SUD ERSED 

AQUATIC PlANTS 

OF THE Tl AL 

T M C RIVER 

By VIRGINIA CARTER, PATRICIA T. GAMMON, 

and NANCY C. BARTOW 

GEOLOGICAL SURVEY BULLETIN 1543 

SUD ERSED 

AQUATIC PlANTS 

OF THE Tl AL 

T M C RIVER 

By VIRGINIA CARTER, PATRICIA T. GAMMON, 

and NANCY C. BARTOW 

GEOLOGICAL SURVEY BULLETIN 1543 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1983 

Second printing 1984 

Library of Congress Cataloging in Publication Data 

Carter, Virginia. 

Submersed aquatic plants of the tidal Potomac River. 

(Geological Survey bulletin; 1543) 

Bibliography: p. 

1. Aquatic plants-Potomac River Estuary. 2. Potomac River Estuary. I. Gammon, Patricia T. 

II. Bartow, Nancy C. III. Title. IV. Series. 

QE75.B9 no. 1543 [QK191] 557.3s [581.9752]82-600122 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22.'304 

1983 
PREFACE 

The tidal Potomac River and Estuary are complex environments inhabited by a variety of plants and animals. These living resources are 

greatly influenced by dynamic physical and chemical processes which 

modify their habitat. They, in turn, modify chemical and physical processes and change the habitat. Plants form the link in the food chain 

between the nutrients in the water column and the sediment and the 

animals-fish, invertebrates, birds, herptiles, and mammals. Hydrologists study submersed plants not only as indicators of water quality and 

health of aquatic ecosystems but also to understand the interrelation 

between plants and other ecosystem components. 

Preface III 

CONTENTS 

Page 

Preface_____________________________________________________________ III 

Introduction--------------------------------------------------------- 1 

Survey of the submersed aquatic plants ----------------------------------- 4 

Wildcelery ------------------------------------------------------- 7 

Redhead-grass --------------------------------------------------- 11 

Curly pondweed -------------------------------------------------- 15 

Coontail --------------------------------------------~------------ 21 

Widgeongrass ---------------------------------------------------- 22 

Sago pondweed --------------------------------------------------- 27 

Slender pondweed ------------------------------------------------ 30 

Eurasian watermilfoil______________________________________________ 33 

Horned pondweed ------------------------------------------------- 37 

Southern naiad --------------------------------------------~------ 41 

Naiad ----------------------------~------------------------------ 43 ~uskgrass_______________________________________________________ 44 

Common elodea --------------------------------------------------- 46 

Water-weed------------------------------------------------------ 50 

Importance of submersed aquatic vegetation ------------------------------ 50 

Historic and present distribution ---------------------------------------- 54 

Factors affecting distribution and abundance ------------------------------ 55 

Natural climate effects----------------------------------------~---- 56 

Excessive nutrients ----------------------------------------------- 56 

Turbidity -------------------------------------------------------- 57 

Pollution -------------------------------------------------------- 57 

Overgrazing ----------------------------------------------------- 57 

Selected references for further reading----------------------------------- 58 

Selected references for additional reading--------------------------------- 58 

Table of Contents V 

SUBMERSED AQUATIC PLANTS OF THE 

TIDAL POTOMAC RIVER 

By VIRGINIA CARTER, PATRICIA T. GAMMON, and NANCY C. BARTOW 

INTRODUCTION 

In terms of drainage area and volume of water, the Potomac River is 

the second largest tributary entering the Chesapeake Bay. The 

Potomac River rises in the Appalachian Mountains in West Virginia, 

flows eastward, and forms part of the Maryland-West Virginia border. 

As it flows eastward from Cumberland, Md., it receives the water from 

many small tributaries and several large rivers such as the South 

Branch of the Potomac, the Shenandoah, and the Monocacy. The 

volume is increased as the river flows eastward. At Little Falls, 300 

miles (483 kilometers) from its source and just below the series of 

rapids which mark the fall line, the Potomac River comes under tidal 

influence. It is tidal from just below Little Falls, near Washiniton, 

D.C., to Point Lookout, Md., 114 mi (183 km) downstream, where it 

enters the Chesapeake Bay about 73 mi (117 km) from the Atlantic 

Ocean. The Potomac River watershed has a drainage area of 14,670 

square miles (38,000 square kilometers) including the District of Columbia and parts of Maryland, Virginia, West Virginia, and Pennsylvania. It contributes about 18 percent of the total freshwater inflow 

to the Chesapeake Bay. 

On the basis of salinity, the tidal Potomac River and Estuary can be 

subdivided into thre~ zones. Between Little Falls and Quantico, Va., 

the Potomac is a relatively narrow freshwater tidal river. Downstream 

from Quantico lies the Potomac Estuary where freshwater and 

saltwater mix. In the upstream end of the estuary, between Quantico 

and the Route 301 bridge at Morgantown, Md., the saltwaterfreshwater interface fluctuates in response to changes in river 

discharge. This zone is often termed the transition zone, or middle 

Potomac. The remainder, from the Route 301 bridge to Point Lookout, 

is called the lower estuary, or the lower Potomac. Some publications 

refer to the entire reach between Little Falls and Point Lookout, including the estuary and the tidal river, as the Potomac Estuary. 

Introduction 1 
- Boundary of Potomac River watershed 

- ··- State boundariE>"' 

----Fall line 

Potomac River Watershed. 

