Decline of Submerged Plants in Chesapeake Bay
This report was
by J. Court Stevenson,
Catherine B. Piper
and Nedra Confer
Credit: Linda Hurley,
Wild celery grass
Since the early 1970s, many people interested in Chesapeake
Bay—waterfowl hunters, fishermen, oystermen, ecologists, researchers,
and waterfront residents have been concerned about the apparent decline
of aquatic grass beds in the Bay. Many people fear that this decline win
force, or has already forced, some species of waterfowl to change their
feeding habits and to move out of this region in search of more suitable
habitats. Others speculate that the decline of theft aquatic plants is
an indication of serious deterioration in the “health” of the Chesapeake.
Still others feel that the loss of the grasses will affect commercial
fisheries in the Bay. In response to. these concerns, "Summary of Available
Information on Chesapeake Bay Submerged Vegetation" was compiled in August
1978 by the University of Maryland Laboratory Center for Environmental
Estuarine Studies at Horn Point, Maryland. This brochure features highlights
from that more comprehensive document.
What Plants Make up Bay Grasses?
Although not true members of the grass family, submerged
aquatic plants are often referred to as “grasses.” Common names such as
eelgrass and widgeongrass have evolved due to their resemblance to true
Eurasian watermilfoil, with its feathery leaves, is probably
the most ubiquitous species, due to its perplexing invasion of the Bay
in the 1950s and early 1960s. From 1960s to 1961, it was estimated that
the area covered by milfoil doubled, from 50,000 to 100,000 acres.
Two pondweed species are common inhabitants of the Chesapeake
Bay. Redhead grass has distinctive oval-shaped leaves that clasp the plant
stem at the leaf base. Sago pondweed, though a relative of redhead grass,
has long, tapering leaves and closely resembles widgeongrass.
Eelgrass is a common inhabitant of the lower Bay, where
salinity is high. it has long, narrow, ribbon-like leaves. Wild celery
is similar to eelgrass, but is found in fresh to slightly brackish waters
and has a light-colored center stripe when held to the light. Horned pondweed
is abundant in late spring and again in late summer, producing seeds twice
a year. its leaves are narrow, fairly short, and rise in pairs from each
joint of the stem. Waterweed has narrow oval leaves that are arranged
by two's and three's at the joints of the stem. This species is familiar
to many people, since it is commonly sold for aquariums. Naiads (eg.,
bushy pondweed) are common in the Susquehanna Flats and other portions
of the Bay with low salinity. They vary in appearance, but tend to have
fully branching stems with short, slender, oval leaves. Coontail is usually
found only in fresh water at the head of the Bay and its tributaries.
Muskgrass (Chara spp.), not illustrated, is an
alga, but is included among the Bay grasses because of its importance
as a food source for waterfowl and its physical resemblance to other grasses.
Muskgrass is commonly found in fresh and brackish areas of the Bay, and
is often characterized by a pungent, skunk-like odor.
Why are Bay Grasses Important?
Primary Production. Aquatic grasses are important primary
producers. Using carbon dioxide and inorganic nutrients as raw materials,
they can transform the sun's energy into carbohydrates and proteins. This
process, called primary production, is of critical importance, since it
forms the basis for food webs in the Bay. Some ducks, fishes, shrimp,
and snails graze directly on the living grasses, while other animals (eg.,
clams and oysters), filter bacteria-laden detritus (dead plant tissue)
from the water to obtain nutrients. It appears that primary production
in the Bay ecosystem has been drastically reduced in recent years because
of the loss of submerged aquatic vegetation.
Relative organic production by submerged aquatic plants
and phytoplankton in the Chesapeake Bay before and after the decline of
the 1970s is shown in figure 2, along with the recent production of North
Carolina sounds. At present, the amount of carbon (a measure of primary
production) produced by the grasses is relatively small—only 6% of the
combined production by submerged aquatic vegetation and phytoplankton.
In 1963, when the grasses were more wide-spread, they accounted for over
40% of this total. in contrast, researchers estimate that submerged aquatic
plants are responsible for 70% of the primary production in the estuarine
sounds of North Carolina, where grass production has not appeared to decline.
