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ОглавлениеChapter 1. Evolution and Ecology
EVOLUTION AND FORM OF AQUATIC PLANTS
It’s not always easy to decide what to include in a list of aquatic plants. We treat 194 macrophytes (large plants) here, including 178 flowering plants, 2 horsetails, 2 ferns, 5 quillworts, 3 mosses, 2 liverworts, and 2 Charophyte algae. Phytoplankton and filamentous algae are not included. Macrophytes range from plants that grow with their roots under water but most of their stems, leaves, flowers, and fruits above the water surface—the emergent flora—to species that are completely submergent.
The difficulty in choosing what to include arises because of the flexibility of some species, which can grow as emergents for part of the time, but survive long periods without standing water. In selecting the species to include it was our intention to focus on plants that typically grow completely beneath the water surface, floating on water, or with at least their roots in standing water. This does not exclude occasional periods of low water when plants that are normally aquatic may be stranded on a muddy shore.
Most of the aquatic plants treated in this book are flowering plants. They reproduce sexually to produce seeds which are contained in a matured ovary (fruit). These plants also have specialized vascular tissues which make possible the distribution of water and dissolved nutrients throughout the plant body, along with an external cuticle to retain moisture. These are features that allowed plants to expand from their aquatic origins to colonize land early in their evolutionary history. Algae have continued to diversify in aquatic environments including both fresh and salt water, and, in fact, dominate some aquatic habitats.
Continuing evolution of the land flora resulted in secondary adaptation to aquatic environments. This “return to water” has occurred in all major groups of land plants: bryophytes, quillworts, horsetails, ferns, and flowering plants (see Figure 1.1 on page 2). Thirty-six families of flowering plants are represented in this book, including members of the basal angiosperms, monocots, and dicots (Table 1.1).
Table 1.1. Representation of Land Plant Lineages in the Aquatic Flora of Pennsylvania (see Appendix for a full list of species)
| TAXONOMIC GROUP | FAMILIES | SPECIES |
| Bryophytes | ||
| liverworts | 1 | 2 |
| mosses | 3 | 3 |
| Quillworts | 1 | 5 |
| Horsetails | 1 | 2 |
| Ferns | 2 | 2 |
| Flowering plants | ||
| basal angiosperms | 2 | 6 |
| monocots | 14 | 110 |
| dicots | 20 | 62 |
Figure 1.1. Evolutionary relationships of major groups of aquatic plants.
Plants have evolved a variety of growth forms to take advantage of the full range of aquatic habitats. Emergent plants occupy lake and stream margins where their roots can be under water but the stems and leaves are largely above the surface. Rooted plants with floating leaves such as water-lilies (Nymphaea and Nuphar spp.) and watershield (Brasenia) are limited to water depths to about 1.5–2 m. The leaf blades are attached by long petioles to rhizomes imbedded in lake or streambed sediments.
Free-floating plants such as the duckweeds (Lemna spp.), watermeals (Wolffia spp.), and water flaxseed (Spirodela punctata) are independent of water depth, but winds and waves usually push them toward the lake or stream margins except on small ponds, where they may cover the entire surface.
Rooted submergent species such as waterweed (Elodea spp.) are limited to depths where light penetration is sufficient to support photosynthesis. This can vary from less than 1 m in very turbid water to 3–4 m or more in exceptionally clear lakes. Other submergent species such as Eurasian water-milfoil (Myriophyllum spicatum), hydrilla (Hydrilla verticillata), and many pondweeds (Potamogeton spp.) typically produce a long stem that only branches when it approaches the water surface. It has been shown that low light promotes shoot elongation and inhibits branching. High light availability has the opposite effect, inhibiting further increase in stem length and stimulating branching. The result is to place the bulk of the leaves just below the water surface where photosynthesis can be most efficient.
Modifications for Life Under Water
An aqueous environment poses challenges different from those encountered on land. Water is denser than air and provides support through greater buoyancy. Consequently, submergent and floating aquatic plants have less need for tissues that provide the stiffening that allows terrestrial species to stand erect. At the same time roots, stems (including rhizomes), and petioles of aquatic plants usually have porous, gas-filled columns of tissue (aerenchyma) that contribute to buoyancy and allow diffusion of carbon dioxide and oxygen throughout the plant.