The tidal Potomac River is relatively shallow and has an average 

overall depth of 19.7 feet (6 meters) The greatest depth, about 120 ft 

(36.5 m), is found near Mathias Point, Md. The tidal river and estuary 

generally have a deep channel flanked on one or both sides by wide 

shallow flats or shoals. Emergent wetlands (marshes) are found in most 

of the tributaries in narrow bands along the shore and more extensive 

areas where the tributary widens into a tidal embayment before entering the main river. There are few emergent wetlands along the shore of 

the main stem of the Potomac River. Those that do exist are generally 

separated from the river by a narrow sand berm, except during floods. 

In some places, there are narrow bands of threesquare (Scirpus 

americanus), a plant that can withstand wave action well, and, in the 

vicinity of Roosevelt Island, in Washington, D. C., large beds of spatterdock (Nuphar luteum) can be seen. 

2 Submersed Aquatic Plants of the Tidal Potomac River 
38' 

VIRGINIA 

Port Tobacco River 

Mathias Point 

MARYLAND 

Morgantown 

I 

.ower Cedar Point 

Wicomico River 

' Chaptico Bay 

Lower Estuary 

10 15 20MlLES 

10 15 20 KILOMETERS 

Map of tidal Potomac River. 

Beds of submersed aquatic plants are common in most estuaries and 

tidal rivers. Submersed aquatic plants are those which grow primarily 

below the water surface, their upper leaves and flowers sometimes extending above the surface. All of the flowering submersed aquatic 

plants mentioned in this publication are rooted, with the exception of 

coon tail. These plant beds provide habitat, cover, and food for a variety 

of aquatic fauna, and they are also a source of food for migrating 

waterfowl. These plants are often referred to as baygrasses or grasses 

in other publications. 

Introduction 3 
Emergent wetland (tidal marsh) in Mattawoman Creek, Md. 

SURVEY OF SUBMERSED AQUATIC PLANTS 

From 1978 to 1981, the U.S. Geological Survey surveyed the 

submersed aquatic plants of the tidal Potomac River and Estuary, 

sampling the shallow flats on the main river and selected tributaries on 

both sides of the river. The U.S. Fish and Wildlife Service cooperated 

in this survey during 1978 and 1979. This formal sampling program 

was conducted using modified oyster tongs to sample the vegetation 

and the substrate on which the plants were growing. In addition to this 

formal sampling program, many field observations were made of plant 

distribution, time of flowering, and seed production. In the tidal 

Potomac River and Estuary, 15 species of submersed aquatic plants occur- two are algae with stemlike and leaflike structures, and the remainder are flowering plants. 

Pictures of all the plants are shown on the following pages, along 

with a description and a map showing the actual (in red) and potential 

(in yellow) distribution. The actual distribution is based upon the results 

of the 4-year survey. The maps may not show the location of all existing 

beds of vegetation because every mile of river was not surveyed. Potential distribution is a best guess based upon the salinity of the water 

which generally determines the range of a species in an estuary. 

4 Submersed Aquatic Plants of the Tidal Potomac River 
Oyster tongs were used to sample the plants and bottom sediments. 

Survey of Submersed Aquatic Plants 5 
Redhead-grass. 

(Photograph by G. M. Haramis, 

U.S. Fish and Wildlife Service.) 

Horned pondweed growing in a sandy substrate. (Photograph by G. M. Haramis, U.S. 

Fish and Wildlife Service.) 

6 Submersed Aquatic Plants of the Tidal Potomac River 
Wildcelery: Vallisneria americana Michx. 

Wildcelery, also called tapegrass or eelgrass, has long slender ribbonlike leaves which grow from white underground stems and rhizomes. 

The leaves, often more than 3 ft (1 m) in length, are finely veined and 

may have a slightly lighter green central stripe. The rhizomes produce 

vegetative runners, a new plant springing from each node. A small 

tuber, or winter bud, is sometimes found attached to the bottom of the 

stem by a long runner, both of which are deeply buried in the substrate. 

Wildcelery flowers in late July through September in the tidal Potomac 

River. The male flowers are borne on short-stalked spathes at the base 

of the plant. The spathe breaks, allowing the flowers to float to the 

water surface. The female flowers are borne on long stems which reach 

Wildcelery: Vallisneria america1W Michx. 

Survey of Submersed Aquatic Plants 7 
Wildcelery: Note the white underground rhizomes with individual plants arising from 

each node. (Photograph by G. M. Haramis, U.S. Fish and Wildlife Service.) 

A single wildcelery plant 

showing winter bud or tuber. 

8 Submersed Aquatic Plants of the Tidal Potomac River 
Bed of wildcelery with flowers and seed pods. (Photograph by G. M. Haramis, U.S. Fish 

and Wildlife Service.) 

Male flowers of wildcelery. Immature seed pods. 

Survey of Submersed Aquatic Plants 9 
Bed of wildcelery in Port Tobacco River, Md. 

the water surface. After fertilization, these stems become coiled, pulling the flowers beneath the surface where the long cylindrical seed 

pods mature. 

This plant is an excellent food for waterfowl, especially canvasbacks 

and redheads. All parts of the plant are eaten, including roots and 

tuberlike winter buds. The canvasback's fondness for wildcelery 

prompted the duck's scientific name, Athya valisneria. Wildcelery harbors insects and other invertebrate organisms and attracts marsh 

birds, shore birds, and muskrats. Its dense beds provide good shelter 

for fish. 

Wildcelery is one of the most abundant submersed aquatic plants 

found in the tidal Potomac River. It is adapted to a narrow range of 

salinity and occurs only in fresh and slightly brackish (salty) water. 