The change in the ratio between the grasses and phytoplankton
may affect the fish species present in Chesapeake Bay. Fishes that feed
on phytoplankton, such as the adult Atlantic menhaden (Brevoortia tyrannus),
have been harvested in increasing numbers. Bluefish, which feed heavily
on forage fishes, including menhaden, have also increased in numbers.
Large invasions of bluefish may eventually out-compete species such as
rockfish (striped bass), whose numbers have been declining in the Bay.
increased catches of menhaden and bluefish have not economically offset
the reduced harvest of rockfish, shad, and herring. The economic effects
of grass decline may become even greater in the future if the populations
of commercially valuable fishes continue to be affected.
provide important habitats for many animal species. Investigators at
the Virginia Institute of Marine Science found up to 33,000 animals
among the submerged aquatic vegetation in lower Chesapeake Bay. In experiments
when artificial grasses were substituted for living plants, the animal
community which developed on the leaves was similar to that found in
living grass beds, indicating that these particular animals depended
on the grasses for the habitat it offered.
The variety of structure that grass beds offer, compared
to unvegetated bottom, provides estuarine-spawning fishes (e.g., shad,
herring, and rockfish), and their offspring with protection from predators.
The shelter provided by the submerged aquatic plants may also be important
to blue crabs that are especially vulnerable to predators during molting
because of their soft shells and sluggish activity.
By baffling currents and reducing wave action, grass beds reduce the
velocity of water flow and cause suspended sediments to settle out of
the water, reducing turbidity. The growth of the grasses is enhanced
both by lower levels of turbidity and by the accumulation of rich organic
sediment. Conversely, a loss of aquatic grasses increases the amount
of turbidity in the estuary. This is similar to the dust bowl effect
which occurred when vegetation was lost in the midwest prairie regions
in the 1930s. As vegetation is lost, the dust (or turbidity, in this
case), increases, which then buries the remaining plants and cuts down
light needed for growth, which in turn results in greater losses of
Submerged aquatic plants can affect the water quality of Chesapeake
Bay by using dissolved nitrogen and phosphorus for their growth. By
withdrawing the nutrients from the water, they make them unavailable
for use by algae, which often reach pea-soup concentrations in summer
in rivers that flow into the Bay. The grasses then convert these nutrients
into plant tissue, which eventually is incorporated into Chesapeake
Bay food webs by animals that consume live plants or detritus. The grasses
thus act as a “nutrient pump,” recycling nitrogen and phosphorus from
the sediments to the Bay and the animals in it. When a large portion
of the Bay bottom is covered with grass beds, this process may simultaneously
increase the Bay's productivity and decrease nuisance levels of algae.
Some researchers speculate that submerged aquatic plants further diminish
algal populations by secreting chemical inhibitors into the water. It
appears that submerged grasses may have been subtly controlling the
algae—both by competition for nutrients and by chemical inhibition.
In previous times, the colonists
used estuarine grasses as mattress filling, bedding for domestic animals,
cattle forage, insulation, fertilizer, mulch, and fuel. Even today,
the seeds of eelgrass are harvested from the Gulf of California by the
Seri Indians and used to prepare a gruel. Several species of submerged
aquatic plants might serve as important renewable natural resources.
Decline of the Bay Grasses
The U.S. Fish and Wildlife Service Migratory Bird and
Habitat Research Laboratory (MBHRL) and the Maryland Department of Natural
Resources have monitored the occurrence of aquatic grasses in Maryland
waters from 1971 to the present. These surveys constitute the main body
of information on Chesapeake Bay grasses. By combining data from over
600 sampling stations in 26 areas, they found that the percent of stations
with grasses decreased from about 28% at the start of the survey in 1971
to about 10% in 1978.
Although no comparable survey has been conducted in Virginia,
spot measurements of submerged grass beds by the Virginia Institute of
Marine Science reflect a similar decline.
Some areas around the Chesapeake Bay show dramatic evidence
of this decline (1971, 1972,
At the head of the Bay where the Susquehanna River enters, the lush grasses
that long provided a prime feeding ground for waterfowl virtually disappearcd
after 1971. In the nearby Sassafras River, seven species of grasses were
documented in the late 1960s, but only two have been found so far in the
1970s. Further down the Bay, the Patuxent River supported at least eight
species in the 1960s, but only four have been found in the 1970s. Around
Curtis and Cove Points near Calvert Cliffs, four species were documented
from 1930 to 1970, but survey teams found no grasses from 1971 to 1976.
