Aquatic plants, also known as hydrophytes, are a diverse group that adapted to live in aquatic environments, whether freshwater or saltwater. These specialized plants are found in various watery habitats, including lakes, rivers, ponds, and wetlands. Aquatic plants possess unique features that allow them to thrive when submerged or partially submerged. This adaptation enables them to form an integral part of aquatic ecosystems, supporting a wide array of other organisms.
Classifying Aquatic Plants
Aquatic plants are commonly categorized based on their growth habit relative to the water surface. Emergent plants, for instance, are rooted in the sediment but have the majority of their stems and leaves extending above the water. Familiar examples include cattails and bulrushes. Floating-leaved plants, such as water lilies, are also rooted in the bottom, but their leaves float on the water’s surface. Free-floating plants, like duckweed, are not anchored to the substrate and drift freely on the water’s surface.
The fourth category, and the primary focus when considering plants that grow completely underwater, are submerged plants. These plants live entirely below the water’s surface, with some being rooted to the bottom sediment and others free-floating within the water column. Submerged plants exhibit distinct adaptations that allow them to absorb nutrients and gases directly from the surrounding water.
Unique Adaptations for Underwater Growth
Plants that grow completely underwater have evolved specific physiological and structural adaptations. One notable adaptation is the absence or reduction of stomata, the small pores used for gas exchange in terrestrial plants, as submerged plants absorb gases directly through their thin epidermal layers. Their leaves often lack a thick, waxy cuticle, which is unnecessary for preventing water loss and would hinder gas and nutrient absorption.
Submerged plants frequently possess thin, highly dissected leaves, which maximize the surface area for efficient absorption of dissolved gases like carbon dioxide and oxygen, and nutrients from the water. Many aquatic plants also develop a specialized tissue called aerenchyma, a spongy tissue with large air-filled spaces throughout their stems, leaves, and roots. Aerenchyma provides buoyancy, allowing the plants to remain upright in the water column, and facilitates the transport of gases, such as oxygen from photosynthetic tissues to submerged roots.
The root systems of submerged plants are often reduced, as their primary function shifts from water and nutrient absorption to anchorage in the sediment. Some submerged plants, like hornwort, are even free-floating and lack roots entirely. Their stems are typically flexible, enabling them to withstand water currents without breaking. Reproduction in submerged plants can involve specialized strategies, including underwater pollination or vegetative propagation through fragmentation, rhizomes, or specialized buds.
Key Examples of Submerged Plants
Many plant species thrive entirely underwater. In freshwater habitats, common examples include Anacharis, which features dense leaves arranged in whorls along its stems. Hornwort (Ceratophyllum demersum) is another prevalent freshwater submerged plant, characterized by its feathery, fan-shaped leaves and lack of a root structure.
Vallisneria, often called eelgrass or water celery, is a popular submerged plant with long, ribbon-like leaves that typically root in the substrate and can form extensive underwater meadows. Cabomba (Cabomba caroliniana), or fanwort, is recognized by its highly dissected, fan-shaped leaves. Various aquatic mosses, such as Java Moss, also flourish completely submerged, often attaching to rocks or driftwood.
In saltwater environments, true flowering plants that grow completely submerged are primarily seagrasses. Examples include Eelgrass (Zostera marina), found in temperate coastal waters, and Turtle Grass (Thalassia testudinum), common in tropical and subtropical regions. These marine plants form underwater meadows, providing habitat and food for numerous marine species.
Ecological Importance
Submerged aquatic plants are integral to the health and functioning of aquatic ecosystems. Through photosynthesis, they release oxygen into the water, which is essential for the survival of fish, invertebrates, and beneficial bacteria.
These underwater plant beds provide habitat and shelter for a variety of aquatic animals, including fish, shellfish, and invertebrates, offering protection from predators and serving as nursery grounds for juvenile organisms. Many aquatic animals also feed directly on submerged plants or the small organisms that live on their surfaces. Submerged plants also contribute to water quality by absorbing excess nutrients like nitrogen and phosphorus, which can otherwise lead to harmful algal blooms. Their root systems help stabilize bottom sediments, preventing erosion and improving water clarity by trapping suspended particles.