Eelgrass (Zostera marina), which looks similar, thrives in saltier water 

and is not presently found in the tidal Potomac River. Wildcelery 

grows best in silty sand from just north of Maryland Point to the 

Wicomico River, growing in narrow zones along the shoreline and in 

tributaries such as Upper Machodoc Creek, Nanjemoy Creek, and Port 

Tobacco River. The most extensive growth occurs in the Port Tobacco 

River where beds may cover several thousand square feet (several hundred square meters). 

10 Submersed Aquatic Plants of the Tidal Potomac River 
3Jl;:' 'JOc_' --~--------o--'~''-------.---- -:.::,"',:: JO'~----,--, 

38130' 

38' 

VIRGINIA 

20 MILES 

f---,--J-,--,J.-,..___..L _ __J 

10 15 

10 15 20KILOMETERS 

MARYLAND 

Potential (yellow) and actual (red) distribution of wildcelery. 

Redhead-grass: Potamogeton perfoliatus (Fern.) Farw. 

Redhead-grass is a large vigorously growing plant with broad 

[0.75-1.5 inches (2-4 centimeters)] oval- to lance-shaped leaves occurring alternately along the white or reddish stem. The base of the leaf 

wraps partly around the stem, thus helping to distinguish this pondweed from the curly-leafed pondweed with which it might be confused. 

It has a dense, tangled, and extensive mass of rhizomes which anchors 

it on exposed sites. 

Redhead-grass is common in the shallow fresh to moderately 

brackish water of the tidal Potomac River and Estuary. It occurs abundantly from Maryland Point to the Wicomico River along the shore and 

Survey of Submersed Aquatic Plants 11 
Redhead-grass. 

Redhead-grass in flower. 

(Photograph by G. M. Haramis, 

U.S. Fish and Wildlife Service.) 

12 Submersed Aquatic Plants of the Tidal Potomac River 
in the tributaries. Extensive beds of redhead-grass lie on the Virginia 

side of the transition zone just above the Route 301 bridge and along 

the littoral zone on the Maryland side from the Route 301 bridge to 

Maryland Point. 

Spikes of flowers are located in the leaf axils or at the end of the 

stems. These spikes rise above the water surface for wind pollination; 

they sink below the water as the fruits mature in early to midsummer. 

The blunt-backed pointed-tipped seeds are about 0.1 in (3 mm) long. 

They detach from the plant and eventually sink to the bottom. New 

plants may either grow from the rhizomes of existing plants or germinate from seed. 

3~r'''~·---------.--------~- ·-----------,----------~'~r~-· ---------.-. 

MARYLAND 

38130' 

VIRGINIA 

"' 

10 " ZOMILES 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of redhead-grass. 

Survey of Submersed Aquatic Plants 13 
Redhead-grass flowers and maturing seeds. 

Redhead-grass takes its name from the species of duck that favors it 

as food source: the redhead duck. This species of waterfowl has declined sharply in the Chesapeake Bay recently, and this decline is a 

result of the loss of abundant growth of redhead-grass and other 

baygrasses. Many waterfowl still feed heavily on redhead-grass and the 

invertebrates it harbors during fall migration and the wintering period. 

14 Submersed Aquatic Plants of the Tidal Potomac River 
Redhead-grass seeds. 

Curly pondweed: Potamogeton crispw L. 

Curly pondweed was introduced into North America from Europe at 

least 150 years ago. It resembles redhead-grass but has a slightly flattened stem and elongated flat opposite leaves with toothed wavy 

margins which give its name. The root or rhizome mass is not as large 

as that of redhead-grass. 

Curly pond weed is found in small populations in the fresh and slightly 

brackish waters of the tidal Potomac River and transition zone. It 

forms isolated patches in Nanjemoy Creek and Port Tobacco River, 

growing in sheltered spots that have a silty-clay substrate. Large beds 

are uncommon. 

Survey of Submersed Aquatic Plants 15 
Curly pondweed. 

16 Submersed Aquatic Plants of the Tidal Potomac River 
3; ·~JOc.._ ' -- -----, ---,- -----,------''-'1':::'"-----,---, 

MARYLAND 

VIRGINIA 

38' 

10 15 20MILES 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of curly pondweed. 

The flowers of curly pondweed are borne on spikes at or slightly 

above the water surface in late July or August. The flowering heads 

and fruit of curly pondweed are smaller than those of redhead-grass, 

and there are fewer flowers and seeds on a spike. Hard winter buds, 

called turions, form in the fall and germinate to form new plants in the 

spring. 

Curly pondweed is considered to be a pollution-resistant species in 

the United States and Europe. It provides good shelter for fish and 

other aquatic organisms, especially during the early part of the summer; it dies back in September. It has only marginal food value for 

waterfowl. 

Survey of Submersed Aquatic Plants 17 
Flower of curly pondweed. 

Seeds of curly pondweed. 

18 Submersed Aquatic Plants of the Tidal Potomac River 
Tips of curly pondweed 

bearing winter buds. 

Germinated winter bud of curly pondweed. 

Survey of Submersed Aquatic Plants 19 
Coontail. 

20 Submersed Aquatic Plants of the Tidal Potomac River 
Coontail showing leaf detail. 

Coontail: Ceratophyllum demersum L. 

Coontail, also called hornwort, is an entirely submersed aquatic plant 

that has no true roots, although the basal ends are often buried in mud. 

The forked leaves occur in whorls and have marginal teeth that make 

them rough to the touch. Dense whorls of leaves near stem tips give the 

bushy coon taillike appearance. The leaves and stems may be stiff with 

encrusted lime. The flowers and nutlike seeds of coontail are found 

singly in the leaf axils. 