The Choptank River on the Eastern Shore had only one station with grasses
in 1975. Before and after 1975, there were several grass beds west of
the town of Secretary to the mouth of the river. In the 1960's surveys
by the Maryland Department of Natural Resources reported many grass beds
upriver from this point, but these no longer exist. At Bloodsworth and
South Marsh islands, near the Virginia border, submerged aquatic vegetation
was severely affected in 1976, when many other areas were improving. Estimates
of total coverage of submerged grasses in the York River, Virginia, show
a decrease of 865 acres from 1971 to 1974. During the same period of time,
grass coverage in the Rappahannock River decreased from 1,730 acres to
about 10 acres.
These surveys (along with accounts of naturalists and
watermen), clearly indicate that there has been a Bay-wide decline in
grasses. Furthermore, there is evidence that all species have declined
rather than a single species, with the exception of Eurasian watermilfoil
which has increased in nuisance proportions. Although eelgrass alone declined
in the 1930s, (possibly due to a disease), it is improbable that ten species
would decline simultaneously due to any natural cause. The only other
comparable accounts in the scientific literature of large-scale grass
losses involve stresses imposed on the system. These stresses involve
pollution by man, salinity, or substrate modifications.
Why are Bay Grasses Declining?
Various factors have been suspect in the decline of Bay
grasses, but little direct scientific evidence is available. With present
knowledge, hypotheses can be formulated, but no one cause can be definitely
proven. The first issue of importance is whether natural or man-induced
perturbations are more likely to have caused the loss of these plants.
Among the most likely natural stresses on the grass system
are overgrazing by animals, the effects of Hurricane Agnes, warming trends
of Bay waters, and natural diseases. Man-induced stresses include the
introduction of pollutants into Bay waters.
Overgrazing by animals.
European carp, cownose rays, and mute swans have all been accused of
damaging aquatic plant beds. Carp and rays uproot plants in search of
hard-shelled mollusks and crustaceans. The mute swan consumes the plants
at an estimated rate of10 pounds per day. Although the feeding habits
of fish and waterfowl are responsible for localized reductions of aquatic
vegetation, it is unlikely that they are the causeof Bay-wide grass
declines. Carp and rays are not new to the Bay and their populations
have shown no noticeable increases in the 1970s. The mute swan, although
a newcomer, is limited mainly to Eastern Bay and to the Little Choptank,
Choptank and Chester rivers. Considering this localized population (between
200 and 300 birds-June 1977 count by the Maryland Wildlife Administration)
and evidence that Eastern Bay and Chester River submerged grasses are
luxuriant compared to many other parts of the Bay, it is unlikely that
swans (or any other grazers) represent a threat serious enough to decimate
the submerged aquatic plants throughout the Bay.
The force with which Hurricane Agnes hit the Bay in June 1972 was comparable
to the record storm that occurred in August 1933. In both instances,
extensive damge resulted from heavy rainfall with subsequent drastic
lowering of the salinity of the Bay. Further monitoring in the 1970s
indicated depressed salinity levels (due to a variety of reasons besides
Agnes), even as late as 1976. However, most of the Bay grasses can survive
lowered salinities and in some cases, germination and growth rates actually
increase. For example, following the 1933 storm, which had more severe
winds and wave action than Agnes, vegetation was reported to have returned
to prior levels within two or three years. Under normal conditions,
the same response would have been expected after Agnes, since bodies
of water such as the Chesapeake Bay are naturally resilient to storms.
However, the expected recovery did not occur. The submerged grasses
are still significantly reduced compared with the pre-Agnes level (fig.
4). The only long-term effect of Agnes was noticeable in the Susquehanna
Flats area where large quantities of sediment buried the submerged grasses.