The vascular tissues of submergent aquatic plants are also greatly reduced. Consequently, these plants tend to be limp and to collapse in a soggy heap when removed from the water. In order to prepare herbarium specimens, it is necessary to float the plants in a tray of water, slide a piece of mounting paper under them, and carefully lift them out of the water so the stems and leaves are spread in a lifelike manner as they are when floating in a column of water.
Terrestrial plants have evolved various coverings (cuticle, surface hairs, etc.) to prevent excess water loss through evapotranspiration, a problem not encountered under water. Submergent aquatics typically have thin leaves with little or no cuticle; as a result they shrivel quickly when removed from water. Floating leaves, on the other hand, typically have a cuticle on surfaces exposed to the atmosphere.
In order to resist the force of moving water, aquatic plants have evolved strong anchoring features such as rhizomes that are buried in lake and stream bottom sediments, or various forms of holdfasts by which species like water moss (Fontanalis spp.) and riverweed (Podostemum ceratophyllum) cling to rocks and other surfaces. Flexibility is another important characteristic in water plants; many species have long flexible stems or leaf petioles that can move with the water currents or waves. The petioles and peduncles of water-lilies are longer than the distance from the water surface to the rhizome to which they are attached, to allow for wave action; in addition, the petioles are firmly attached near the middle of the leaf blade. The rounded shape and smooth margins of the leaf blades also reduce resistance. The underwater leaves of many species such as the water-crowfoots (Ranunculus spp.), coontail (Ceratophyllum sp.), bladderworts (Utricularia spp.), and water-milfoils (Myriophyllum spp.) are finely divided, thus offering less resistance to water currents and also increasing the surface area for absorption of carbon dioxide.
Stomata are pores that allow for the movement of gasses such as carbon dioxide and oxygen in and out of leaves. In terrestrial plants stomata are typically on the lower leaf surfaces; floating leaves of aquatic plants have their stomata on the upper surface, providing access to atmospheric carbon dioxide. Submersed leaves have few or no functional stomata because carbon dioxide and oxygen dissolved in water enter the leaf, or in some cases, the roots, by diffusion.
Emergent aquatics are more like terrestrial plants in many ways; however, their roots must be able to withstand constant inundation and the stress of wave action. Many are rhizomatous, providing a network of anchoring structures. Porous tissues (aerenchyma) that allow oxygen to diffuse through stems, rhizomes, and roots are often present. Even the leaves of emergent species show adaptations such as blades with an arrowhead shape that offer less resistance to the forces of wind and water.
Some emergent species have evolved leaf surfaces that shed water. This characteristic is most notable in American lotus (Nelumbo lutea), goldenclub (Orontium aquaticum), and northern mannagrass (Glyceria borealis). The hydrophobic quality is the result of tiny rounded, wax-covered projections called papillae that cover the leaf surface (Neinhuis and Barthlott 1997). Engineers in Germany have used the leaf surface of lotus as a model to design a paint that sheds water and dirt.
Variability in Form
Aquatic plants are notoriously variable in form, which can make identification challenging. Variation in water depth is a major cause. For example, common bur-reed, which is normally an emergent plant with stiffly erect leaves, will grow in deeper water, but does not flower, and the leaves are less rigid and bend over and float at the tip. Several arrowheads (Sagittaria graminea and S. rigida) grow vegetatively as short sterile rosettes of narrow, pointed leaves in deep water. Sagittaria graminea can even flower under water, but in the absence of flowers it is impossible to tell the rosettes of these species apart visually.
Plants like the water-crowfoots (Ranunculus spp.), false-mermaid (Proserpinaca spp.), and most of the water-milfoils (Myriophyllum spp.) and pondweeds (Potamogeton spp.) have both underwater and floating or emersed leaves. The floating or emersed leaves are simpler and sturdier compared to the submersed leaves of the same plant. In addition, many of the pondweeds vary as to whether floating leaves are produced. Species such as Potamogeton bicupulatus and Potamogeton diversifolius can grow as submergents in deeper water, but often produce floating leaves in shallow water.
Reproduction
Sexual reproduction—Aquatic angiosperms (flowering plants) may produce their flowers under water, at the water surface, or elevated above the water surface. Pollination strategies vary accordingly. For species that hold their flowers above the water surface, insect or wind pollination can proceed as for terrestrial plants. Underwater flowers, such as those of the waternymphs (Najas spp.) and some pondweeds must depend on water currents to transport pollen, much like wind-pollinated plants on land. For annuals like the waternymphs (Najas spp.) and waterworts (Elatine spp.) seed production is essential to maintain populations from year to year. The pollination success rate for perennial species is not quite as critical, since the plants usually live from year to year. In addition, most aquatic perennials also have effective asexual or clonal growth mechanisms (see below).