Coontail has a very localized distribution in the fresh to slightly 

brackish water of the transition zone and its tributaries. Small populations are found in the A quia-Potomac Creek area, and a large bed of 

Survey of Submersed Aqu· tic Plants 21 
Coontail bed in Nanjemoy Creek. 

coontail, Eurasian watermilfoil, and curly pondweed is located in Nanjemoy Creek. Coontail can also be found in small open pockets in the 

marshes in Mattawoman Creek. Coontail is not an important food for 

waterfowl. It rarely flowers or produces seeds and may shade other 

plants more preferred by waterfowl. 

Widgeongrass: Ruppia maritima L. 

Widgeongrass, also called ditch-grass, has narrow threadlike alternate leaves that grow from short zigzag rootstocks or rhizomes that 

lack tubers. When not in seed, widgeongrass resembles sago pondweed 

and horned pondweed, but its leaves are arranged in less bushy 

clusters, and the sheathing base of the leaves has a firm rounded tip. 

Widgeongrass bears its flowers on umbels at the water surface, usually 

in late summer and early fall. Widgeongrass flowers are initially 

enclosed in sheaths at the leaf base; the flower stalks elongate during 

22 Submersed Aquatic Plants of the Tidal Potomac River 
3~~·,~·--------~--------~~77~---------r----------~"'~30_' ---------,-, 

MARYLAND 

38130' 

VIRGINIA 

38' 

10 15 20 MILES 

10 15 20KILOMETERS 

Potential (yellow) and actual (red) distribution of coontail. 

maturation. The oval black fruits are 0.05 to 0.1 in (1.5-3 mm) long 

with a pointed tip. They are widely distributed by the water. 

Widgeongrass tolerates ocean-strength salinity but is most productive in slightly to moderately brackish estuarine waters. It may be 

found throughout the Potomac Estuary below Maryland Point. Despite 

its wide distribution, widgeongrass currently is not very abundant in 

the estuary; the largest beds are found in the Wicomico River. 

Because of its broad distribution in the estuary, widgeongrass is one 

of the most important submersed plants in the Potomac Estuary and in 

the Chesapeake Bay. In the past, widgeongrass was an extremely important waterfowl food, particularly to the American widgeon, or 

baldplate, after which it was named. 

Survey of Submersed l quatic Plants 23 
Widgeongrass. 

Widgeongrass (bottom) and 

horned pondweed (top). 

24 Submersed Aquatic Plants of the Tidal Potomac River 
Widgeongrass plant with male 

and female flowers. 

Mrue flower. 

Survey of Submersed Aquatic Plants 25 
Widgeongrass seeds: Female flower. 

MARYLAND 

VIRGINIA 

36' 

10 15 20MILES 

10 15 20KILOMETERS 

Potential (yellow) and actual (red) distribution of widgeongrass. 

26 Submersed Aquatic Plants of the Tidal Potomac River 
Widgeongrass growing from seed. 

Sago pondweed: Potamogeton pectinatus L. 

Sago pondweed is a vigorous plant with much branched stems and 

numerous threadlike leaves spreading out in fanlike fashion. The leaf 

tips have long tapering points; the length and thickness of the leaves is 

Sago pondweed. 

Survey of Submersed Aquatic Plants 27 
Bushy specimen of sago pondweed in flower. 

28 Submersed Aquatic Plants of the Tidal Potomac River 
,, \ 

Sago pondweed showing bean-shaped tuber. 

quite variable. When not in bloom or in seed, it resembles 

widgeongrass, but its leaves are in bushier clusters, and the rootstocks 

are long and straight and often have tubers. Sago pondweed has a 

pointed tip (or "bayonet") attached to its stipules which also 

distinguishes it from widgeongrass. 

Sago pondweed with seeds. 

Survey of Submersed Aquatic Plants 29 
Close-up showing flowers (right) and seeds (left). 

Sago pondweed blooms early in the growing season. The flowers are 

born like beads on the long slender stalk. The seeds are oval, 0.1 in 

(3 mm) or longer, with a pointed tip. 

Sago pondweed, one of the most desirable species of submersed 

aquatic plants because of its food value to waterfowl, is found in 

isolated populations in the fresh to moderately brackish waters of the 

Potomac Estuary. It grows in the transition zone on both sides of the 

Potomac River and in Nanjemoy Creek and other tributaries. The 

nutlets (seeds) and starchy tubers of sago pondweed make it an excellent and important food for diving ducks. Once widely distributed in 

the tidal Potomac River and Estuary, sago pondweed is not as abundant as it once was. 

Slender pondweed: Potamogeton pusillus L. 

Slender pondweed is entirely submersed with alternate linear grasslike leaves, 0.01 to 0.1 in (0.3-2.4 mm) wide, pointed at the tip. The leaf 

blades are entire rather than toothed, and there are generally small 

glands at the unsheathed base of the leaves. 

30 Submersed Aquatic Plants of the Tidal Potomac River 
MARYLAND 

38130" 

VIRGINIA 

38' 

10 20MlLES 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of sago pondweed. 

Slender pondweed has a relatively spotty distribution in the tidal 

Potomac River; it is seldom very abundant. It is found in fresh and 

slightly brackish water in the transition zone and tributaries-Nanjemoy Creek and Port Tobacco River on the Maryland side and Nomini 

Bay on the Virginia side. It is a good food for waterfowl when found in 

sufficiently large concentrations. 

The flowers are born in whorls in terminal or axillary spikes. The 

seeds are small and rounded on the back. Winter buds are also produced. Slender pondweed can be confused with leafy pondweed 

(Potamogetonfoliosus) which is found in small tributaries emptying into 

the Piscataway Creek. Leafy pondweed lacks the small glands at the 

base of the leaves, and its seeds have an undulate ridge along the back. 