After 6 years, however, the lack of grass recolonization in this area
raises suspicion that other factors have prevented the retum of the
There is evidence that eelgrass declined along the North American and
Danish coasts in the 193Os when water temperatures increased over several
years. A similar hypothesis has been proposed in explaining the recent
eelgrass declines in the lower portion of the Chesapeake Bay. Since
the Chesapeake Bay is close to the southernmost extent of the eelgrass
range, temperature fluctuations, especially warm winters, might adversely
affect its growth and reproduction. Water temperatures recorded at Solomons
and Baltimore, Maryland, indicate less significant warming trends in
the late 1970s than those which occurred in the 1930s. The relatively
cold winters of 1977 and 1978 have not been accompanied by increases
in the submerged aquatic vegetation; instead, further decreases have
occurred (fig. 4). In addition, the ten species in the upper Bay which
declined in the 1970s all have ranges extending farther south. So warmer
temperatures, which may have contributed to the eelgrass decline, cannot
be regarded as a leading factor in the overall decline of the grasses
in Chesapeake Bay.
Upon examining dead plants after a conspicuous die-off of submerged
aquatic grasses, plant pathologists have isolated a number of pathogenic
organisms (em.e., bacteria, fungi, and viruses). The slime mold, Labyrinthula,
was often cited as the reason for the eelgrass decline in the 1930s,
but subsequent attempts to infect healthy plants with the isolated pathogen
were generally unsuccessful, suggesting that this organism was only
a secondary cause of infections. Researchers in Australia found that
Labyrinthula was normally associated with healthy eelgrass as
well as with 'diseased' plants. Therefore, disease seems an unlikely
explaination in the Chesapeake Bay decline—especially since a very
wide spectrum pathogen would be necessary to attack ten different species
of submerged grasses.
Pollution—point and nonpointsources
Because the grasses grow near shore in shallow water,
they are especially vulnerable to pollutants
from adjacent land areas. Pollutant sources can be classified into two
general categories: point sources, involving discharge from specific outlets
(usually industrial or municipal sewage treatment plants); and nonpoint
sources such as failing septic systems, runoff from agricultural lands
(including sediment, fertilizer, and herbicides), or runoff from urban-suburban
Point-source pollutants are most concentrated around
large cities associated with manufacturing and include nutrients from
sewage, chlorine (used to control pathogenic organisms), heavy metals,
and organic toxins, (e.g., kepone). To determine whether point sources
were more important than nonpoint sources in the decline of submerged
aquatic plants, the locations of submerged grasses in 1971 to 1976 were
plotted on maps to detect whether the losses were in progressively radiating
concentric rings around major pointsource epicenters. The analysis revealed
no obvious pattern. Furthermore, Bay grass populations were intact in
the Patapsco River downstream from Baltimore's large industrial center
at Sparrows Point. Oyster-producing areas in the Chester River affected
by industrial discharge had healthy grass populations. It is more probable
that the overall grass decline is attributable to numerous nonpoint sources,
rather than to individual point sources. Chlorine represents a special
case which involves both nonpoint and point sources.
Chlorine enters Chesapeake Bay from sewage and water-treatment
facilities, electric power plants, and, sometimes, from nonpoint sources,
(bound with agricultural pesticide runoff). Based on data from Martin
Marietta Corporation, approximately 29.2 million pounds of chlorine entered
Chesapeake Bay in 1973. It is estimated that at least 3% of this amount
could produce persistent by-products—chlorinated hydrocarbons. These
substances, in concentrations of less than one part per billion, are known
to kill shrimp. Substantial yearly increases in chlorine loading correlate
with the grass declines from 1971 to the present. Recent work by the Ecological
Services Laboratory of the National Park Service indicates that elodea,
wildcelery, and sago pondweed show symptoms of growth retardation, loss
of chlorophyll, and collapse when grown in tanks with total chlorine levels
ranging from 0.05 to 0.125 parts per million, levels which are commonly
found in the upper Potomac River. Until further research has been done,
the possibility that chlorine is a causal factor in the Bay-wide decline
cannot be discounted.
Increasing levels of turbidity in shallow waters have
long been regarded as a factor inhibiting growth of submerged aquatic
plants. Turbidity negatively affects plant growth through the reduction
of light. Based on field data collected in the plant survey from 1972
to 1978, there is no indication that turbidities have increased significantly
in the upper Bay. Some areas, such as the Honga, Manokin, Severn, and
Patapsco Rivers, show increases in turbidity over the 5 years covered
by the survey. Other areas, for example, the Chester, Choptank, and Nanticoke
rivers and Susquehanna Flats show turbidity decreases. Maryland Water
Resources data for 1968 Ito 1976 corroborate turbidity decreases in the
Chester, Nanticoke, and Choptank rivers. Even though decreases in turbidity
have been documented in these watersheds, all rivers have experienced
a decline in their grass populations during the 1970s. Since reduced light
is a fundamental problem for submerged grasses, turbidity is probably
an important contributing factor in the plant loss in some areas.