Some aquatic plants have evolved unique pollination strategies utilizing the water surface as a stage. Species in the Frog’s-bit Family (Hydrocharitaceae) are especially interesting in this regard.
Water-celery (Vallisneria americana) has one of the most unusual pollination methods. Water-celery is dioecious, meaning that male and female flowers are produced on separate plants. Male flowers are released from an inflorescence at the base of the plant to float freely to the water surface where they are dispersed by wind. Meanwhile female flowers on long peduncles just reach the water surface where their tips produce a dimple that attracts the floating male flowers to where the pollen can be transferred directly from anther to stigma (Figure 1.2). We have seen lakes white with male flowers at the leeward end from a large blooming population of water-celery (Figure 1.3). Once pollination has occurred, the peduncle coils tightly, pulling the developing fruit farther under the water surface where it can mature.
The waterweeds (Elodea canadensis and E. nuttallii) and hydrilla (Hydrilla verticillata), also in the Frog’s-bit Family, have similar mechanisms. Pollen is released onto the water surface from male flowers, explosively in the case of hydrilla, where it is carried by wind or water currents to the stigmas of the female flowers which are positioned at the surface.
Figure 1.2. Pollination in water-celery (Vallisneria americana) occurs when free-floating male flowers drift into contact with female flowers positioned at the water surface.
Figure 1.3. Lake surface covered with white male flowers of water-celery (Vallisneria americana) in Sullivan County, PA.
Asexual reproduction—Asexual, or clonal, reproduction is very common in aquatic plants. Many perennial species produce rhizomes, modified stems that grow horizontally in the substrate, sending up shoots, leaves, or flowers at intervals. Water-lilies, many pondweeds, and most emergent species are in this category. Clonal colonies can expand to cover large areas, forming dense patches of genetically identical plants. In addition, pieces of rhizome that become detached can start a new colony at another location.
Fragmentation is not limited to rhizome segments. Pieces of the stems of many aquatic plants break off and float; these may continue to grow and even flower and fruit as floating fragments. In addition, through the formation of adventitious roots, these fragments can start new colonies. Water-milfoils (Myriophyllum spp.), waterweed (Elodea spp.), coontail (Ceratophyllum spp.), and bladderworts (Utricularia spp.) are examples of plants that reproduce this way.
The formation of dormant buds, or turions, is another way aquatic plants can reproduce. Turions are usually formed at the end of the growing season, resulting in a vegetative structure that can survive the winter or other periods of unfavorable growing conditions. Turions are generally dense and sink to the bottom. When the water warms in the spring they resume active growth. Species that form turions include duckweeds (Lemna spp.), waterweeds (Elodea spp.), bladderworts (Utricularia spp.), and some pondweeds (Potamogeton spp.).
Dispersal
Lakes and ponds are islands of habitat; although streams connect some to watersheds, others are completely isolated. Even streams contain a variety of growing conditions based on variables such as the degree of shading and speed of water movement. Each body of water, connected or not, seems to have a slightly different set of plants. The differences are partly due to the water chemistry (more on that later), but the random movement of seeds and plant fragments (propagules) is also involved.
How do aquatic plants move? Flowing water connects some lakes and ponds to watersheds and carries seeds or plant fragments downstream. Some seeds, such as those of the cat-tails (Typha sp.) are dispersed by wind. But waterfowl and other animals are important vectors of many aquatic species.
Charles Darwin was one of the first to study the potential for birds to disperse seeds (Browne 1995). More recent studies have documented that many waterfowl feed on the fruits and seeds of aquatic plants and serve as effective dispersal agents (Charalambidou and Santamaria 2002). Seeds can also be carried externally on the feet or feathers of birds. Small plants such as the duckweeds and watermeals can also adhere to birds or other animals such as beaver, otter, muskrats, or turtles and be carried from one site to another.
Humans too, spread plants from site to site. Some of this is inadvertent, through seeds or plant fragments that cling to the exterior of boots, boats, motors, oars, or fishing gear. Some of it is more deliberate, such as dumping the contents of an aquarium into a local stream, canal, or lake. Cultivated water gardens can also be a source of plant introductions resulting from overflow during heavy rains, the careless handling of garden waste, or bird-mediated dispersal. The deliberate introduction of species to lakes or ponds, usually for ornamental value, is yet another source.