Survey of Submersed Aquatic Plants 31 
Slender pondweed. Slender pondweed. 

Slender pondweed showing seeds. 

32 Submersed Aquatic Plants of the Tidal Potomac River 
3~~·30~·---------,--------~-'T'-----------,----------~"'=----------,-, 

MARYLAND 

38~30' 

VIRGINIA 

"' 

10 15 20 MILES 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of slender pondweed. 

Eurasian watermilfoil: Myriophyllum spicatum L. 

Eurasian watermilfoil was introduced to the United States from Asia 

in the early 1900's. It is easily recognized by its featherlike leaves which 

grow in whorls of four along the stems. It has short, almost woody, 

lower stems with numerous long roots and colonizes mud, sand, and 

detritus substrates. 

Eurasian watermilfoil bears flowers in spikes which rise above the 

water surface in mid- to late summer. The abundant fruits look like 

four joined nutlike bodies. Eurasian watermilfoil is a perennial plant 

that spreads rapidly from small fragments which break off and form 

roots, drifting until stranded. The seed beads also float to new locations, or seeds are eaten and later excreted by waterfowl. 

Survey of Submersed Aquatic Plants 33 
Eurasian watermilfoil. 

Eurasian watermilfoil shoMng leaf detail. 

34 Submersed Aquatic Plants of the Tidal Potomac River 
urasian watermilfoil flowering in Nanjemoy Creek. (Photograph by G. M. Haramis, U.S. 

Fish and Wildlife Service.) 

At present, Eurasian watermilfoil occurs in widely scattered beds in 

the transition zone and tributaries including the Wicomico River. 

However, in the early 1960's, it enjoyed a brief population explosion in 

the tidal Potomac River and its tributaries, blocking many channels 

and backwater areas. It declined naturally and reached an apparently 

stable population level in the late 1960's. The plant provides excellent 

cover for fish fry and crabs, and its seeds may be locally important 

waterfowl food. In some areas, watermilfoil may outcompete more 

desirable waterfowl food plants. 

Survey of Submersed Aquatic Plants 35 
Eurasian watermilfoil bed in Nanjemoy Creek. (Photograph by G. M. Hararnis, U.S. Fish 

and Wildlife Service.) 

36 Submersed Aquatic Plants of the Tidal Potomac River 
3;.r~JO:__' -------,---------,;--'j-77' _____ ---, _ _ _ _ --:.; "'.::."'- ------,---, 

MARYLAND 

38130' 

VIRGINIA 

38' 

10 15 20MILES 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of watermilfoil. 

Horned pondweed: Zannichellia palustris L. 

Horned pondweed is a slender branching plant growing from extensive creeping rootstocks or rhizomes. It has numerous slender vertical 

stems with opposite or nearly opposite narrow threadlike leaves by 

which it may be distinguished from widgeongrass or sago pondweed. 

The base of the leaf is covered by a membraneous sheath or stipule. 

When exposed to wave action, it often forms mats of grasslike prostrate growth. 

Horned pondweed flowers early in the growing season and develops 

conspicuous cresent-shaped horn-like fruits which aid in distinguishing 

it from widgeongrass. It dies back in late July and August in the tidal 

Potomac River. At this time, plants disintegrate, and vegetation and 

Survey of Submersed Aquatic Plants 37 
Horned pondweed, prostrate form. (Photograph by G. M. Haramis, U.S. Fish and Wildlife 

Service.) 

seeds are dispersed throughout the estuary. New plants may be found 

during late September and October. 

Horned pondweed is one of the most widely distributed submersed 

aquatic plants in the tidal Potomac River, growing in fresh to 

moderately saline water along the shore. It can be found up the river as 

far as National Airport in Washington, D.C., and as far down the river 

as the St. Mary's River. It grows in many of the tributaries, including 

the Wicomico River. 

38 Submersed Aquatic Plants of the Tidal Potomac River 
Horned pondweed, upright plant. (Photograph by G. M. Haramis, U.S. Fish and Wildlife 

Service.) 

Survey of Submersed Aquatic Plants 39 
Mass of horned pondweed found floating in Wicomico River. (Photograph by G. M. 

Haramis, U.S. Fish and Wildlife Service.) 

Seeds of horned pondweed. 

40 Submersed Aquatic Plants of the Tidal Potomac River 
3~r~~---------.--------~-'T''-----------.----------~"·F·----------r-o 

MARYLAND 

38'30' 

VIRGINIA 

"' 

10 15 20MILES 

10 15 20KILOMETEFI.S 

Potential (yellow) and actual (red) distribution of horned pondweed. 

Southern naiad: Najas guadalupensis (Spreng.) Magnus 

Southern naiad has coarse roots and slender much branched stems 

with opposite narrow toothed ribbonlike leaves enlarged at the base. 

The flowers and fruits are born singly in the leaf axils with papery walls 

tapered to a short style. Seeds are 0.08 to 0.2 in (2-5 mm) long, round 

in cross section, and tapered at each end. The seeds are coarsely and 

deeply pitted. 

Southern naiad is found in very small populations in the tidal 

Potomac River. It occurs with other species in the fresh to slightly 

brackish water of the Wicomico River. Its stems, foliage, and nutlets 

are eaten by waterfowl. 

Survey of Submersed Aquatic Plants 41 
.... 

Southern naiad. 

Leaf detail of southern naiad. 

42 Submersed Aquatic Plants of the Tidal Potomac River 
3~~~~·---------- --------~~,·~---------,----------~"r·~ --------,-, 

MARYLAND 

38'30' 

VIRGINIA 

,.. 

