, at Deep Creek Lake (M. Naylor, Maryland Department of Natural
Resources, pers. comm. 2000). Discovered recently at three sites in Pennsylvania:
in scattered stands in the Schuylkill River, downtown Philadelphia (P.
Madeira, USDA/ARS, Aquatic Weed Control Research, pers. comm. 1996); at Highland
Lake, a 28 acre impoundment on Southwick Creek (Colangelo 1998); and at Lake
Nockamixon, where plants are abundant in a drowned portion of Haycock Creek
[E. Zacharias and A. Schuyler 8345 (PH), 1998]. Common in Delaware ponds
(AREC 1995), especially in southern Sussex Couty where it has spread to nearly
1200 acres in 14 ponds and portions of the Nanticoke River (C. Martin, Delaware
Dept. of Natural Resources, 1999). Present since the late 1980s at two ponds
in southeastern Connecticut (Balcom 1997),
where plants were originally misidentified due to the absence of midrib teeth
(Les 1996). New to Massachusettes from a
pond on Cape Cod (B. Hellquist pers. comm. 2001). New to the state of Maine
in 2002 in the Saco drainage at Pickerel
Pond, York County, where plants are established and abundant throughout the
pond (R. Bouchard, ME DEP, pers comm. 2003). First documented in 2003
for the state of New Jersey in the Lower
Delaware drainage at Lake Mallard in the Pinelands National Reserve (Sullivan
s.n. DOV, FLAS). This small lake is one of several connected ponds. As of
April 2003, hydrilla had not been detected in any of the other linked ponds
(G. Sullivan, Allied Biological, Inc., pers. comm. 2003).
West of Mississippi River -
Appearing more frequently at lakes and reservoirs in Louisiana,
especially along Highway 1 as it stretches diagonally across the state, in canals
and bayous of the Atchafalaya Basin, and through the coastal marsh region south
of Interstate 10 (C. Biggar, Louisiana Department of Wildlife and Fisheries,
pers comm 1998). Also present at Lake Bruin (indicated by the dot in the eastern
Tensas drainage) which is disjunct from other
infested regions in Louisiana. Known from over 80 Texas
reservoirs, residing in drainages that extend from north-central to eastern
Texas, and south to the Rio Grande, at the Mexico border (Helton and Hartman
1997; E. Reyes, U.S. Fish and Wildlife Service, pers comm 1998). Eradicated
in the mid 1980s from two ponds in Phoenix, Arizona,
where no new infestations have been found (E. Hall, Arizona Dept.of Agriculture,
pers. comm. 1996). Eradicated from private ponds and several reservoirs in nine
California counties; presently occurring
at less than 50 sites in Imperial, Tulare, Madera, Mariposa, Calaveras, Yuba,
Lake and Shasta Counties (PPDC 1997). Recent infestations in Clear Lake threaten
the highly productive Sacramento/San Joaquin River Delta area (Anderson 1996).
Occurring in Washington, in a lake east
of Puget Sound, where early biomass measurements reflect high growth potential
in the northwestern climate (K. Hamel, Washington State University, pers. comm.
1995). New to the state of Arkansas, established
in Lake Ouachita and DeGray Lake, in the Ouachita Headwaters and the Upper Ouachita
drainages, respectively. First identified in Lake Ouachita in 1999 during creel
surveys. Lake Ouachita is a 42,000 acre lake, with an approximate hydrilla infestation
size of 4,000 acres. DeGray Lake is nearly half the size of Lake Ouachita and
has 3-4 confirmed acres of hydrilla infested waters. Hydrilla has been verified
growing in waters of 7.3 m in depth with expected growth to be in the 9.1 -
10.7 m level in the near future due to light pentration to 12.2 m in depth and
to the sedimentary composition of the lake. Lake Ouachita personnel are working
on a project to initiate biological control efforts in the lake (R. Stokes,
USACE, Manager, Lake Ouachita, pers. comm. 2003).
Means of Introduction: The dioecious strain was imported to the United States in the early 1950s for use in aquariums. It entered Florida's inland water system after plants were discarded or planted into canals in Tampa and in Miami (Schmitz et al 1988). The monoecious strain was a separate introduction, first found decades later in the Potomac Basin (Environmental Laboratory 1985).
Hydrilla is mainly introduced to new waters as castaway fragments on recreational boats, their motors and trailers and in live wells. Stem pieces root in the substrate and develop into new colonies, commonly beginning near boat ramps. Once established, boat traffic continues to shatter and spread hydrilla throughout the waterbody. Both types propagate primarily by stem fragmentation, although axillary buds (turions) and subterranean tubers are also important. Tubers are resistant to most control techniques (Schardt 1994) and may be viable as a source of reinfestation for years (Van and Steward 1990).
Hydrilla may be unknowingly transplanted into private ponds as a contaminant on watergarden plants. It is often found spreading after extensive 2,4-D use in public waters once heavily populated with Eurasian water-milfoil (Myriophyllum spicatum) (Bates and Smith 1994).
Status: Recorded
from over 690 water bodies within 190
drainage basins of 21 states.
Impact of Introduction: Once established, hydrilla results in an array
of ecosystem disruptions. Changes often begin with its invasion of deep, dark
waters where most plants can not grow. Hydrilla grows aggressively and competatively,
spreading through shallower areas and forming thick mats in surface waters that
block sunlight penetration to native plants below (van Dijk 1985). In the southeast,
hydrilla effectively displaces beneficial native vegetation (Bates and Smith
1994) such as wild-celery (Vallisneria americana) and coontail (Ceratophyllum
demersum) (van Dijk 1985; Rizzo et al. 1996).
It has been shown to alter the physical and chemical characteristics of lakes.
Colle and Shireman (1980) found sportfish reduced in weight and size when hydrilla
occupied the majority of the water column, suggesting that foraging efficiency
was reduced as open water space and natural vegetation gradients were lost.
Stratification of the water column (Schmitz et al. 1993; Rizzo et al. 1996),
decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996)
have been documented. Changes in water chemistry may also be implicated in zooplankton
and phytoplankton declines (Schmitz and Osborne 1984; Schmitz et al. 1993).
Hydrilla seriously affects water flow and water use. Infestations in the Mobile
Delta are reducing flow in small tidal streams and creating a backwater habitat
(J. Zolcynski pers. comm. 1998). Its heavy growth commonly obstructs boating,
swimming and fishing in lakes and rivers and blocks the withdrawl of water used
for power generation and agricultural irrigation.
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Author: C.C. Jacono and M.M. Richerson
Revision Date: 18 December 2003