ROLE OF PLANTS IN AQUATIC ECOSYSTEMS
Photosynthesis
Green plants play a basic role in aquatic ecosystems because of their ability to carry out photosynthesis. Plants, including macrophytes, phytoplankton, and filamentous algae, plus cyanobacteria, are the primary producers that drive aquatic food chains (Figure 1.4). Powered by the sun, they fix carbon, producing energy-rich sugars and starches. Other members of the aquatic community that eat plants are termed herbivores. They include fish, turtles, snails, insects, and some birds. Carnivores in turn depend on the herbivores for food. Predatory birds such as osprey, eagles, herons, and egrets are at the top of the food chain in these systems (Figure 1.5). Directly or indirectly all are dependent on plants.
Sugars and starches are not the only product of photosynthesis; the process also releases oxygen. In aquatic systems the oxygen produced by green plants is important in meeting the respiratory needs of plants and animals.
Limiting factors—The need for light to drive photosynthesis limits the depth to which plants can grow in a lake; this is especially true for rooted submergent species. Light does not travel as far through water as through air. In addition, plankton and suspended sediments create turbid conditions that limit light penetration. The clearer the water, the greater the depths at which rooted submergent plants will be found. See also discussion of stem length and branching patterns in the section on growth habit above.
Emergent, floating-leaf, and free-floating plants have unimpeded access to sunlight. However, they can form a canopy that reduces the amount of light reaching lower layers of aquatic vegetation, just as in a forest.
Figure 1.4. An aquatic food chain: powered by the sun, plants and algae in and around the lake (primary producers) photosynthesize, producing sugar and starch. Plants are food for many small fish, insects, and other invertebrates such as snails (herbivores), which are in turn consumed by carnivores. Larger fish and birds of prey are often the top carnivores in freshwater systems.
Figure 1.5. The Great Blue Heron and other fish-eating birds are top predators in aquatic ecosystems.
Carbon dioxide is the source of carbon for photosynthesis in terrestrial plants; in aquatic systems the carbon source can be either dissolved carbon dioxide (CO2), bicarbonate , or carbonate . Sources include the atmosphere, geology, and carbon dioxide resulting from the respiration of aquatic organisms. The form in which carbon is available depends on the pH (acidity or alkalinity) of the water. Bicarbonate and carbonate are dominant at pH 6.4 and above. Dissolved CO2 is more abundant at pH values below 6.4.
Dissolved CO2 is used by most aquatic macrophytes and algae. Some algae and submersed macrophytes, including mosses and quillworts, can use only CO2. Submersed, rosette-type plants, including water lobelia (Lobelia dortmanna) and quillworts (Isoetes spp.), obtain up to 90 percent of their carbon dioxide directly from lakebed sediments through their roots. Interestingly, these plants have stiff leaves with a cuticle, which prevents loss of CO2 through diffusion into the surrounding water. Other plants, such as waterweed (Elodea spp.), can switch to bicarbonate when free CO2 is in low supply; however, photosynthesis under those conditions is less efficient.
Another modification seen in hydrilla, Brazilian waterweed, and common waterweed is similar to the C4 photosynthesis seen in warm season grasses and other terrestrial plants in high light/high temperature conditions. This allows the plants to capture carbon dioxide in the dark, later converting it to typical C3 sugars (Casati et al. 2000).
Decomposers
Detritus feeders and decomposers that feed on decaying plants and other organic matter form another important link in the system. These organisms release minerals to be utilized in a new cycle of growth. Decomposition may be slow in aquatic systems because of a lack of oxygen at the lake or stream bottom; very acidic conditions also inhibit decomposition, leading to a buildup of peat.
An excessive amount of organic matter, such as occurs when high nutrient levels stimulate an algae bloom, may create anoxic conditions as decomposers deplete the oxygen level, depriving other organisms of an adequate source.
Habitat Structure
The zone in which aquatic plants can grow is called the littoral zone (Figure 1.6). Greater water depth, where light cannot reach the bottom to support photosynthesis, is referred to as the benthic zone. The littoral zone often shows strong zonation with emergent plants such as pickerel-weed (Pontederia cordata), arrowhead (Sagittaria spp.), bur-reeds (Sparganium spp.), and spike-rushes (Eleocharis spp.) closest to the shore (Figure 1.7). Beyond them are the water-lilies and other rooted floating-leaf plants. Rooted submergents can be found at greater depths and throughout the littoral zone, although a dense stand of water-lilies (Nymphaea odorata and Nuphar spp.) or watershield (Brasenia schreberi) may limit the light too much to allow other plants to grow beneath them.