10 15 2tl MILES 

1 0 15 20 KLOMETERS 

Potential (yellow) and actual (red) distribution of southern naiad. 

Naiad: Najas gracillima Magnus 

This naiad is a very slender and delicate plant. It has much branched 

stems and long straight linear leaves with a sheathing base. The 

enlarged base has relatively long spiny teeth. The seeds are linear and 

slightly cresent shaped with rows of small pits. Naiad has only been 

found in freshwater in the upper reaches of the Mattawoman Creek. 

Survey of Submersed Aquatic Plants 43 
Naiad. 

Naiad showing seed. 

Muskgrass: Chara spp. 

Muskgrasses, sometimes called stoneworts, are actually algae. The 

plant body or stem has alternating short nodes and long internodes 

with whorls of simple undivided branchlets occurring at the nodes. 

44 Submersed Aquatic Plants of the Tidal Potomac River 
3;l·;;'~'-------,----- -;:-.:., 77'-- -------,-- -- -"""'c;'r:"'--. ----------,r-"1 

38~30' 

"' 

MARYLAND 

VIRGINIA 

10 15 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of naiad. 

Muskgrass. 

Muskgrass with filamentous algae 

covering stems and leaves. 

Survey of Submersed Aquatic Plants 45 
~:r·~~·---------.--------~-'T''-----------,----------~"r~·----------r-o 

MARYLAND 

38130' 

VIRGINIA 

38' 

10 15 20 Kl OMETERS 

Potential (yellow) and actual (red) distribution of muskgrass. 

Rhizoids from the base of the stem hold the plant firmly on the 

substrate. Reproduction is by spores rather than flowers and seeds. 

Muskgrasses are bottom-dwelling plants that grow in small populations or isolated clumps in the fresh to slighly brackish water of the 

tidal Potomac River and its tributaries. Significant numbers of plants 

are found in spring in the Wicomico River but disappear in midsummer. 

Common elodea: Elodea canadensis Michx. 

Common elodea is often found in home aquariums; it is only locally 

abundant in the tidal Potomac River. It has branching stems with 

leaves in whorls of two or three at the nodes, and the leaves are closely 

46 Submersed Aquatic Plants of the Tidal Potomac River 
Common elodea. 

Common elodea, leaf detail. 

Survey of Submersed Aquatic Plants 47 
Bed of common elodea in Nomini Bay. (Photograph by G. M. Haramis, U.S. Fish and 

Wildlife Service.) 

Elodea in flower. 

crowded near the tip of the stem. The leaves are veinless and oneribbed, linear, or narrowly oval with blunt tips and no leafstalk, varying 

in width and size at different locations. The leaves have microscopic 

teeth along the margins. 

48 Submersed Aquatic Plants of the Tidal Potomac River 
3Jl~30c_' --------.--------;:-':;-''------,--------'-l"r=-"'-----~ 

MARYLAND 

38~30' 

VIRGINIA 

36' 

10 15 

10 15 20 KILOMETERS 

Potential (yellow) and actual (red) distribution of common elodea. 

Common elodea seldom blooms in our area, reproduces vegetatively, 

and is often found broken loose and free-floating. When it blooms, male 

flowers are on a spathe 0.4 to 0.5 in (11-13 mm) long, and female 

flowers have a long threadlike tube that reaches the surface for pollination. 

Common elodea is found only occasionally growing in the tributaries 

of the tidal Potomac River in fresh to moderately brackish water and 

generally in association with other plants. Large populations are only 

found in extremely sheltered areas with clear water as in N omini Bay 

or the tiny subtributaries, such as those entering Piscataway Creek. It 

is generally a poor food for ducks because it produces little or no seed in 

this area. Elodea can achieve extremely dense growth which provides 

excellent refuge to an abundance of aquatic life. 

Survey of Submersed Aquatic Plants 49 
Water-weed. 

Water-weed: Egeria densa Planch. 

Water-weed, also know as Anacharis, was introduced to this country 

from South America. It is similar to common elodea but is a larger 

coarser plant_ It has branching stems with leaves in whorls of four or 

more at the nodes, closely crowded near the tip of the stems. The leaves 

are linear to narrowly lance shaped with microscopic teeth along the 

margins. Flowers have not been observed in our area. 

Water-weed has been found only in the freshwater tributaries to the 

tidal river, growing in extremely clear flowing water under tidal influence. 

IMPORTANCE OF SUBMERSED AQUATIC VEGETATION 

Submersed aquatic plants are an important component of the aquatic 

environment. In the Chesapeake Bay and the Potomac River, aquatic 

plant communities have many functional values. Growing in beds along 

the shallow flats and margins, the plants create a habitat that provides 

50 Submersed Aquatic Plants of the Tidal Potomac River 
MARYLAND 

VIRGINIA 

38' 

10 15 

10 15 20KILOMETERS 

Potential (yellow) and actual (red) distribution of waterweed. 

food and cover for large numbers of invertebrate species-insect larvae, mollusks, crustaceans, and others-which are eaten by fish, waterfowl, and larger invertebrates. The plants themselves (seeds, leaves, 

stems, tubers, and attached epiphyte populations) are consumed by 

such diverse herbivores as ducks, muskrats, snails, turtles, and fish. 

The vegetation provides valuable spawning sites for some fish and 

refuge for fish fry, many invertebrate species, and juvenile and molting 

crabs. Plant beds also improve water quality by absorbing nutrients 

from the water column, by providing a source of oxygen as a byproduct 

of photosynthesis, and by settling particulate matter. The stems and 

leaves retard current flow, and the roots stabilize the bottom, thus 

slowing erosion and encouraging the accumulation of sediment. In 

Importance of Submersed Aquatic Vegetation 51 
Aquatic plants growing on the shallow margins of the Port Tobacco River, Md. 

