Submerged Aquatic Vegetation

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Submerged aquatic vegetation

Submerged aquatic vegetation consists of a taxonomically diverse group of plants that lives entirely beneath the water surface. This diverse group of aquatic plants includes species of angiosperm vascular plants, mosses, and liverworts, and macroalgae (seaweeds). Their underwater growth habit separates them from other kinds of aquatic plants that are free-floating, have floating leaves, or are emergent above the water surface.

Almost all species of submerged aquatic plants live in freshwater ponds, lakes, rivers, streams, and wetlands , and shallow marine waters. They can occur in ponds with an acidic pH less than 4 or in alkaline waterbodies with pH greater than 10, but they tend to be most rich in species at pHs of 6 to 8. Only a few angiosperm species occur in brackish estuarine or marine habitats, including the eelgrass (Zostera marina ), widgeon grass (Ruppia maritima ), and turtle-grasses (Thalassia species). No aquatic mosses or bryophytes occur in saline waters.

Macroalgae, commonly known as seaweeds, are also classified as submerged aquatic vegetation though they are not biologically part of the kingdom Plantae. They are members of the kingdom Protista and live within the marine environment . They, too, are beneficial to the aquatic habitat .

Both grasses and macroalgae absorb nutrients which can be a source of pollutants in aquatic habitats. Submerged vegetation also is a major food source for many different species of organisms including waterfowl, sea turtles , and manatees . These plants and seaweeds add oxygen to the water, and grass roots help stabilize shorelines against erosion . Additionally, submerged aquatic vegetation provides shelter and food for organisms. For example, crustaceans, especially the blue crab, and juvenile or larval fish use submerged vegetation as protective nurseries and a means to hide from predators. Some organisms like barnacles and bryozoans attach themselves to plant surfaces to live. Other species use submerged vegetation as place to lay eggs and hatch new offspring.

Though communities of submerged aquatic vegetation help improve water quality , their own survival depends on maintaining good water quality. These plants and macroalgae are sensitive to environmental changes brought about by agricultural runoff , industrial waste, and global warming. Increased temperature, salinity , and depth of coastal waters in particular can contribute to marked habitat change for these species, thus threatening their distribution and abundance.

Submerged aquatic plants are most abundant in relatively shallow water, where they have access to enough sunlight to engage in photosynthesis at a high enough rate to survive. Waterbodies with poor visibility may support few or no submerged aquatic plants, and only at very shallow depths. Waterbodies may have poor visibility because of an excessive abundance of phytoplankton , or they may be highly turbid because of suspended clays, or they may be brown-colored because of dissolved organic matter leached from nearby bogs or peaty soil . Waterbodies with exceptionally clear water may have submerged aquatic plants growing on the bottom as deep as about 19 feet (6 m). Shallow, moderately fertile, mesotrophic or eutrophic waterbodies may support especially large populations of submerged aquatic plants, where they may even be regarded as nuisance "weeds".

Some of the most widespread and familiar species of angiosperm submerged aquatic plants include the tape-grass or wild celery (Vallisneria americana ), the waterweed (Elodea canadensis ), the ribbon-leaf pondweed (Potamogeton epihydris ), the slender water-nymph (Najas gracillima ), the pipewort (Eriocaulon septangulare ), the greater bladderwort (Utricularia vulgaris ), and the lake quillwort (Isoetes lacustris ). Examples of bryophytes that are submerged aquatic plants include an aquatic peatmoss (Sphagnum macrophyllum ), an aquatic moss (Fontinalis antipyretica ), and an aquatic liverwort (Ricciocarpus natans ).

A few species of submerged aquatic plants are used as ornamentals in aquaria and outdoor water-gardens. There are even some clubs of aficionados of the aesthetics of these cultivated aquatic plants. Examples include the Amazon sword (Echinodorus amazonicus ), the tropical hornwort (Ceratophyllum submersum ), the temple plant (Hygrophila corymbosa ), the Java moss (Vesicularia dubyana ), and a tropical aquatic liverwort (Riccia fluitans ). Submerged aquatic plants are also used as ecosystem components in the constructed wetlands that are sometimes used to treat human sewage and other wastes.

A few submerged aquatic plants, particularly several non-native invasive species, can be sufficiently abundant that they are considered serious weeds because of their effects on the use of waterbodies for recreation and transportation . Some of the most important invasive weeds of this kind in North America are the Eurasian water-milfoil (Myriophyllum spicatum ), the Brazilian waterweed (Egeria densa ), and the hydrilla (Hydrilla verticillata ). Attempts are sometimes made to reduce infestations of these species using mechanical harvesters or herbicides.

[Bill Freedman Ph.D. ]