Plants provide cover for young fish, tadpoles, and salamander larvae. Many algae grow attached to the surfaces of larger plants as epiphytes. Insects and other invertebrates such as fresh water sponges also live on underwater plant surfaces. Emergent plants provide sites for dragonflies, and other insects with aquatic larval stages, to crawl up out of the water in order to emerge as aerial adults (Figure 1.8).
Figure 1.6. Diagram of the littoral zone showing bands of emergent, floating-leaf, and submergent plants.
Figure 1.7. Zonation in littoral zone vegetation at a lake in Wayne County, PA; note emergent plants closest to the shoreline with floating-leaf species farther out and open water beyond.
Figure 1.8. Shed skin (exuvia) left clinging to emergent vegetation following metamorphosis of a dragonfly from its aquatic larval stage.
TYPES OF AQUATIC ECOSYSTEMS
Lakes
All lakes, whether they are natural or created by damming, are on a trajectory that will result in filling and eventual conversion to swamp or marsh and then dry land. This process is accelerated by the production of abundant organic matter in the lake itself, but is slower in less productive systems.
Most of the natural lakes in Pennsylvania are found in the glaciated regions of the northeastern and northwestern corners of the state (Figure 1.9). Some are isolated kettle lakes that lack an outlet; depending on the depth and slope of the depression, they may be surrounded by a bog mat.
Bogs develop in shallow basins where acidic, low-nutrient conditions favor the growth of sphagnum mosses and associated bog vegetation around the lake margins. These lakes are termed dystrophic. Their water is typically stained brown by organic acids that leach out of the peat and other decaying vegetation. They will eventually become bogs, bog forests, and finally terrestrial forests as they fill with peat and dry out over time (Figures 1.10, 1.11).
Glacial lakes with low nutrient conditions, exceptionally clear water, and sparse growth of aquatic plants are termed oligotrophic. Some of these lakes drop off quickly to depths up to 30 m; as a result the littoral zone is narrow and sediment accumulation is low. Well developed bogs are not present. These lakes may be isolated kettle lakes, or water bodies with both inlets and outlets. Some appear to have been formed due to damming of streams by glacial deposits; but all are characterized by low within-lake biomass production. Many of the aquatic plants classified as endangered and threatened in Pennsylvania are found in lakes at the oligotrophic end of the spectrum (see below).
Other natural lakes in Pennsylvania would probably be classified as mesotrophic. These are lakes with moderate nutrient availability that support the growth of a variety of plants. Mesotrophic conditions may reflect natural inputs from the underlying geology or moderate enrichment due to watershed conditions. Glacial lakes in northwestern Pennsylvania are in this category due to the limestone content of the underlying glacial deposits.
Figure 1.9. Distribution of lakes in Pennsylvania with respect to the southern boundary of the Wisconsinan (most recent) glaciation; lake location data provided by the Pennsylvania Department of Environmental Protection Clean Lakes Program.
Figure 1.10. Succession from lake to bog and eventually dry land in a glacial kettle lake.
Figure 1.11. A glacial kettle lake surrounded by a floating bog mat in Sullivan County, PA.
Eutrophic lakes, at the other end of the spectrum, are systems that have undergone nutrient enrichment with nitrogen and phosphorus derived from external sources such as agriculture, sewage systems, or fertilized lawns in the immediate watershed.
In addition to natural lakes, Pennsylvania has many lakes that were formed by humans damming a stream or wetland. These lakes are usually shallow (except perhaps in the former channel) and support abundant aquatic growth throughout. When coupled with nutrient enrichment, they become eutrophic. Abundant plant growth results in rapid build-up of sediments and accelerated successional development toward marsh and terrestrial conditions.
Rivers and Streams
Rivers and streams provide a variety of habitats for aquatic plants, from rapids, where riverweed (Podostemum ceratophyllum) and aquatic mosses cling to the rocks, to shoreline shallows and backwaters where a diversity of emergent and submergent species may be found. Factors such as water quality and rate of flow affect the occurrence of aquatic vegetation. Lack of light may limit the growth of some aquatic species in small streams in forested areas. In urban and suburban landscapes scouring due to heavy flow during and after rainstorms may limit the growth of rooted submergent species.