(Photograph by G. M. Haramis, U.S. Fish and Wildlife Service.) 

Patches of curly pondweed growing in a shallow bay in Nanjemoy Creek, Md. 

(Photograph by G. M. Haramis, U.S. Fish and Wildlife Service.) 

52 Submersed Aquatic Plants of the Tidal Potomac River 
Wildcelery plant parts eaten by waterfowl. (Photograph by M. C. Perry, U.S. Fish and 

Wildlife Service.) 

Canvasbacks on Chesapeake Bay. (Photograph by G. M. Haramis, U.S. Fish and Wildlife 

Service.) 

turn, plants provide a valuable source of detritus that serves as food for 

numerous organisms in the detritus-based food chain of the estuary and 

coastal zone. 

Importance of Submersed Aquatic Vegetation 53 
HISTORIC AND PRESENT DISTRIBUTION 

Historically, the Potomac Estuary and tidal river contained an abundance of submersed aquatic vegetation. A map of the river at low tide, 

VIRGINIA 

Submersed 

.Aquatic 

- Vegetation 

Emergent Plants 

MARYLAND 

1STA1'UTE MILES 

0 1 2 3 

I I ', ,' ' ', 

012345 

KILOMETERS 

Upper part of the tidal Potomac River at low water showing distribution of aquatic 

vegetation (reproduced from Cumming and others, 1916). 

54 Submersed Aquatic Plants of the Tidal Potomac River 
published in 1916, shows a narrow channel and wide shallow vegetated 

margins and flats containing curly pondweed, coontail, and wildcelery. 

The transition zone and lower estuary were also well vegetated; 

eelgrass (Zostera marina) beds were found in shoal zones as far up the 

river as Cobb Island at the mouth of the Wicomico River. 

Today, large beds of submerged aquatic plants occur primarily in the 

transition zone. There is almost no submersed vegetation in the 

freshwater tidal river, except for one large bed of wild celery in the ship 

channel behind Haines Point in Washington, D.C., and a bed of 

wildcelery, slender pondweed, and naiad in Mattawoman Creek 5 km 

upstream of the tributary mouth. There are small healthy beds of 

elodea, water-weed, and curly pondweed in tiny tributaries emptying 

into the Potomac River, and there is a large stand of horned pondweed 

in the tidal pond behind National Airport. Broad expanses of the lower 

estuary are completely unvegetated, and eelgrass no longer occurs in 

the Potomac River. 

FACTORS AFFECTING DISTRIBUTION AND ABUNDANCE 

Submersed aquatic plants occur where the physical and biological environment is favorable to their growth and reproduction. Each species 

has its own requirements for and tolerances to salinity, temperature, 

light, substrate type, water velocity, and other factors. In addition, 

submersed aquatic plants must compete for light, space, and nutrients 

in the aquatic system. Plant populations in the Potomac River and the 

Chesapeake Bay vary in size naturally from year to year, depending 

upon the water temperature, the salinity, the amount of seed set the 

previous year, and the successful germination of seeds. Distribution of 

annual plants, such as horned pondweed, may fluctuate greatly if 

storms scour the bottom and wash away the seeds. Large beds of 

perennial plants such as wildcelery and sago pondweed growing from 

rhizomes and tubers may reappear in the same location year after year. 

A general decline in populations of submersed aquatic plants in the 

Chesapeake Bay has been noticeable in recent years. However, the 

plants in much of the tidal Potomac River and Estuary disappeared in 

the 1920's and 1930's and have not reappeared. By 1939, the loss of 

plants from the tidal river and the lack of eelgrass in the lower estuary 

had been noticed and documented by scientists interested in waterfowl 

food. Various theories have been suggested to account for the disappearance of submersed aquatic plants in the tidal river and lower 

estuary and for the decline throughout the Chesapeake Bay, but sound 

scientific support for these theories has been generally lacking. The 

factors which caused the disappearance of these plants from the tidal 

river may or may not be operating today. Natural stresses such as 

Factors Affecting Distribution and Abundance 55 
overgrazing by animals or climatic changes and manmade stresses, 

such as pollution, could act together or separately to affect plant survival. 

NATURAL CLIMATE EFFECTS 

Severe storms, such as Hurricane Agnes in 1972 and the March 

storm of 1936, caused by early snowmelt and heavy precipitation may 

have scoured the sediment from some parts of the river or deposited 

large quantities of sediment over the vegetation beds, as happened in 

the Susquehanna Flats area of the Chesapeake Bay after Hurricane 

Agnes. However, plant populations are generally fairly resilient, and 

recolonization, although it might take several years, should be expected 

to occur. Warming trends and disease have been suggested as possible 

causes for the eelgrass disappearance in the Potomac River during the 

1930's. 

EXCESSIVE NUTRIENTS 

Decline in submersed aquatic vegetation may be associated with the 

long-term increase in nutrient enrichment from metropolitan wastes, 

agricultural runoff, and other sources. The introduction of nutrients 

may have changed the balance between phytoplankton and aquatic 

plant growth and favored a major ecological shift in primary productivity. Scientists have shown that increased nutrient loads cause heavy 

epiphyte and filamentous algae growths that shade out aquatic plants. 

An initial decline in plant abundance followed by continued shading 

Epiphytes growing on submersed aquatic plants in the tidal Potomac River. (Photograph 

by G. M. Haramis, U.S. Fish and Wildlife Service.) 