The upper Potomac River provides
an example of the effects of nutrient loading on a river. Since shortly
after the turn of the century, water quality in the upper Potomac River
has been continually degraded by untreated or partially treated municipal
wastewater associated with increasing population in the Washington metropolitan
area. Wastewater discharges resulted in high levels of phosphorus and
nitrogen, and low levels of dissolved oxygen in the river, promoting the
growth of large nuisance populations of blue-green algae that produce
unpleasant odors and affect the recreational uses of the area.
In the 1920s, the Potomac River, as well as other areas
in Chesapeake Bay, was infested with an undesirable plant, water chestnut
(Trapa natans). During the 1950s, the Potomac was overrun by dense
beds of Eurasian watermilfoil. Heavy growths of water chestnut and milfoil
competed with other aquatic plants for light and nutrients and often crowded
out other plant species. Since 1960, the upper Potomac River has been
experiencing massive summer blooms of algae (Anacystis), promoted
by high levels of phosphorus and nitrogen. Excessive blooms effectively
shade out submerged grasses, and may be part of the reason why the upper
Potomac River no longer supports dense grass beds.
Herbicide use has increased dramatically in the Bay area
as land use patterns and agricultural practices have changed. Herbicides
can adhere to soil particles which may then be washed into nearby streams
during spring and summer thunderstorms. Herbicides either become dissolved
in water or adsorbed onto bottom sediments, becoming available for uptake
by submerged grasses. Since herbicides are specifically designed to kill
a broad spectrum of plant species, they might be expected to have an adverse
effect on aquatic grasses. Laboratory studies concerning herbicide effects
upon redhead grass indicate that one in ten parts per million of the herbicide
atrazine can be toxic. Further conclusions cannot be drawn until an extensive
survey of atrazine and other herbicides in the Bay is conducted.
Petrochemicals enter Chesapeake Bay from tankers, refineries,
municipal and industrial effluents, boats, and urban and land runoff.
Approximately 47% of this amount comes from urban and river sources; municipal
and industrial sources account for about 44%. The remaining petrochemical
inputs come from oil spills, ship-generated wastes, and boats.
The impacts of oil and oil-contaminated sediments on
submerged vegetation are largely unknown. Oil may coat plants, blocking
nutrient assimilation and gas exchange. The relatively low incidence of
oil pollution in Chesapeake Bay would suggest that it is not a major factor
in the recent demise of the submerged grasses, however, more information
is necessary about the biological impact of petrochemicals in an estuarine
Dredging and boat traffic.
Dredging, either for the purpose
of harvesting clams or increasing channel depths, is known to physically
disrupt or destroy existing grass beds. The immediate physical effects
are generally confined to the area of dredging, but downcurrent turbidity
and disposal of spoil in open water reduce the light needed by the grasses
for growth. Boat traffic can also constitute a significant threat to aquatic
grasses. In shallow areas, boat propellers physically damage plants and
disrupt bottom sediments, causing turbidity. Excessive boat traffic may
be involved in grass declines, but not all areas that are showing declines
are popular with boaters. It is unlikely that this particular activity
is responsible for Bay-wide vegetation losses, although it may contribute
to the problem in isolated areas.
Future prospects for Bay grasses
The future status of submerged aquatic plants in Chesapeake
Bay has been given a high priority by the EPA Cheapeake Bay Program. Accordingly,
a number of studies have been funded to seek answers for the recent decline
of the Bay grasses.
As results of these studies become available, the interrelationships
among water quality, the grasses and other aspects of the Bay ecosystems
may be determined.
The submerged grasses are credited with a key role in
maintaining the health of the Bay. A better understanding of this role
through research and protection of existing grasses through management,
are vital to ensuring that the Chesapeake Bay ecosystems will continute
to provide man with the benefits which he has enjoyed in the past.