Delaware Estuary
The tidal influence of the Atlantic Ocean extends up the Delaware River to the fall line, where elevation of the river exceeds the elevation of the high tide line. The Pennsylvania portion of the estuary extends from the Pennsylvania/Delaware state line to Morrisville in Bucks County. Twice a day the water level in the estuary rises and falls as much as 2 m, creating tidal marshes along the shoreline of the river and its tributaries (Figure 1.12). The tidal influence also extends up tributary streams; however, in some cases, such as the Schuylkill River and Neshaminy Creek, dams have truncated its extent.
Figure 1.12. Freshwater tidal marsh on the bank of the Delaware River in Bucks County, PA.
Classified as freshwater tidal marshes, despite the fact that the water in the estuary is slightly brackish, the marshes support a group of highly specialized aquatic plants. Many freshwater tidal marsh plants are classified as endangered, threatened, or rare by the Pennsylvania Natural Heritage Program, including water-hemp ragweed (Amaranthus cannabinus), swamp beggar-ticks (Bidens bidentoides), showy bur-marigold (Bidens laevis), American waterwort (Elatine americana), dwarf spike-rush (Eleocharis parvula), mud-plantain (Heteranthera multiflora), long-lobed arrowhead (Sagittaria calycina), subulate arrowhead (Sagittaria subulata), river bulrush (Schoenoplectus fluviatilis), Smith’s bulrush (Schoenoplectus smithii), Walter’s barnyard-grass (Echinochloa walteri), and wild-rice (Zizania aquatica).
Freshwater tidal marshes are an endangered habitat in Pennsylvania. They are limited to the narrow strip of Atlantic Coastal Plain in the state, and development of Philadelphia and the adjacent riverfront areas of Delaware and Bucks Counties has taken a heavy toll. Several plants that once grew along the tidal shores of the Delaware are believed to be extirpated. Parker’s pipewort (Eriocaulon parkeri), awl-shaped mudwort (Limosella australis), and Nuttall’s mud-flower (Micranthemum micranthemoides) have not been seen in Pennsylvania in over 50 years. Nuttall’s mud-flower is believed to be extinct throughout its range.
Threats to freshwater tidal marshes include riverbank erosion, leading to loss of fine sediments (Figure 1.13) and colonization by non-native, invasive plants. Common reed (Phragmites australis) is the most troublesome of the invaders. In addition, Chinese lobelia (Lobelia chinensis), a low-growing plant from the Asia-Pacific region, has also become common along the tidal shores of the Delaware River.
Figure 1.13. Tidal riverbank showing absence of fine sediments and tidal marsh vegetation except in the area protected by a discarded section of dredge pipe.
Sea level rise, which is already occurring, is another threat. To survive, tidal marshes will have to accrete (grow upward) due to the gradual increase in elevation brought on by sediment deposition, or migrate inland. However, rates of sediment deposition are too slow to keep up with the current increase in sea level, which is expected to be approximately one meter by the year 2100. Unfortunately, a recent study has revealed that in very few cases is land available for freshwater tidal marshes in Pennsylvania to migrate inland (Titus et al. 2009).
MANAGEMENT OF AQUATIC ECOSYSTEMS
Endangered, Threatened, and Rare Species
About 60 species of aquatic plants are classified as endangered, threatened, rare, undetermined (candidate), or watch list by the Pennsylvania Natural Heritage Program at this time (PNHP 2010). One species, northeastern bulrush (Scirpus ancistrochaetus), is also listed as threatened under the Federal Endangered Species Act (Figure 1.14; Table 1.2).
Because of their rarity, these plants make an important contribution to the overall biological diversity of aquatic ecosystems. Many, such as water lobelia (Lobelia dortmanna), bayonet rush (Juncus militaris), floating-heart (Nymphoides cordata), slender water-milfoil (Myriophyllum tenellum), and horned bladderwort (Utricularia cornuta) are associated with oligotrophic glacial lakes; Pennsylvania populations represent the southern limit of range of these species.
An additional twelve species are believed to be extirpated in Pennsylvania. Of these, four are plants that were found in the freshwater intertidal zone.
PNHP-listed plants are protected under the Pennsylvania Code, Title 17, Chapter 45, Conservation of Pennsylvania Wild Plants and implementing regulations (Commonwealth of Pennsylvania 1993). The program is administered by the Pennsylvania Natural Heritage Program and the Bureau of Forestry of the Department of Conservation and Natural Resources.