56 Submersed Aquatic Plants of the Tidal Potomac River 
from phytoplankton may greatly suppress or eliminate the aquatic 

plant populations. Fluctuations of salinity in the transition zone may 

limit the growth of epiphytes and algae and allow the submersed 

aquatic plants to survive, even if nutrient concentrations are high. 

TURBIDITY 

Increasing turbidity from suspended sediment or algae causes a 

reduction in the light available for photosynthesis and generally acts to 

confine plant growth to shallower water. Many scientists believe that 

turbidity has been a factor suppressing plant growth in the tidal 

Potomac River. At present, there is no evidence that turbidity is higher 

today than in the early 1900's or that turbidity was responsible for the 

disappearance of plants from the tidal river. Turbidities in the transition zone where plants are abundant are very similar to those in the 

tidal river. Observations have shown that plants presently existing in 

the Potomac River are those that grow to or near the water surface, 

thus avoiding the effects of turbidity. 

POLLUTION 

Submersed aquatic plants may be sensitive to increased loads of 

pollutants such as chlorine, herbicides, or such natural substances as 

sediment. It has been suggested that chlorine from sewage and watertreatment facilities and powerplants could have an adverse affect upon 

aquatic plants. The Ecological Services Laboratory of the National 

Park Service has found that elodea, wildcelery, and other species are 

adversely affected by chlorine concentrations at levels near those 

presently found in the tidal river. It is not clear, however, that chlorine 

caused the original disappearance of the submersed vegetation, nor is 

there sufficient evidence to state that its presence prevents the 

reestablishment of the plants. Herbicide usage has increased in the entire Chesapeake Bay area, and herbicides may adhere to silt particles 

and be washed into adjacent streams. Laboratory and field studies 

would be needed to establish whether concentrations of herbicides in 

bottom sediments or water are sufficient to affect the aquatic vegetation . .The same is true for heavy metals and chemicals which are both 

present in the sediment and water of the Potomac River. The impact of 

oil and oil-contaminated sediments on submersed aquatic vegetation is 

not known, and the concentrations of heavy metals in water seem to be 

lower than those found to affect the growth of aquatic plants. 

OVERGRAZING 

Carp, cownose rays, waterfowl, and muskrat have all been accused of 

destroying or damaging aquatic plant beds. Plants transplanted to the 

Factors Affecting Distribution and Abundance 57 
tidal river in Washington, D.C., and Maryland survive if wire cages or 

exclosures are placed around them; they are cut off or uprooted when 

unprotected. It is possible that predation or overgrazing is preventing 

the recolonization of the spallow flats in the reach from Chain Bridge to 

Quantico. 

SELECTED REFERENCES FOR ADDITIONAL READING 

Bartsch, A. F., 1954, Bottom and plankton conditions in the Potomac River in the Washington 

Metropolitan area-Appendix A, in A report on water pollution in the Washington 

Metropolitan area: Inter-State Commission on the Potomac River Basin, 57 p. 

Heal, Ernest, 1977, A manual of marsh and aquatic vascular plants of North Carolina, with 

Habitat data: The North Carolina Agricultural Experiment Station, 298 p. 

Carter, Virginia and Harris, G. M., 1980, Distribution and abundance of submersed aquatic 

vegetation in the tidal Potomac River-Implications for waterfowl: Atlantic Naturalist, 

v. 33, p. 14-19. 

Cummings, H. S., Purdy, W. C., and Ritter, H. P., 1916, Investigations of the pollution and 

sanitary conditions of the Potomac watershed: Treasury Department, U.S. Public Health 

Service Hygienic Laboratory Bulletin, no. 104, 231 p. 

Hotchkiss, Neil, 1967, Underwater and floating-leaved plants of the United States and 

Canada: U.S. Fish and Wildlife Service Resource Publication 44, Washington, D.C., 124 

p. 

Linduska, J. P., 1959, Waterfowl and the Potomac River: Maryland Conservationist, v. 36, 

no. 6, p. 4-8. 

Lippson, A. J., and others, 1979, Environmental Atlas of the Potomac Estuary: Martin 

Marietta Corp., Baltimore, Md., 279 p. 

Martin, A. C., and Uhler, F. M., 1939, Food of game ducks in the United States and Canada: 

U.S. Department of Agriculture Technical Bulletin 634, Washington, D.C., 308 p. 

Orth, R. J., Moore, K. A., and Gordon, H. H., 1979, Distribution and abundance of 

submerged aquatic vegetation in the lower Chesapeake Bay, Virginia: Environmental Protection Agency Report Number 600/8-79-029/SAV 1, Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C., 198 p. 

Stevenson, J. C., and Confer, N. M., 1978, Summary of available information on Chesapeake 

Bay submerged vegetation: U.S. Fish and Wildlife Service, Office of Biological Services, 

FWS/OBS-78/66, 335 p. 

Stevenson, J. C., Confer, Nedra, and Pieper, C. B., 1979, Decline of submerged aquatic 

plants in Chesapeake Bay: U.S. Fish and Wildlife Service, Oflice of Biological Services, 

FWS/OBS-79/24, 12 p. 

Stewart, R. E., 1962, Waterfowl populations in the upper Chesapeake region: U.S. Fish and 

Wildlife Service Special Scientific Report on Wildlife, no. 65, 208 p. 

Wood, R. D., 1967, Charophytes of North America: Stella's Printing, West Kingston, R.I., 

72 p. 

58 Submersed Aquatic Plants of the Tidal Potomac River 

)~ U.S. GOVEl{Ni'lENT PRINTING OFFICE: 1984-421-594:405