Figure 1.14. Northeastern bulrush (Scirpus ancistrochaetus), a federally threatened aquatic plant that occurs in Pennsylvania.
Table 1.2. Endangered, Threatened, and Rare Aquatic Plants of Pennsylvania
a Global ranks: G5 = secure, G4 = apparently secure, G3 = vulnerable; state ranks: S3 = vulnerable, S2 = imperiled, S1 = critically imperiled, SU = status uncertain, SX = apparently extirpated.
b PNHP status: PE = Pennsylvania endangered, PT = Pennsylvania threatened, PR = Pennsylvania rare, PX = extirpated in Pennsylvania, TU = tentatively undetermined, N = not listed, SP = special population, W = watch list.
c Federal rank: LT = listed threatened.
Problem Vegetation
Exotic invasive species—Non-native plants have invaded aquatic ecosystems as well as terrestrial habitats (Table 1.3). In low nutrient systems they are usually not a problem, as most serious invasives require high resource availability to grow vigorously. Where nutrients are not limiting, species like European water-chestnut (Trapa natans) (Figure 1.15), Eurasian water-milfoil (Myriophyllum spicatum), hydrilla (Hydrilla verticillata), fanwort (Cabomba caroliniana), or curly pondweed (Potamogeton crispus) can become dominant. Invasive species are also more likely to become a problem in recently created impoundments where a native flora has not yet become established, or in lakes where excessive use of herbicides has eliminated the native plants. Dense growth of species like water-chestnut or Eurasian water-milfoil not only interferes with native plant growth, but also interferes with recreational uses such as boating and swimming.
Table 1.3. Non-native, Invasive Aquatic Plants in Pennsylvania
| COMMON NAME | SCIENTIFIC NAME | % OF LAKESa |
| long-stem waterwort | Elatine triandra | 15.7 |
| curly pondweed | Potamogeton crispus | 13.0 |
| Eurasian water-milfoil | Myriophyllum spicatum | 9.6 |
| cultivated water-lilies | Nymphaea spp. | 9.6 |
| yellow iris | Iris pseudacorus | 4.3 |
| water-chestnut | Trapa natans | 3.5 |
| hydrilla | Hydrilla verticillata | 3.0 |
| fanwort | Cabomba caroliniana | 2.6 |
| waternymph | Najas minor | 1.8 |
| Brazilian waterweed | Egeria densa | <1.0 |
| European water-clover | Marsilea quadrifolia | <1.0 |
| flowering-rush | Butomus umbellatus | <1.0 |
| mudmat | Glossostigma cleistanthum | <1.0 |
| parrot-feather | Myriophyllum aquaticum | <1.0 |
| watercress | Nasturtium officinale | streams and springs |
| water-starwort | Callitriche stagnalis | streams and springs |
a Based on surveys of 115 lakes between 2000 and 2007.
Prevention is far more effective than attempts to control an invasive plant after it has become established. Prevention can take the form of limiting access by boats from other areas that might carry seeds or plant fragments and educating lake users to prevent deliberate or accidental introductions of non-native plants. Another approach is to protect water quality; nutrient enrichment (eutrophication) will exacerbate problems with over-abundant vegetation.
Figure 1.15. An infestation of the non-native, invasive water-chestnut (Trapa natans) at a lake in Bucks County, PA.
Physical control efforts such as pulling, cutting, or raking may provide temporary relief; however, the ability of many aquatic plants to propagate themselves from detached fragments should be kept in mind.
Over-abundant native species—Not all problems arising from excessive growth of aquatic vegetation are caused by non-native species. Under favorable growing conditions some native species can also form dense stands that interfere with recreational uses of lakes. Shallow water and high nutrient availability are most often the causes.
Opportunistic species that have created problems include fragrant waterlily (Nymphaea odorata) (Figure 1.16), Farwell’s water-milfoil (Myriophyllum farwellii), common water-milfoil (M. humile), broad-leaved water-milfoil (M. heterophyllum), waterweed (Elodea nuttallii), purple bladderwort (Utricularia purpurea), inflated bladderwort (Utricularia inflata), and coontail (Ceratophyllum demersum).
Prevention of problems caused by explosive growth of native species should focus on reducing nutrient inputs.
Figure 1.16. Water surface nearly covered by native fragrant water-lily (Nymphaea odorata) at a shallow pond in Lackawanna County, PA.
